31179A17- pb.doc

[CN] Chapter 7

[CT] Softswitch

[A HD] Introduction...........................................................................................................................3[A HD] What is a Softswitch?............................................................................................................4[A HD] A Distributed Architecture....................................................................................................5[A HD] Access....................................................................................................................................7

[B HD] PC to PC and PC to Phone................................................................................................7[B HD] Phone-to-Phone VoIP........................................................................................................8

[A HD] VoIP Gateways....................................................................................................................12[B HD] Enterprise Gateways........................................................................................................15[B HD] Carrier Grade Gateways..................................................................................................16

[A HD] Softswitch Technology........................................................................................................19[B HD] Applications Server.........................................................................................................21[B HD] Softswitch Applications..................................................................................................22[B HD] IP Centrex........................................................................................................................23[B HD] Class 5 Exchange.............................................................................................................24[B HD] Softswitch Replacing Class 4 Switches...........................................................................28[B HD] Softswitch Replacing Class 5 Switches...........................................................................31

[A HD] Transport.............................................................................................................................33[B HD] Converged and Converging Architectures......................................................................33[B HD] IP Networks.....................................................................................................................36[B HD] TDM................................................................................................................................38[B HD] ATM................................................................................................................................39

[FIG] Figure 7-1 Softswitch Components...........................................................................................5[FIG] Figure 7-2 PC to PC and PC to Phone......................................................................................8[FIG] Figure 7-3 IP Phones on an IP Network..................................................................................10[FIG] Figure 7-4 SOHO grade gateway............................................................................................14[FIG] Figure 7-5 Enterprise gateways on a VoIP network................................................................16[FIG] Figure 7-6 IP Centrex using a Class 5 switch.........................................................................17[FIG] Figure 7-7 IP PBX also known as soft PBX...........................................................................23

Convergence+ Guide To Convergence Technologies ISBN# 0619131179

-1 4/12/2023

[FIG] Figure 7-8 IP Centrex using a Class 5 switch.........................................................................26[FIG] Figure 7-9 IP Centrex with Softswitch....................................................................................27[FIG] Figure 7-10 Architecture of Class 4 and Class 5 switches with VoIP gateways....................29[FIG] Figure 7-11 Class 4 replacement Softswitch Note: Class 4 TDM switches are absent...........30[FIG] Figure 7-12 Class 5 replacement Softswitch...........................................................................31[FIG] Figure 7-13 Legacy network: data, voice &SS7 networks......................................................34[FIG] Figure 7-14 Converging networks: combined circuit and packet switched networks............35[FIG] Figure 7-15 A converged network data and voice on same network......................................36

[OBJ HD] After reading this chapter and completing the exercises you will be able to:

[BEG OBJ]

Understand and describe the concept of softswitch and why it is the next generation technology

Describe differences between the PSTN and IP networks relative to distributed and centralized

switching systems

Discuss VoIP gateways and describe their functions

Explain the relationship between; gatekeeper and media gateway controller, and softswitch and the

media gateway.

Understand a signaling gateway

Compare and explain how the softswitch can replacement a PSTN class 4 switch

Compare and explain how the softswitch can replacement a PSTN class 5 switch

Understand the components and functions of a packet switched network and contrast them with the

circuit switched network components and functions.

[END OBJ]


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[A HD] Introduction

[BT A HD]

You will recall that the PSTN (Public Switched Telephone Network) performs the following functions;

access, switching, and transport. The softswitch network performs the very same functions. If they

perform the same functions, what distinguishes one from the other? Primarily the two networks differ

only in their architectures. The softswitch architecture is a distributed architecture whereas the PSTN is

a centralized architecture. You will explore the architecture for softswitch solutions and the

components that comprise this new and flexible architecture. Many of the rules change between the

two architectures to accommodate the shift from circuit switched to packet-switched client-server

architecture, but the basic functions remain the same. The current softswitch architecture must

accommodate the need to protect the current investments in legacy equipment and support. The

softswitch bridges the legacy PSTN technology to the next level known as the next generation

technologies. The legacy equipment will only be replaced by softswitch architectures, when the

economics of the legacy equipment become favorable for the next generation of technology. New

installations have the advantage from the start without the weight of integrating with the older

technology. Those who are deep into old technology will delay in purchasing and installing

architecture that is cheaper, simpler, smaller, and more convenient to use. Many will not be quick to

change until the economics show acceptable cost savings. Economic factors; equipment depreciation

cost of service, and growth will be heavily considered before migration to the softswitch architecture

will take place. You will discover that migration to softswitch can be accomplished slowly, since the

softswitch has been designed to integrate with the legacy PSTN.


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[A HD] The Softswitch

[BT A HD]

A Softswitch is a device used to control connections between circuit switched and packet switched

networks. A device that contains both the switching logic and fabric is used. It is common that the

device is treated today as two parts; a Media Gateway, and a Call Agent Gateway.

The Call Agent manages functions like; call routing, signaling, and billing so on and acts as a control

center. A single Call Agent can control a number of Media Gateways in geographically isolated areas

over a TCP/IP network.

The Media Gateway connects various types of digital media creating an end-to-end path. The media

gateway can have interfaces connecting to PSTN using DS1 or DS3 ports, it can interface to ATM

networks and it has Ethernet interfaces to connect VoIP calls. The call agent controls and instructs the

media gateways to connect media between these interfaces to complete the call.

[A HD] A Distributed Architecture

[BT A HD]

Softswitch unlike the PSTN is a distributed architecture as shown in Figure 7-1. This network model


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separates the packet-switching hardware from the call control, service logic, and service creation used

in the PSTN model. The distributed architecture enables flexibility when adding new services as well

as provides choices of hardware without requiring changes in the switching structure. This architecture

provides opportunities for developers to innovate market niche products and services. The lower layer

physically transports both voice and data traffic. This layer is serviced by media gateways.

SGCP (Simple Gateway Control Protocol) controls VoIP gateways by a call control component known

as a call-agent.

[FIG] Figure 7-1 Softswitch Components


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This is client-server architecture and is what provides developer opportunities. The distributive

architecture is what enables a service provider to start out small and grow with the economic demand

as opposed to putting out a huge upfront investment in Class 4 switches.


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Signaling Gateway

Feature Server

Softswitch

Universal Media

Gateway

SGCP

SIP

MGCP

MGCP (Media Gateway Control Protocol)SGCP (Simple Gateway Control Protocol) call agentSIP ( Session Initiation Protocol)

MGCP (Media Gateway Control Protocol)SGCP (Simple Gateway Control Protocol) call agentSIP ( Session Initiation Protocol)

[A HD] Access

[BT A HD]

Softswitch and PSTN architectures each address three key functions:

[BEG BL]

Access - how a subscriber becomes attached to the network

Switching - how a call is controlled across the network

Transport - how a call is moved across the network

[END BL]

In the case of accessing a VoIP (Voice over IP) network, access can be gained from an IP source by

either a PC or IP phone, or from an analog handset through a media gateway. In a residential setting

your ordinary analog phones using the existing internal wiring can be attached to a very inexpensive

ATA (analog terminal access) device which is attached to your broadband DSL or cable service.

[B HD] PC to PC and PC to Phone

[BT B HD]

Early on, the QoS (quality of service) on the IP network left much to be desired. As a result this form

of access, PC voice to PC voice, did not catch on immediately in the market place. The first VoIP

applications used PCs equipped with speakers and microphones to access a VoIP network. It is now

also possible to complete phone calls PC to telephone using a softphone application on the PC. PC-to-

PC and PC to telephone applications are now used most widely for long-distance bypasses as you


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recall from an earlier chapter, (see Figure 7-2) for a graphic of the PC to PC and the PC to telephone

design. The primary users of access have been saving money on long distance calling, in particular on

international long distance. Some of the applications like Skype allow you to make long distance calls

world wide for free PC to PC. SkypeOut provides a PC to PSTN phone service to most countries for a

very small per minute fee like .017 € per minute. Today 10 € equals about 12 U.S. dollars.

[FIG] Figure 7-2 PC to PC and PC to Phone

[B HD] Phone-to-Phone VoIP

[BT B HD]

As the voice quality of VoIP improved it was not long before the industry recognized the benefits of a


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IP Network

Voice GatewayVV

Class 4 Switch

Telephone

PC PC

PC as a phone. From there the IP handset was born. The IP handset includes the necessary computer

hardware to make an IP phone call possible. The IP phone certainly removes cultural objections to

VoIP calls using the computer as a phone. The IP handset looks and functions like an ordinary

telephone instead of a PC. IP handsets are stand-alone devices and provide the option for an IP

desktop-to-desktop solution. A GUI (graphic user interface) on the enterprise IP phone enables

numerous features much easier than with a 12-key conventional telephone handset with its long list of

codes to memorize.

The key advantage of an IP phone to an enterprise is that the phone requires a minimum of network

configuration and management compared to a PBX. Each employee equipped with an IP phone can

move their own phone anywhere that they can get onto the network with possibly no manual

reconfiguration of the phone or the network. Wherever an IP handset is connected on an IP network

(see Figure 7-3) the phone is recognized through its own IP and MAC addresses. In an enterprise

setting, a worker can disconnect their IP phone and move to another cubicle, building, or state, or even

a hotel (may require an ATA device or an 800 number to connect into the company network) and the

phone will function with no reprogramming necessary. With a circuit-switched PBX (private branch

exchange), tools for managing moves, adds, or changes are very time consuming and difficult to say

the least. And IP networks ease of use makes it very expensive to administer. According to some

estimates, it may cost from $300 to $400 per phone on a PBX to; move, add, or change it in any way. It

can occasionally take a number of days for a change to be implemented on a PSTN Centrex by the

telephone company.


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[FIG] Figure 7-3 IP Phones on an IP Network

Two VoIP Protocols will be discussed in a later chapter; SIP (Session Initiation Protocol), and H.323.

An IP phone connects to the network via an Ethernet connection (RJ-45). In a business environment,

an Ethernet switch serves to concentrate VoIP phone lines. This makes the VoIP service provider very

happy since the SP (service provider) does not need to maintain a Class 4 or 5 switch or even a VoIP

gateway.


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IP Telephone

IP Telephone

IP Telephone

IP Network

Data

Softswitch

Server

In the PSTN voice network, service providers would have to purchase and maintain large Class 4 or 5

switches. In a VoIP network, it is generally assumed that the subscriber will purchase their own CPE

(customer premises equipment), therefore, the cost per DS0 (a single 64 kbps port) for the service

provider is zero dollars.

SIP and Java programs on an IP network enable possibilities for future applications that would not be

feasible with circuit-switched architectures. These applications can be separated into three categories:

[BEG BL]

WTI (web-telephony integration) applications.

Occupation specific and industry specific applications

personal productivity applications

[END BL]

The LCD (liquid crystal display) screens with GUIs on IP phones provide numerous expanded

functions when compared to the analog handset. Using a conventional POTS handset, the user must

memorize a long list of numbered codes to perform functions like as call transfer, call forwarding,

camp on, conferencing, voice mail retrieval, and so on. Most users limited their list to only a handful of

the possible features available on a PBX. When needed they may know the feature exists but do not

know the code to activate it. This barred them from being as efficient in their communications as they

could be. An IP phone with a GUI enables access to all the phone features.


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IP phones today are yet very expensive (~$500) compared to a conventional business handset (~$150).

That high cost is very unattractive to the residential market, but remember that in a residential setting

they have the option of using their legacy analog phones using an ATA to interface then to the IP

network. Price competition historically has driven the price of new technologies, it is expected the

same will happen with IP phones bringing the prices into an affordable range.

In review of the distributed architecture, the leading advantage of IP phones to a service provider is

that they do not need to invest in a switch or gateway. From the SP’s point of view the subscriber will

cover the phone investment by purchasing their own IP phones. Service providers’ profits are earned

today on high margins from service feature offerings. Many new features not possible via circuit

switched telephone networks will become new market niches. And the expectation is very high for

many new features that the consumer will buy into. The IP phone GUI interfaces make services easier

to use, resulting in a greater marketability of those new services. It is evident that the future voice

network will consist of IP phones that connect to IP networks where the intelligence for that service is

provided by softswitches located throughout the network instead of only on centralized computer

offices.

[A HD] VoIP Gateways

[BT A HD]

The commercial type of IP access is via VoIP gateways. A gateway provides a connection between an

endpoint on a data network to the edge of the PSTN, switched-circuit network. The gateway’s job is to

translate transmission formats and communication procedures that are used on each side (the data


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network and the PSTN). Gateways can be provisioned as standalone devices or they can be

provisioned to integrate to other systems. An existing telephone handset interfaces to a gateway via; a

direct connection, a PBX, or a Class 5 switch. The gateway is responsible for packetizing the voice and

routing it over the IP network.

Located on the edge of a network is the media gateway, it interfaces between TDM (Time Division

Multiplexing) and IP networks. Here is where analog or digital signals from a handset (PBX or Class 4

or Class 5 switch) are digitized (if analog), packetized, and compressed for transmission over an IP

network. The mechanism that accomplishes this is called a codec (in this case a speech coder/decoder).

The media gateway is capable of interfacing directly with a TDM switch (PBX or Class 4 or Class 5

switch).

A gateway includes three elements:

[BEG BL]

an interface for the TDM side of the network (described in terms of DS0s or T1s)

an interface for the packet side of the network (usually an Ethernet connection)

the necessary signal processing between these two sides.

[END BL]

Signal processing is done using a DSP (digital signal processors) on a circuit board designed to support

voice. Signal processing functions include echo cancellation, coding/decoding of the analog signals

adapting the digitally encoded information into a series of IP datagrams that are then transmitted over


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the network to their destination.

Initially, in the early VoIP days, the gateway had to contain a lot of intelligence to make voice calls

possible. A media gateway can be a part of the switching and routing depending on the intelligence

contained in the gateway. The advancement of gatekeeper technology into a carrier-grade softswitch

has moved the intelligence from the gateway onto the softswitch. The trend is toward placing more

intelligence in the softswitch and less intelligence in the media gateways.

Of all its capabilities, scalability is the most important factor for media gateways. The number of ports,

DS0s, in one chassis determines its classification. Depending on its density, the number of DS0s, a

media gateway fits into one of these classifications:

[BEG BL]

residential or SOHO gateway

enterprise gateway

carrier grade gateway

[END BL]

SOHO stands for (Small office Home office). Figure 7-4 Illustrates a residential or SOHO grade VoIP

gateway.

[FIG] Figure 7-4 SOHO grade gateway


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[B HD] Enterprise Gateways

[BT B HD]

Enterprise gateways can combine legacy telephony to interface with VoIP networks (see Figure 7-5).

This is done by connecting a gateway to the trunk side of a PBX. This will allow the users will retain

their existing handsets. The company will not have to require new training to utilize their existing

telephone handsets. This will provide businesses with the option to retain its expensive PBX and the

PBX handsets while moving toward or converting to IP networking technology. Enterprise gateways

are usually configured with multiple T1 or El cards in a single chassis that interface on the trunk side

of the PBX. On the trunk side of the gateway is an Ethernet connection to a router. The T1 trunks


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IP Network

SoftswitchData

Server

Telephone Telephone Telephone Telephone Telephone

connect to the line side of the gateway.

educes or eliminates interoffice phone bills The advantage is that the company.

[FIG] Figure 7-5 Enterprise gateways on a VoIP network

[B HD] Carrier Grade Gateways


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IP Network

Voice Gateway

VV

Server

Data

Softswitch

PBX Digital Telephone

Voice Gateway

VV

PBXDigital

Telephone

[BT B HD]

Initially VoIP gateways were used for international long-distance as a bypass and enterprise interoffice

long distance. These applications began to show value and that has led to a demand for larger (greater

capacity) gateways. This translates into more ports per gateway, for the carrier operations. (see Figure

7-6). These gateways needed to have enough DSOs to allow Application Servers to interface with

Class 4 and Class 5 switches. The numbers here are upward of 100,000 DS0s in a single node with an

OC-3 trunk-side interface. These gateways absolutely have to offer PSTN five 9s reliability to

interface with circuit switches.

[FIG] Figure 7-6 IP Centrex using a Class 5 switch


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IP Network Voice Gatewa

y

VV

Server

Data

Softswitch

Telephone

Voice Gatewa

y

VV

TelephoneClass 4/5Switch

Class 4/5Switch

NEBS 3 compliance, yet another requirement is that the switches must be certified NEBS 3, a

firm requirement for any platform to be installed in a central office. NEBS (Network

Equipment Building Standards) address the switches physical reliability. All carriers in North

America require that equipment in their switching locations go through rigorous NEBS testing.

The tests include evaluations for;

electrical safety

bonding and grounding

immunity from electromagnetic emissions

lightning

power failure recovery.

Equipment must also meet physical standards which includes;

fire resistance

earthquake resistance

altitude testing

temperature

humidity

battery of other rigid tests

Other Strategies required to become NEBS compliant:

Backup and disaster recovery strategies,

and UPS (uninterruptible power supply) to prevent network outages due to power failure,


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ensuring access to mirror sites,

fire and waterproof storage facilities for critical databases and configuration backup

information

Softswitching, an advanced technology, has empowered VoIP to darken TDM as the telephony

technology. Until the development of the softswitch, VoIP was in a stall lacking intelligence necessary

to route calls across the network. The progress to an IP alternative for the PSTN would likely not have

happened in the absence softswitch technology.

Two very important requirements for softswitch are reliability and scalability. The service must be

available and work when needed and it must be able to scale small to medium to large environments

without disruption to ongoing services, and downscale as well.

[A HD] Softswitch Technology

[BT A HD]

The intelligence in a network is provided by the softswitch. It has to coordinate; call control, signaling,

and the other features that manage a call across networks. Call controls first and foremost

responsibility is to manage call setups and teardowns this is the softswitches principally. When a call is

established, connection control is responsible to control that the call stays up until it is released by

either the calling party or called party. Call control and service logic are the functions that process a

call and provide telephone features. Examples of call control and service logic functions include:

[BEG BL]

recognizing that a party has gone off hook and that a dial tone should be provided


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interpreting the dialed digits to determine where the call is to be terminated

determining if the called party is available or busy

recognizing when the called party answers the phone

recognizing when either party subsequently goes on-hook

and lastly records the above actions for billing.

[END BL]

In order to set up a call, a common protocol must be used that is understandable at each end of the

network and throughout disparate networks. A softswitch uses peer to peer for call control, and

softswitch for media control. Signaling protocols are SIP, H.323, and SS7 (Signaling System 7).

Out of H.323 technology evolved gatekeeper technology. H.323 was designed for local area networks

an H.323 gatekeeper manages activities in a zone. A zone is a group of one or more gateways managed

by a single gatekeeper. Think of a gatekeeper as a logical function and not a physical function. Its

functions are:

[BEG BL]

address translation - a name or email address for a terminal or gateway and a transport

address admissions control - authorizing access to the network.

[END NL]

As the VoIP network grows larger and larger they become more complex and management solutions

with higher intelligence became necessary. As the network grows, greater call-processing power

became necessary. To interface signaling between IP networks and the PSTN requires VoIP to support


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signaling protocols to SS7. The need to integrate features across the PSTN the IP network using

disparate voice over protocols became a key softswitch obligation.

The softswitch controls the number and type of features provided to the network. It interfaces with an

application server coordinating; conferencing, call forwarding, and other call features. Softswitch

software hosted on a server includes call control applications. The greater the demand the larger the

server required. An SS7 signaling gateway serves as a protocol intermediary, and a translator between

the PSTN and IP networks when a call originates in an IP network. A translation from the H.323

signaling protocol to SS7 is necessary in order to set up the call. Signaling gateways are used to

terminate signaling links from PSTN networks or other signaling points. Billing, administrative, and

tracking functions are available from usage statistics, important management features of the softswitch.

[B HD] Applications Server

[BT B HD]

The application server provides call forwarding, conferencing, voice mail, forward on busy, camp on,

call transfer, and so on. The softswitch provides these application programs to the appropriate

subscribers as requested.

A softswitch is designed on open standards, whereas the Class 4 or Class 5 switch offers only

proprietary. Hardware and software compatibility is an issue with PSTN components, hardware or

software, from one vendor are not compatible with products from another vendor. The softswitch open

standards are deliberate to free service providers from vendor dependences.


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Softswitch enables service providers to provide integrate third-party applications or the opportunity to

write their own applications. This is the ultimate sales point for a service provider that is offered by the

softswitch technology.

[B HD] Softswitch Applications

[BT B HD] We will explore how softswitch measures up with the various levels of PSTN switches.

IP PBX - a VoIP gateway can be placed on the trunk side of a PBX. The gateway packetizes the voice

and routes it over an IP network which can save the business in long-distance costs. This solution uses

the existing PBX's set of features like; call forwarding, conferencing, and so on. It provides investment

protection by leveraging the legacy PBX with a VoIP solution. The intelligence for this solution is

provided by software known as gatekeeper. The gatekeeper is the predecessor of the softswitch. This

design is illustrated in Figure 7-5. Developers have designed a soft PBX, which can replace the legacy

PBX (illustrated in Figure 7-7). These soft PBXs known as IP PBXs are significantly lower priced

than a hardware. The IP PBX may be called an enterprise-grade softswitch.

[FIG] Figure 7-7 IP PBX also known as soft PBX


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[B HD] IP Centrex

[BT B HD]

IP Centrex - The RBOCs (Regional Bell Operating Companies) realized the threat softswitches was

to on their circuit switched Centrex. Should large companies route their interoffice voice traffic over a

corporate WAN (wide area network) using an IP PBX, that would diminish the need for the RBOC’s

circuit switched Centrex services. Although lagging a bit behind the the IP PBX the IP Centrex

services became available. Centrex is a specialized business solutions for voice service. The equipment

is owned by the service provider and is located in the service provider's premises. Centrex service

caters to removing the burden of cost and responsibility of telephony equipment ownership for a

business.

[B HD] Class 5 Exchange


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IP Network

IP PBX

IP Telephone

IP Telephone

[BT B HD]

Class 5 exchanges, also known as central offices, house the Class 5 switch. The Class 5 switch is

concerned with subscriber type of activities; generation of dial-tone, comfort sounds like ring echo and

touch tone sounds, handling of network services such as advice of call duration, charges, etc. A single

Class 5 can serve up to 10,000 subscribers it can cover an area of a small city of less than 10,000, a a

group of small towns, or support several villages.

IP provides improved utilization of access by carrying simultaneous calls over a single broadband

service. For an analog Centrex, one pair of copper wires is needed to serve each analog telephone,

regardless of whether the phone is active or not. When phone goes on hook, the bandwidth of those

wires is unused.

IP Centrex service can support an IP network when using a Class 5 Switch platform, and support

traditional POTS (Plain Old Telephone Service) and ISDN lines. This works using a CPE media

gateway and a GR-303 gateway which is co-located with the Class 5 switch (see Figure 7-8). The

media gateway size can range from an IP phone to a large carrier-grade media gateway. The GR-303

gateway translates all signaling information it receives from the customer's media gateway,

depacketizes the voice for delivery to the switch translating signaling messages from the switch into

the appropriate IP telephony protocols; H.323, SIP, or MGCP, packetizing the voice for delivery to the

customer's media gateway.

GR-303(Generic Requirement-303) is a Telcordia standard interface to a Class 5 telephone switch


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from a DLC (digital loop carrier). In order to connect directly to the PSTN, IP phones and IP

telephony gateways must adhere to the GR-303 standard.

[FIG] Figure 7-8 IP Centrex using a Class 5 switch

The customer's media gateway, the GR-303 gateway, and the IP network connected together appear to

the Class 5 switch as a DLC (Digital Loop Carrier) system. And the telephone sets connected to the

customer gateway appear as ordinary phone lines. The IP Centrex appears as a DLC system to the

Class 5 switch, it can deliver the same features to the IP Centrex users that it delivers to an analog or

an ISDN Centrex user. As a result PSTN features are available to IP Centrex users without an upgrade

to the Class 5 switch.

IP Centrex can replace the Class 5 switch with a softswitch (Compare Figure 7-9 to Figure 7-8). A


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IP Network

Voice Gateway

VV

Telephone

Telephone

Class 5 Switch

GR303 Gateway

VV

softswitch actually is an application specific to telephony running on a large network server. The

softswitch provides call control and service logic just as a Class 5 switch does. Class 5 switches are

responsible for transport or switching of the packetized voice, but the softswitch in not. Because these

functions are now performed by a Gateway.

Softswitch and IP Centrex signal each other using IP telephony protocols like SIP or H.323. The call

setup information is gathered and the softswitch figures out where the called party resides. If the called

party is a member of the Centrex group, then the softswitch will inform the originating media gateway

or IP phone, and the terminating media gateway or IP phone to route the call directly from one to

another. If the called party not a member of the Centrex group, but is served by the PSTN, then the

softswitch will inform the originating media gateway or IP phone to route the call to a trunking

gateway connecting it to the PSTN.

[FIG] Figure 7-9 IP Centrex with Softswitch


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Here the trunking gateway’s codec packetizes and compresses the voice so that it can be transmitted

over a circuit-switched facilities. The trunking gateway must work with a signaling gateway that is

used to exchange SS7 signals with the PSTN. The signaling and the trunking gateways receive their

instructions from a softswitch.

[B HD] Softswitch Replacing Class 4 Switches

[BT B HD] Class 4 replacement by softswitch began with the long-distance bypass industry. Long

distance bypass operators use VoIP gateways for international transport. This technology enabled

bypass to be competitive with the incumbent long-distance companies. Technical challenges for these

operators grew as demand for service grew. With growth it was soon discovered that the media

gateways were not dense enough for the levels of traffic they were handling. A similar growth situation


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IP Network

Voice Gateway

VV

Server

Data

Softswitch

PBX Telephone

Voice Gateway

VV

PBX Telephone

was occurring with the gateways that controlled these gateways they too were limited in their

capability to handle increasing traffic. And a most challenging situation was that the international

traffic required interfacing to different country variations of SS7 signaling, all countries do not have

exactly the same rules governing SS7.

Necessity, scalability and intelligence in the bypass network, became the mother of invention which

came in the form of a Class 4 replacement softswitch. This softswitch is made up of more densely

populated gateways and manages with more intelligence. This involved installing a dense gateway on

the trunk side of a Class 4 switch. Similar to the PBX solution, the media gateway packetized the voice

from the Class 4 switch and routed it over an IP network. This resulted in the service provider saving

money on its long distance transport.

Subsequently the circuit-switched Class 4 switch would be removed from that architecture altogether.

Now the Class 5 switch connected directly to a media gateway to route the call over an IP network.

The call control, signaling, and features were now controlled by a softswitch. It would follow that the

softswitch, as a Class 4 replacement switch, would now compete directly with the Class 4 switch.

Figures 7-10 and 7-11 make obvious this evolution in architecture.

[FIG] Figure 7-10 Architecture of Class 4 and Class 5 switches with VoIP gateways


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[FIG] Figure 7-11 Class 4 replacement Softswitch Note: Class 4 TDM switches are absent


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IP Network

Voice Gateway

VV

Server

Data

Softswitch

Telephone

Voice Gateway

VV

Telephone

Class 4 Switch

Class 4 Switch

Class 5 Switch

Class 5 Switch

IP Network

Softswitch

Data

Server

Voice Gateway

VV

Voice Gateway

VV

Class 5 Switch

Class 5 Switch

Telephone

Telephone

[B HD] Softswitch Replacing Class 5 Switches

[BT B HD]

Softswitch technologies next targeted the Class 5 switch for replacement (see Figure 7-12). The ability

of the softswitch industry to replace the Class 5 provides the finality of the legacy telecommunication

PSTN infrastructure. A Class 5 switch cost millions of dollars and requires at large chunk of real

estate. A Class 5 replacement softswitch can provide incredible positive financial implications for the

local telephone service providers.

[FIG] Figure 7-12 Class 5 replacement Softswitch


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IP NetworkVoice

Gateway

VV

Server

Data

Softswitch

Telephone

Voice Gateway

VVTelephone

Obviously there are concerns and even objections to replacing a Class 5 switch with a softswitch.

These concerns include E911 and CALEA (Communication Assistance for law Enforcement Act) of

1994. Another concern is the perception that softswitch will not be able to match Class 5 switch

features. One Class 5 manufacturer is reported to have some 3,500 features that have been developed

over a 20+ year time frame. A number of successful Class 5 replacement softswitch installations have

taken place and this segment of the industry by all counts is growing rapidly.

E911 (Enhanced 911 service) is a North American telephone network feature that automatically

associates the physical address with the calling party's telephone number. This provides emergency

responders with the location of the emergency without the person calling for help having to provide it.

Enhanced 911 have been deployed in most metropolitan areas in the United States. One of the 911

issues relates to phone mobility, physically moving the phone from one geographic location to another

(a different address). If you move your phone from one geographic location to another even just to a

different floor in a building, 911 will think that you are at the previous location.

The U.S. Congress passed the CALEA (Communications Assistance for Law Enforcement Act) to aid

law enforcement in its effort to conduct surveillance of digital telephone networks. The act obligates

telephone companies to make it possible for appropriate law enforcement agencies to tap phone

conversations carried out over its networks, as well as making call records available. The act also

stipulates that it must not be possible for a user to detect that his conversation is being monitored.

The softswitches that replace PBXs, Centrex, Class 4 and Class 5 switches are very diverse in scale.


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Scale refers to their processing power and is measured in calls per second, the number of busy-hour

call attempts handled. The capability to handle features from a feature server and interface different

signaling protocols are additional diversities. A softswitch is actually software that resides on a server.

The limitations are the processing power of the server and the complexity of the software. The

softswitch concept which is fundamentally the separation of call agents and applications from the

underlying media transport is what will make it possible for AOL (America Online Inc.) or Yahoo or

Microsoft or others to one day be your full-service Telco.

[A HD] Transport

[BT A HD]

[B HD] Converged and Converging Architectures

[BT B HD]

There are three modes of voice transport in use today: IP, TDM, and ATM (Asynchronous Transfer

Mode). IP networks bypassing TDM and ATM networks long-distance service providers dramatically

reduced their costs of long-distance transport. This allowed them to be more profitable than their TDM

and ATM competitors. Softswitch and VoIP made their debut on the transport side of the network.

Long distance service providers badly needed an intelligence that would provide call control over an IP

network used for transport. And the softswitch needed a SS7 interface into the IP network. Lastly it

had to control the transmission of features across an IP network. Accomplishing this gave birth to the

Class 4 replacement softswitch.


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[FIG] Figure 7-13 Legacy network: data, voice &SS7 networks

Convergence technology is the integration of voice, data, video and other broadband services provided

over a single network based on IP. The subscriber as a result enjoys highly efficient IP services

desktop to desktop. This can be referred to as a converged network. The Class 4 and Class 5 switch

was designed when voice and data were distributed over separate channels. These are known as legacy

networks (see Figure 7-13), the legacy infrastructure of Class 4 and 5 switches remains at the periphery

of the network while the core of the network is IP which provides proficient voice transport. A

converged network is where voice and data are handled on one network (see Figure 7-15).


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Class 5 Switch

Telephone

Data Network

voice Network

SS7

Class 4 Switch Class 5

Switch

Class 4 Switch

Telephone

[FIG] Figure 7-14 Converging networks: combined circuit and packet switched networks

In the converged environment, voice switching is performed by Class-less switches (meaning no Class

4 or Class 5 switches). The limitations of geography defined a Class 5 switch as providing local service

and a Class 4 switch as providing long distance service. If geography is not relevant, then a Class

designation must also be described the same.


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Telephone

IP Network

Class 4 SwitchClass 5

Switch

Universal Media

Gateway

Universal Media

Gateway

SoftswitchSS7

Server

Class 5 Switch

Telephone

Telephone

[FIG] Figure 7-15 A converged network data and voice on same network

[B HD] IP Networks

[BT B HD]

Because of its wide availability IP is becoming the preferred means of transport for data and the

preferred means of transport for voice. Originally designed for DARPANET, IP became the protocol

for the Internet as well as the transport medium of choice for most data networks. In the late 1990s


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IP Network

Server

Softswitch

IP Telephon

e

Telephone

Telephone

Telephone

Enterprise Media Gateway

SOHO Media Gateway

Carrier Grade Media Gateway

IPIP

IP

IPIP

VoIP was introduced and the telecommunications industry began to concentrate on converging voice

and data networks. TDM was designed specifically for voice and does not handle data as well as IP.

New demands for email, instant messaging, video conferencing, and World Wide Web applications

were indicators of a growing demand for IP data based communications, as opposed to circuit switched

communications. The cost of the PSTN dual systems to handle communications seems grossly

inefficient. Advantages of converging voice and data networks:

[BEG BL]

IP offers a world wide universal addressing scheme

the integration of voice and data applications

less bandwidth

lower equipment costs

lower operations and maintenance costs

[END BL]

Large service providers operate; the PSTN, the ATM network, the Frame Relay network, the IP

network, the ISDN and DSL overlay networks, wireless and X.25 networks. Each of these networks

requires their own OAM&P (Operations, Administration, Maintenance, and Provisioning) to provide

the facilities and personnel required to manage each network. And IP strongly appears to be the

approach for replacing these dissimilar systems with an efficient networking technology.

TCP/IP is omnipresent throughout the world. IP is supported by all personal computers, LANs and

WANs, dial-up Internet access, handheld devices, wireless web, and other next-generation


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applications. With IP as core there is an opportunity to link voice and data networks by a combination

of public and private infrastructures. IP has the advantage of offering a world wide universal

addressing scheme.

The integration of technology devices and software, and the convergence of voice and data services as

well as multimedia are accelerating rapidly. Service providers now offer an ever expanding repertoire

of high-margin services that separate them from circuit-switched service providers. Examples include:

[BEG BL]

“follow me” services - (like a cell phone, your number follows you, not fixed to a single

geographic location – a challenge for 911.

meet me – net meeting and instant messenger, VoIP conferencing, and so on.

[END BL] , email address as substitute for telephone

numbers, and video conferencing over IP. These are all benefits of converged voice and data networks.

The anticipation of nearly unlimited demand for IP based services in the late 1990s initiated the

creation of IP backbone providers. But the anticipated level of demand did not materialize, but instead

resulted in over delivery of IP backbone capacity. The winners of this were the IP consumers as well as

the VoIP service providers.

[B HD] TDM

[BT B HD]

Time division multiplexing is circuit-switched legacy PSTN telephony. The TDM softswitch is


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associated with VoIP. It should be noted that comparable architecture is possible with a TDM network.

Instead of the mainframe computer architecture encompassing the Class 4 and Class 5 switches, a

TDM softswitch uses client-server architecture. Instead of VoIP gateways, it uses TDM gateways,

which can be just as scalable as a Class 4 or Class 5 switches.

[B HD] ATM

[BT B HD]

ATM is the principal long-distance voice transport in North America. It replaced TDM due to its

superior efficiency and has yet to be outdated by IP due to its reputation for excellent QoS, a capability

built into its adaptation layer.

The ATM adaptation layer examines customer traffic as it arrives at the switch. Next, based on the type

of traffic, the ATM adaptation layer classifies the traffic on its QoS requirements. These requirements

are based on three parameters:

[BEG BL]

whether the traffic is connectionless or connection-oriented

whether it requires a fixed or variable bit rate

whether an explicit timing relationship exists between the sending and receiving devices

[END BL]

Following that process, ATM assigns a class to the cells that make up the traffic stream. It then

delivers the cells to the network, knowing that ATM's highly reliable connection oriented transport will


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ensure that the QoS mandate from the sending device will be accommodated on a network wide basis.

Many incumbent service providers are faced with the challenge of interworking their ATM and IP

networks as they shift traffic to the efficient IP networks.

[EOC A HD] Summary

From earlier chapters and reinforced here you discovered that the PSTN is made up of three main

essentials: access, switching, and transport. IP also manages these three essential functions.

Softswitch technologies have enabled the telecommunications market to open the switching aspect of

the business to competition.

Softswitch makes it possible for a service provider to economically bypass the central office and Class

5 and Class 4 switches.

The only path to competition in telecommunications markets is for each service provider to own and

operate their own switching facilities. Softswitch, given its lower costs of ownership and operation

coupled with its distributed architecture, makes it possible for new market entrants to compete

favorably with incumbent service providers.


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Softswitch is troublesome to legacy networks (RBOCS and IXCs) and equipment vendors (makers of

PBXs and Class 4 and Class 5 switches) because elements of softswitch are grossly cheaper, simpler,

smaller, and more convenient to use in the network than their predecessors.

You have learned how the economics and of softswitch will reduce telephony costs and provide

communications features that otherwise could not be possible with the centralized PSTN system.

You are now familiar with the differences between distributed [PSTN] and distributive [IP networks].

Gatekeepers and gateways have evolved and taken on various functions over time. The softswitch has

become more and more intelligent as the earlier intelligence in the gateways has been moved to the

softswitch to provide more efficiency in the overall network.

The importance of a Media Gateway (call agent) is to control telecommunications equipment in order

to bridge call transmission between circuit-based networks and IP-based networks.

You have explored how the PSTN Class 4 switch and Class 5 switch can be replaced with softswitches.

The components of a packet switched network and the circuit switched network components have been

discussed emphasizing the efficiencies gained by exploiting a distributed packet switched IP networks.


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It was shown that a customer does not have to go cold turkey when moving from PSTN to an IP network.

Since the migration path using softswitches allows both systems to co-exist allowing a gradual migration.

[EOC A HD] Key Terms

[BEG KT]

Application Server - is a server computer in a computer network dedicated to running certain

software applications.

ATA (analog terminal adapter) is a device used to allow analog phones to be connected to an IP

network. Voyage and other VoIP residential service providers use CPE ATAs to connect analog

phones to a broadband IP network. Broadband is typically provided by DSL or Cable services.

ATM - This is the international standard for cell relay in which multiple service types (such as voice,

video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to

occur in hardware, thereby reducing transit delays

CALEA - The U.S. Congress passed the Communications Assistance for Law Enforcement Act

(CALEA) to aid law enforcement in its effort to conduct surveillance of digital telephone networks.

The act obliges telephone companies to make it possible for appropriate law enforcement agencies to

tap phone conversations carried out over its networks, as well as making call records available. The act

also stipulates that it must not be possible for a user to detect that his conversation is being monitored.


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Call Agent - Intelligent entity in an IP telephony network that handles call control in an MGCP model

voice over IP network. Call agent is known as a MGC (Media Gateway Controller).

Carrier-Grade Gateways – are gateways that uphold the PSTN carrier reliability standards 99.999%

uptime availability.

CPE – is communication equipment residing on the end user's side, demark, of the network interface

boundary .

DLC – is a multiplexing/de-multiplexing system that typically has a high bandwidth fiber connection

to a Central Office out one side, and provides copper interfaces to many residences out the other

side. DLC's are deployed where there is fiber installed to a neighborhood, but copper to the

homes. DLC's are less expensive to deploy than Fiber to the Home or Fiber to the Curb because there's

only one optoelectric conversion required.

DS0 - In T-carrier systems Digital signal 0 (DS0) is a basic digital signaling rate of 64 kb/s,

corresponding to the capacity of one voice-frequency-equivalent channel.

E911 - Enhanced 911 services or E911 service is a North American telephone network feature that

automatically associates the physical address with the calling party's telephone number. This is

generally done by a form of reverse telephone directory that is supplied by the telephone company.

This provides emergency responders with the location of the emergency without the person calling for

help having to provide it. Enhanced 911 has been deployed in most metropolitan areas in the United

States.

enterprise gateways - are customer premises equipment that provides conversion between the audio

signals carried on telephone circuits and data packets carried over the Local Area Network. Enterprise


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Gateways are very similar to Media Gateways, but are scaled and configured to meet the needs of the

enterprise rather than a carrier.

gatekeepers – along with MGCs (media gateway controllers) were precursors to the softswitch.

GR-303 - A set of technical specifications from Bellcore that define the interface between the Digital

Loop Carrier (DLC) and a Class 5 switch.

GUI - A computer terminal interface, such as Windows that is based on graphics instead of text.

H.323 - is an umbrella recommendation from the ITU-T that defines the protocols to provide audio-

visual communication sessions on any packet network.

IP Centrex - is a set of specialized business solutions where the equipment providing the call control

and service logic functions is owned and operated by the service provider and is located on the service

provider's premises.

IP Network - a network in which transmission of information is done using IP protocol.

IP PBX - The Internet Protocol Private Branch eXchange (IP PBX) is telephone switching equipment

that resides in a private business instead of the telephone company. An IP PBX delivers employees

dial-tone, the ability to conference, transfer, and dial other employees by extension number as well as

many other features. Voice transmissions are sent via data packets over a data network instead of the

traditional phone network.

LCD (Liquid Crystal Display) - is a display that consists of two polarizing transparent panels and a

liquid crystal surface sandwiched in between. Voltage is applied to certain areas, causing the crystal to

turn dark. A light source behind the panel transmits through transparent crystals and is mostly blocked

by the dark crystals.


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Media Gateway - Software that acts as a "call agent" to control telecommunications equipment in

order to bridge call transmission between circuit-based networks and IP-based networks. A Media

Gateway acts as a translation unit between disparate telecommunications networks such as PSTN;

Next Generation Networks; 2G, 2.5G and 3G radio access networks or PBX. Media Gateways enable

multimedia communications across Next Generation Networks over multiple transport protocols such

as ATM and IP.

MGCP (Media Gateway Control Protocol) was developed by Telcordia and Level 3 Communications,

is one of a few proposed control and signal standards to compete with the older H.323 standard for the

conversion of audio signals carried on telephone circuits (PSTN) to data packets carried over the

Internet or other packet networks.

QoS - Short for Quality of Service, a networking term that specifies a guaranteed throughput level.

One of the biggest advantages of ATM over competing technologies such as Frame Relay and Fast

Ethernet is that it supports QoS levels. This allows ATM providers to guarantee to their customers that

end-to-end latency will not exceed a specified level.

RBOCs (Regional Bell Operating Companies) - Originally seven local telephone companies formed

upon the divestiture of AT&T in 1984. The seven were: NYNEX, Bell Atlantic, BellSouth,

Southwestern Bell, US WEST, Ameritech, and Pacific Telesis. Today there are only four: VERIZON,

SBC, BELL SOUTH, and QWEST

SGCP - Controls Voice over IP gateways by an external call control element (called a call-agent).

SIP - is a multimedia and telephony protocol that provides services including call forwarding, number

delivery, authentication and other telecoms applications. The protocol can set up, control and tear

down sessions including internet telephone calls and multimedia conferences.


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Softswitch - also called a Proxy Gatekeeper, Call Server, Call Agent, Media Gateway Controller, or

Switch Controller. Software used to bridge a public switched telephone network and voice over

Internet by separating the call control functions of a phone call from the media gateway (transport

layer). Softswitch performs call control functions such as protocol conversion, authorization,

accounting, and administration operations.

SOHO (Small Office/Home Office) - is the fastest growing market for computer hardware and

software. So-called SOHO products are specifically designed to meet the needs of professionals who

work at home or in small offices.

TDM (Time Division Multiplexing) - is a communication system technique that interleaves

information from several channel inputs onto a carrier in specific time slots.

[END KT]

[EOC A HD] Review Questions

[BEG RQ]

1. Which of the following is an architectural feature of a softswitch network that is not a

feature of a Public Switched Telephone Network?

a. It provides access.

b. It provides switching

c. It is a distributed architecture


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d. It provides transport.

2. What is the device called that takes care of billing, call routing, and signaling in a

softswitch network?

a. Media gateway

b. Softswitch gateway

c. Call agent

d. Soft agent

3. Which of the following are possibilities for completing a Voice over IP telephone call?

(Choose all that are correct.)

a. PC to Phone

b. PC to PC

c. Phone to PC

d. PC to softswitch

4. What is the chief advantage of IP phone to the service provider?

a. They can move their points of presence without reprogramming

b. They do not have to purchase a Class 4 or 5switch

c. They do not have to provide customer service

d. They have to provide a special switch that is inexpensive.


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5. What device is located between a VoIP network and a legacy telephony network and acts as

an interface between the two networks?

a. Media gateway

b. Data base

c. Transformer

d. GUI receivers

6. Switching equipment for major carries must be tested by ___________.

a. National Security Agency

b. Network Equipment Building Standards

c. International Standards Organization

d. Cisco

7. A(n)_______________________ aggregates several business legacy telephony lines for

interface with a Voice over IP network.

a. Enterprise gateway

b. SOHO gateway

c. PSTN gateway

d. Trunk consolidator

8. ______________ is the phrase used for establishing a telephone connection.

a. Call switch


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b. Call setup

c. Call teardown

d. Call connection control

9. What coordinates routing between networks of signaling messages?

a. PSTN switch

b. Analog/digital converter

c. A hub

d. A softswitch

10. What is the function of an H.323 gateway? (Choose all that are correct)

a. Address translation

b. Authorization of access

c. SS7 signaling

d. Manages activities in a zone

11. A(n) ________________________________ provides for call forwarding, conferencing,

voice mail, and forward on busy.

a. Application server

b. Client

c. MGCP

d. NEBS


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12. The RBOC’s offer a(n) ____________________ to provide a set of business solution

primarily for voice service to their customers

a. IP PBX

b. Media gateway

c. IP Centrex

d. Class 5 switch

13. Which of the following is (are) voice over IP signaling protocols? (Choose all that are

correct)

a. SIP

b. H.323

c. SS6

d. SS7

12. A(n) ________________________ packetizes/depacketizes voice so that it can be

transmitted over circuit-switched facilities.

a. Trunking gateway

b. IP PBX

c. Hub

d. Class 3 switch


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13. A converged network typically contains a mix of _____________ and _______________.

(Choose all that are correct.)

a. IP equipment, PSTN equipment

b. Voice, data

c. IP traffic, TDM traffic

d. CALEA, SIP

14. Which of the following is an advantage of a converged network? (Choose all that are

correct.)

a. Lower equipment costs

b. Integration of voice and data applications

c. No legacy telephone equipment is used

d. Lower maintenance costs

15. _______________ is the principal long-distance voice transport in North America.

a. ATM

b. TDM

c. NEBS

d. QoS


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Thinking Questions

1. Many of the CLECs (Competitive Local Exchange Carriers) of the telecom boom of the late

1990s failed because they were forced to compete with incumbent carriers by having to deploy

the same expensive PSTN switches as the incumbents. What do you think would have been the

CLECs fate had softswitches been available to them? Why?

2. The deployment of softswitch services to the last mile will have a strong positive impact on

local economic development while expanding people's quality of life through improved

education, entertainment, and communication. Do you agree? Explain why or why not. Provide

examples.

3. Sam has been supporting telephony switch equipment for over 30 yours. He has had training

on his legacy telephony equipment as new equipment has been added, and he is considered “the

best in the business.” His company now wants to move into implementing a Voice over IP

solution in one of their offices, and they want Sam to do the work. What should Sam do to

prepare? Do you think that taking Voice over IP training will provide him enough information

to accomplish the job? In reality, how different is the new system going to be? Should he push

for hiring a Voice over IP expert? Support your conclusions with evidence that your have

found in the library or on the web.


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4. The Hale Corporation likes to regularly move its employees from place to place within the

corporation to promote camaraderie. Three people have been supporting the legacy PSTN

telephony network within the corporation, but one is scheduled to retire in a 6 months. The

president of the company has retained you as telephone consultant to determine whether it

would make more sense to change to a voice over IP system so that he doesn’t have to place the

retiring employee. What sorts of things would you need to consider in making this

determination?

5. Given the cost of a Voice over IP telephone handset, how are service providers offering

telephone service over cable modem from the home at a cost-effective price point? Does the

home-owner have to purchase Voice over IP handset for each extension in the home? Why or

why not?

6. In light of advances in Voice over IP telephone devices, describe the type of computer and

telephony equipment possibilities for future workers. Will workers need both a voice handset

and a computer?

7. What are the advantages and disadvantages of CALEA?

[END RQ]


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