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Chapter 11PPP and ISDN

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PPPISDNISDN BRI Configuration Example

Contents

Review Questions

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PPP

The IETF defined PPP to provide point-to-point,router-to-router, host-to-router, and host-to-hostconfigurations. PPP is considered a

peer technology.

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PPP (cont.)

PPP is commonly used over dial-up or leased linesto provide connections into IP networks. PPP also supports other Network layer protocolssuch as Novell IPX and AppleTalk.

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PPP (cont.)

Due to its flexibility, PPPis the mostly used WAN connection method today. SLIP (Serial Line Internet Protocol) was the predecessor to PPP; it only supports TCP/IP connection.

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PPP (cont.)

SLIP offers no encryption,compression, or error correction. It is an analog protocol limited 56-Kbps transmission. PPP overcomes all of

SLIP’s limitations.

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PPP (cont.)

PPP can be used over several different physical interfaces, including:

Asynchronous serialISDN synchronous serial

High-Speed Serial Interface (HSSI)

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PPP in the Protocol StackMost WAN protocols

operate at the Data Link layer of OSI model. However, PPP also relies on Network layer servicescalled NCPs at layer 3 of the OSI model.

PPP (cont.)

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PPP in the Protocol Stack (cont.)NCPs (Network Control

Protocols) allow the simultaneously use of multiple Network layerprotocols and are required for each protocol that uses PPP.

PPP (cont.)

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PPP in the Protocol Stack (cont.)Fig. 11-1 indicates the

location of PPP in the protocol stack; notice that the NCPs function at the Network layer.

PPP (cont.)

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Frame FormatPPP, like many WAN

technologies, is based onHDLC protocol. The frame structures of PPP andHDLC are shown in Fig. 11-2.

PPP (cont.)

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PPP (cont.)

Frame Format (cont.)The protocol field allows PPP to simultaneously support many protocols by allowing it to indicatewhich protocol it is encapsulating.

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PPP (cont.)

PPP uses the LCP field to establish, configure, maintain, and terminate connections.

Cisco’s HDLC is the default encapsulationtype for serial interfaces on Cisco’s routers .

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PPP (cont.)

Frame Format (cont.)The LCP field of the PPP

packet can contain many different pieces of information such as compression, authentication, magic number, LQM, andmultilink.

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PPP (cont.)

Frame Format (cont.)Authentication: Two

authentication protocolsare available- (Password Authentication Protocol) PAP and (Challenge Handshake Authentication Protocol) CHAP.

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PPP (cont.)

Frame Format (cont.)Magic number: They are

unique numbers added by the router to a packet, which allow it to detect alooped-back link.

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PPP (cont.)

Frame Format (cont.)Link Quality Monitoring

(LQM): Check the reliability of the link by monitoring the number of errors, latency between requests, connection retries, and connection failures on the PPP link.

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PPP (cont.)

Frame Format (cont.)Multilink: Allows multiple

transmission devices to send data over separate physical connection. For example, two64-Kbps ISDN channels can be combined to provide an effective throughput of 128-Kbps.

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PPP (cont.)

LCP Link ConfigurationThe LCP link

Configuration process modifies and enhances the default characteristicsof a PPP connection. It includes the following actions:

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PPP (cont.)

Link establishmentAuthentication (optional)Link-quality determination(optional)Network layer protocol configuration negotiationLink termination

LCP Link Configuration (cont.)

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PPP (cont.)

LCP Link Configuration (cont.)Link establishment: PPP must open and configure the PPP connectionbefore any data can be transmitted over the link.

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PPP (cont.)

LCP Link Configuration (cont.)Authentication: CHAP or PAP can be used to verifythe identity of the devices that are establishing the connection.

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PPP (cont.)

LCP Link Configuration (cont.)LQM: Checks the quality of the link and monitors its reliability.

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PPP (cont.)

LCP Link Configuration (cont.)Network layer protocol

configuration negotiation: Identifies the appropriateNetwork layer protocolfor the connection.

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PPP (cont.)

LCP Link Configuration (cont.)Link termination: When the call is complete, or the specifications defining the call are no longer met, the call isterminated.

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PPP (cont.)

Establishing PPP Communication Three of the five link

configuration actions are involved in establishing PPPconnections: link establishment, optional authentication, and Network layer protocol configuration negotiation.

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PPP (cont.)

Establishing PPP Comm. (cont.)The link establishment

phase involved the configuration and testing of the data link.

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PPP (cont.)

Establishing PPP Comm. (cont.)The (optional)

authentication phase can use two authentication types with PPP connections: PAP and CHAP.

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PPP (cont.)

Establishing PPP Comm. (cont.)Most administrators

configure their devices to use CHAP because it is the stronger authentication method of the two.

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PPP (cont.)

Establishing PPP Comm. (cont.)PPP uses a simple two-way handshake methodto establish the link.PPP transmits a clear textusername and password across the link between hosts to establish the link.

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PPP (cont.)

Establishing PPP Comm. (cont.)Compared with PAP,

CHAP provides a much more sophisticatedauthentication process.

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PPP (cont.)

Establishing PPP Comm. (cont.)

Like PAP, CHAP providesusername and password authentication serviceduring the initial link establishment.

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PPP (cont.)

Establishing PPP Comm. (cont.)However, CHAP employs a three-way, rather than two-way handshake. to establish the link.Once the link is established, the local router queries the remote router with a packet known as a challengechallenge.

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PPP (cont.)

Establishing PPP Comm. (cont.)PPP is an encapsulation type for serial interfacecommunication. Fig. 11-3 shows an example of configuring a PPP connection.

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PPP (cont.)

Establishing PPP Comm. (cont.)The third phase of the establishing process is Network layer protocol configuration negotiation.

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PPP (cont.)

Establishing PPP Comm. (cont.)After LCP has finishednegotiating theconfiguration parameters, Network layer protocolscan be configured individually by the appropriate NCP.

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PPP (cont.)

Configuring PPP AuthenticationYou can choose to enable CHAP, PAP, or both on your PPP connection, in either order. (see Fig. 11-4)

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PPP (cont.)

For example, to set therouter to first use CHAP, and then go to PAP(assuming that CHAP is not available)

Configuring PPP Auth. (cont.)

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PPP (cont.)

Configuring PPP Auth. (cont.)Fig. 11-5 illustrates a

configuration and the commands required for that configuration to operate using CHAP authentication.

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PPP (cont.)

If you want to configure the same host name and password for CHAPauthentication on several routers, you can also do so. (see Fig. 11-6)

Configuring PPP Auth. (cont.)

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PPP (cont.)

Once you have completed configuring your PPPinterface, you can verify your changes using the show interface command.

Confirming PPP Comm.

Router# sh int s0

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PPPISDNISDN BRI Configuration Example

Contents

Review Questions

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ISDN

ISDN is a circuit-switched service fromISP to allow voice, data, video, and audiotransmissions over existing telephone lines.

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ISDN is often used as a low-cost alternative toFrame Relay or T1 connections.

ISDN (cont.)

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ISDN (cont.)

The service is offered attwo different levels:Basic Rate Interface (BRI) and Primary Rate Interface (PRI) .

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ISDN (cont.)

BRI (Basic Rate Interface)由 2個 B通道加上 1個 16 Kbps的 D通道組成，即2B2B++1D1D。2個B通道可合起來提供128 Kbps的傳輸速率或分開運作，一條用來上網，

一條用來打電話。

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PRI (Primary Rate Interface)在北美和台灣為配合T1規格，使用 2323B B ++11DD(使用64Kbps的D通道)；在歐洲地區為配合E1規格，使用3030BB + + 11DD 。

ISDN (cont.)

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ISDN (cont.)

In both BRI and PRI, a single D-channel is used for signaling information and the B-channels are used to carry data.

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ISDN (cont.)

The B-channels are used to make phone calls for voice, video, and data connections.

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ISDN (cont.)

The usage of D-channel is signaling (setting up and tearing down circuit-switched connections), frame synchronization, clocking, and physical layer framing.

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ISDN (cont.)

Because the control communications are conducted on a channelseparate from the data transfer, ISDN is said to use out-of-band signaling.

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ISDN (cont.)

The Data link layer protocol that manages thesignaling exchange on the D-channel is Link AccessProcedure-D (LAPD).

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ISDN (cont.)

The ITU maintains several standards on ISDN. These standards are organized into ITU-T group, which are organized by three different letter designations: E, I, and Q. (see Table 11-1)

ISDN Standards

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ISDN (cont.)

ISDN OperationsISDN can use HDLC or PPP as its WAN protocol. As previously mentioned, it uses the LAPD to pass thesignaling messages between the routers and the ISDN switch at the local telcocentral office.

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ISDN (cont.)

ISDN Standards (cont.)The data travels between routers on the B-channelsvia HDLC or PPP encapsulation, as shown in Fig. 11-7.

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ISDN (cont.)

Specification about a BRI callThe D-channel between the router and the local ISDNswitch comes up.The ISDN switch uses the SS7 signaling technique to set up a path to a remote switch.

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ISDN (cont.)

Specif. about a BRI call (cont.)

The remote switch sets up the D-channel link to a remote router.The B-channels are then

connected end-to-end.

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ISDN (cont.)

ISDN Standards (cont.)ISDN standards use

function groups and reference points to describe the various components that can be used in making an ISDNconnections.

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ISDN (cont.)

ISDN Standards (cont.)Function groups describe a set of functions that are implemented by a deviceand software.The terminal adapter (TA)is an example of a function groups.

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ISDN (cont.)

ISDN Standards (cont.)Generally speaking, the

connection between two function groups (including the cabling) is called a reference point. (see Fig. 11-8)

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ISDN (cont.)

ISDN Standards (cont.)The function groups are described as follows:

Terminal Adapter (TA)Terminal Equipment 1 (TE1)Terminal Equipment 2 (TE2)Network Termination 1 (NT1)Network Termination 2 (NT2)

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ISDN (cont.)

Functions and ReferencesTA: A converter devicethat allows non-ISDN devices to operate on an ISDN network.

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ISDN (cont.)

TE1: A device that that supports ISDN standardsand that can be connected directly to an ISDN network connection.

For example, ISDN telephone, PCs, routers with ISDN interfaces, or videophonescould function as TE1s.

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ISDN (cont.)

Functions and References (cont.)TE2: A non-ISDN device, such as analog phone or modem, or a PC, which requires a TA to connect to an ISDN network.

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ISDN (cont.)

Functions and References (cont.)NT1: This device provides power for the ISDN service. It terminates the two-wire connection from the CO and connects to a TA to connect to the four-wire custom network.

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ISDN (cont.)

Functions and References (cont.)NT2: A device that

multiplexes and switches signals between various network devices at thecustomer site. A multiplexer or PBX would be an NT2 device.

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ISDN (cont.)

Functions and References (cont.)The reference points in Fig. 11-8 include the U, R, S, T, and S/T.

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ISDN (cont.)

Functions and References (cont.)U: The point that defines the demarcation between the user network and the ISP’s ISDN facility. The U-interface is the actual two-wire cable, also called the local loop.

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ISDN (cont.)

Functions and References (cont.)R: The point between the non-ISDN equipment (TE2) and the TA. The R-interface is the wire orcircuit that connects TE2to the TA.

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ISDN (cont.)

Functions and References (cont.)S: The point between the ISDN customer’s TE1 or TAand the NT1 or NT2. The S-interface is a four-wire cable from TE1 or TA to the NT1 or NT2 .

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ISDN (cont.)

Functions and References (cont.)T: The point between NT1 and NT2. The T-interface is used to divide the normal telephone company two-wire cable into four wires, which then allow the connection of up to 8 ISDN devices.

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ISDN (cont.)

Functions and References (cont.)S/T: When NT2 is not used, the connection from the router or TA to the NT1connection is typically called S/T. (It is not typically used in the US)

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ISDN (cont.)

ISDN functions and reference points

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SPIDISDN switches require

Service Provider Identifiers (SPIDs) for dial-in access.

SPIDs are frequently referred to as ISDN phone numbers because their functions are similar.

ISDN (cont.)

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ISDN (cont.)

An ISDN device can access each ISDN channel via its SPID number.The ISDN provider must assign the SPID numbers for each channel, which are normally 8- to 14-digitnumbers.

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SPID (cont.)

ISDN (cont.)

Most modern ISDNsystems now have a feature called autoSPID. This eliminates the need to configure SPIDnumbers on the routers.

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SPID (cont.)

ISDN (cont.)

Fig. 11-9 illustrates anISDN configuration.

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SPID (cont.)

ISDN (cont.)

Potential switch typesinclude dms-100, ni1, net3, ntt, and so on. (Please see p. 391 in detail)

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MultiLink PPP

ISDN (cont.)

MultiLink PPP allows you to combine the individual bandwidths of several modems and ISDNchannels to increase the bandwidth of a single connection.

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MultiLink PPP (cont.)

ISDN (cont.)

MultiLink provides load balancing, packet fragmentation andreassembly, andsequencing for packets sent across WAN links.

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DDR

ISDN (cont.)

The dial-in-demand routing (DDR) feature that is available on Ciscorouters allows you to use bandwidth as needed.

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DDR (cont.)

ISDN (cont.)

This feature can save organization money on connections because DDRautomatically connects and disconnects the lineas needed.

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DDR (cont.)

ISDN (cont.)

With DDR, all traffic is classified as either interesting or uninteresting. Interesting traffic forces the router to connect toremote routers.

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DDR (cont.)

ISDN (cont.)

Uninteresting traffic will not force the router to connect; however,uninteresting traffic is passed if the line is already active.

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DDR (cont.)

ISDN (cont.)

The dialer-list command,in conjunction withaccess-list commands, are used to define what isinteresting and what is not.

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DDR (cont.)

ISDN (cont.)

Fig. 11-10 defines the DDR connection parameters and the connection link.

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DDR (cont.)

ISDN (cont.)

The 1st command tells the dialer to wait no longer than 15 sec for the ISDN provider to answer during a DDR connection attempt.

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DDR (cont.)

ISDN (cont.)

The 2nd command tells the dialer to hang up the connection if it does not pass any interesting traffic for 300 sec.

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DDR (cont.)

ISDN (cont.)

The 3rd command tells the dialer to only dial additional lines when any channel is transferring at a load value of 50, either inbound or outbound.

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DDR (cont.)

ISDN (cont.)

Load values can range from 1 (not loaded) to 255 (fully loaded), so a load value of 50 would indicate the line being about 20%loaded (50/255).

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DDR (cont.)

ISDN (cont.)

The 4th command maps the dialer to a specific host name, IP address, speed, and phone number.

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Sometimes dialer-listcommand is in conjunction with access-list commands. Dialer-group commands are used to associate dialer-listcommand with the correct interface.

ISDN (cont.)

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DDR (cont.)

ISDN (cont.)

Fig. 11-11 illustrates how you can use dialer-list, dialer-group, and access-list commands to permit IP traffic on your link but deny IGRP traffic.

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PPPISDNISDN BRI Config. Example

Contents

Review Questions

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ISDN BRI Config. ExampleFig. 11-12 shows two routersthat will be used to create a temporary ISDN BRI with connection.To simplify and correctlyimplemented it by usingNetSim tool, several modifications should be made at this example.

(Config-if)# isdn spid1 32177820010100

(Config-if)# dialer-string 7782001

(Config-if)# isdn spid1 32177820020

(Config-if)# dialer-string 7782002

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ISDN BRI Config. Example (cont.)

Digital LinesISDN is only one type of connection that you can obtain from a telecom providers. Other types of digital connections are available.

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ISDN BRI Config. Example (cont.)

Digital Lines (cont.)

The following list contains some currently available services:

T1: 24-ch, 1.544 Mbps.E1: 30-ch, 2.048 Mbps.Fraction E1 and Fraction T1

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ISDN BRI Config. Example (cont.)

Digital Lines (cont.)Digital Subscriber Line

(DSL): DSL connectionsare generically referred to as xDSL, because they are several different DSL technologies.

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ISDN BRI Config. Example (cont.)

Digital Lines (cont.)Examples of xDSL include ADSL, HDSL, IDSL, SDSL, and VDSL.

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Real World Scenario

Should we really still use ISDN?Only if you can’t get anything else! If you only have a 56 Kbps modem, then yes, ISDN will be better.But if you can get DSL, a Cable modem, Frame Relay, or even a wireless connection, you will be much better off.

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PPPISDNISDN BRI Config. Example

Contents

Review Questions

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Review QuestionsIf you want to use CHAP authentication, which protocol would you employ?

Ans: B

A. MultilinkB. PAPC. Frame RelayD. PPPE. None of the above

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Review QuestionsIn ISDN communications, what does the functional group TE2 represent?

Ans: B

A. A terminal equipmentB. A non-ISDN equipmentC. A device often used

with ISDN PRI serviceD. None of the above

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Review Questions (cont.)What is the ISDN indicator for a converter device that allows non-ISDN devices to operate on an ISDN network?

Ans: D

A. TE2B. NT1C. UD. TA

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Review Questions (cont.)Which two of the followingprotocols could be used when establishing a connection over ISDN?

Ans: A, B

A. HDLCB. PPPC. PAPD. CHAP

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Review Questions (cont.)

What is the Data Link layerprotocol used on the ISDND-channel ?

Ans: A

A. LAPDB. PPPC. PAPD. CHAPE. HDLC

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Review Questions (cont.)

Which of the following is anNCP? (Choose all that apply.)

Ans: A, B, C

A. IPXCPB. IPCPC. ATCPD. SDLCE. HDLC

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Review Questions (cont.)

What is a common reasonfor looping an interface?

Ans: D

A. To achieve greater bandwidthB. To drop nonessential framesC. To increase packet sizeD. TestingE. None of the above

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Review Questions (cont.)

Which of the following employs the strongest encryption technique?

Ans: C

A. Plain textB. PAPC. CHAPD. Clear textE. None of the above

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Review Questions (cont.)Which of the following is a unique number that helps devices discover looped interfaces?

Ans: D

A. MACB. LCPC. BandwidthD. Magic numberE. None of the above