Chapter 7 Network Architectures

Chapter 7

Network Architectures

Instructor: Nhan Nguyen Phuong

Guide to Networking Essentials, Fifth Edition 2

Contents

1. Putting Data on the Cable: Access Methods

2. The Ethernet Architecture

3. Ethernet Standards

4. Ethernet Frame Types

5. The Token Ring Architecture

6. The AppleTalk Environment

7. The Fiber Distributed Data Interface (FDDI) Architecture

8. Networking Alternatives


1. Putting Data on the Cable: Access Methods

1.1. Function of Access Methods

1.2. Major Access Methods

1.3. Choosing an Access Method


• Given that network architectures communicate in a number of different ways, some factors in network communications must be considered– How computers put data on the cable– How they ensure that the data reaches its destination

undamaged


1.1. Function of Access Methods

• The way in which computers attached to a network share the cable must be defined

• A collision results from two or more devices sending a signal along the same channel at the same time– Splitting data in small chunks helps prevent collisions

• Channel access methods specify when computers can access the cable or data channel– Ensure that data reaches destination by preventing

computers from sending messages that might collide– Every computer on a network must use the same

access method


1.2. Major Access Methods

• Channel access is handled at the MAC sublayer of the Data Link layer in the OSI model

• Five major types of channel access– Contention– Switching– Token passing– Demand priority– Polling


1.2.1. Contention

• In early networks based on contention, computers sent data whenever they had data to send

• As networks grow, outgoing messages collide more frequently, must be sent again, and then collide again

• To organize contention-based networks, two carrier access methods were created– CSMA/CD– CSMA/CA


a. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)


b. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

• When the computer senses that no other computer is using the network, it signals its intent to transmit – Other computers with data to send must wait when

they receive the “intent-to-transmit” signal and send their “intent-to-transmit” only when channel is free

• The overhead created by intent-to-transmit packets reduces network speed significantly

• Used in wireless LANs with an access point– Wireless NIC tells access point its intents to transmit– Access point hears transmissions from all devices,

so it can determine whether it’s okay to transmit


1.2.2. Switching

• Switching: nodes are interconnected through a a switch, which controls access to the media– Contention occurs only when multiple senders ask to

reach the same receiver simultaneously or when the simultaneous transmission requests exceed the switch’s capability to handle multiple connections

• Advantages: fairer, centralized management (enables QoS), switch can have connection ports that operate at different speeds

• Disadvantage: higher cost


1.2.3. Token Passing


1.2.4. Demand Priority

• Demand priority: channel access method used solely by the 100VG-AnyLAN 100 Mbps Ethernet standard (IEEE 802.12)– 100VG-AnyLAN runs on a star bus topology– Intelligent hubs control access to the network

• Hub searches all connections in a round-robin fashion

• When an end node has data to send, it transmits a demand signal to the hub

• The hub then sends an acknowledgement that the computer can start transmitting its data

– The major disadvantage of demand priority is price


1.2.5. Polling


1.3. Choosing an Access Method

2. The Ethernet Architecture

2.1. Overview of Ethernet

2.2. Ethernet Operation



• 1960s and 1970s: many organizations worked on methods to connect computers and share data– E.g., the ALOHA network at the University of Hawaii– 1972: Robert Metcalf and David Boggs, from Xerox’s

PARC, developed an early version of Ethernet• 1975: PARC released first commercial version (3

Mbps, up to 100 computers, max. 1 km of total cable)

• DIX developed standard based on Xerox’s Ethernet (10 Mbps)

• 1990: IEEE defined the 802.3 specification– Defines how Ethernet networks operate at layers 1-2


2.1. Overview of Ethernet

• Ethernet is the most popular network architecture – Advantages: easy to install, scalable, broad media

support, and low cost– Supported transmission speeds: 10 Mbps to 10 Gbps– Uses the NIC’s MAC address to address frames– Ethernet variations are compatible with one another

• Basic operation and frame formatting is the same

• Cabling, speed of transmission, and method by which bits are encoded on the medium differ


2.2. Ethernet Operation

• Ethernet is a best-effort delivery system– It works at the Data Link layer of the OSI model

• Relies on the upper-layer protocols to ensure reliable delivery of data

• Understanding the following concepts is important:– How Ethernet accesses network media– Collisions and collision domains– How Ethernet handles errors– Half-duplex and full-duplex communications


2.2.1. Accessing Network Media

• Ethernet uses CSMA/CD in a shared-media environment (a logical bus)– Ethernet device listens for a signal or carrier (carrier

sense) on the medium first– If no signal is present, no other device is using the

medium, so a frame can be sent– Ethernet devices have circuitry that detects collisions

and automatically resends the frame that was involved in the collision


2.2.2. Collisions and Collision Domains


2.2.3. Ethernet Error Handling

• Collisions are the only type of error for which Ethernet automatically attempts to resend the data

• Errors can occur when data is altered in medium – Usually caused by noise or faulty media connections– When the destination computer receives a frame,

the CRC is recalculated and compared against the CRC value in the FCS

– If values match, the data is assumed to be okay– If values don’t match, the data was corrupted

• Destination computer discards the frame

• No notice is given to the sender


2.2.4. Half-Duplex Versus Full-Duplex Communications

• When half-duplex communication is used with Ethernet, CSMA/CD must also be used

• When using a switched topology, a computer can send and receive data simultaneously (full-duplex communication)– The collision detection circuitry is turned off because

collisions aren’t possible– Results in a considerable performance advantage

3. Ethernet Standards

3.1. 100 Mbps IEEE Standards


3.3. Gigabit Ethernet: IEEE 802.3ab and 802.3z Standards

3.4. What’s Next for Ethernet?



• Each Ethernet variation is associated with an IEEE standard

• The following sections discuss many of the standards, some of which are obsolete or had limited use

• Keep in mind that Ethernet over UTP cabling has been the dominant technology since the early 1990s, and will likely to continue to be for the foreseeable future



• The most widely accepted Ethernet standard today is 100BaseT, which is also called fast Ethernet– The current IEEE standard for 100BaseT is 802.3u

• Subcategories:– 100BaseTX: Two-pair Category 5 or higher UTP– 100BaseT4: Four-pair Category 3 or higher UTP– 100BaseFX: Two-strand fiber-optic cable

– Because of its widespread use, the cable and equipment in fast Ethernet are inexpensive

– Architecture of choice for all but heavily used servers and multimedia applications


3.1.1. 100BaseTX

• 100BaseTX is the standard that’s usually in mind when discussing 100 Mbps Ethernet

• Requires two of the four pairs bundled in a Category 5 twisted-pair cable

• Although three cable types are available for 100BaseT, 100BaseTX is the most widely accepted– Generally called fast Ethernet


3.1.2. 100BaseT4

• 100BaseT4 Ethernet uses all four pairs of wires bundled in a UTP cable

• Advantage: capability to run over Category 3 cable– One of the biggest expenses of building a network is

cable installation, so many organizations with Category 3 cabling chose to get the higher speed with the existing cable plant by using 100BaseT4 instead of 100BaseTX


3.1.3. 100BaseFX

• 100BaseFX uses two strands of fiber-optic cable

– Advantages:• Impervious to electrical noise and electronic

eavesdropping• Can span much greater distances between devices

– Disadvantage: far more expensive than twisted-pair– Rarely used as a complete 100BaseTX replacement

• Used as backbone cabling between hubs or switches and to connect wiring closets between floors or buildings

• Connect client or server computers to the network when immunity to noise and eavesdropping is required


3.1.4. 100BaseT Design Considerations



• Four major implementations of 10 Mbps Ethernet– 10Base5: Ethernet using thicknet coaxial cable– 10Base2: Ethernet using thinnet coaxial cable– 10BaseT: Ethernet over UTP cable– 10BaseF: Ethernet over fiber-optic cable

• Of these 10 Mbps standards, only 10BaseT and 10BaseF are seen today

• 10Base2 and 10Base5 are essentially obsolete


3.2.1. 10BaseT


3.2.2. 10BaseF


3.3. Gigabit Ethernet: IEEE 802.3ab and 802.3z Standards

• Gigabit Ethernet implementations– 802.3z-1998 covers 1000BaseX specifications,

including the L (long wavelength laser/fiber-optic), S (short wavelength laser/fiber-optic), and C (copper jumper cables)

– 802.3ab-1999 covers 1000BaseT specifications, which require four pairs of 100 ohm Category 5 or higher cable


3.3.1. 1000BaseT


3.3.2. 1000BaseLX


3.3.3. 1000BaseSX


3.3.4. 1000BaseCX


3.3.5. 10 Gigabit Ethernet: 10 Gbps IEEE 802.3ae Standard

• Defined to run only on fiber-optic cabling, both SMF and MMF, on a maximum distance of 40 km– Provides bandwidth that can transform how WAN

speeds are thought of

• Runs in full-duplex mode only– CSMA/CD is not necessary

• Primary use: as network backbone– It also has its place in storage area networks (SANs) – Will be the interface for enterprise-level servers


• Standards– 10GBASE-SR: Runs over short lengths (between 26

and 82 meters) over MMF• For high-speed servers, SANs, etc.

– 10GBASE-LR: Runs up to 10 km on SMF• For campus backbones and MANs

– 10GBASE-ER: Runs up to 40 km over SMF• Primary applications are for MANs

– 10GBASE-SW: Uses MMF for distances up to 300 m– 10GBASE-LW: Uses SMF for distances up to 10 km– 10GBASE-EW: Uses SMF for distances up to 40 km


3.4. What’s Next for Ethernet?

• Implementations of 40 Gbps Ethernet are underway

• Ethernet could increase tenfold every 4-6 years– 100 Gbps Ethernet available by 2006 to 2008, terabit

Ethernet by 2011, and 10 terabit Ethernet by 2015

• In October 2005, Lucent Technologies demonstrated for the first time the transmission of Ethernet over fiber-optic cable at 100 Gbps– It will be able to transfer data across the city faster

than today’s CPUs can transfer data to memory– This level of speed has major implications for the

entertainment industry and many other areas

4. Ethernet Frame Types

4.1. Ethernet 802.3

4.2. Ethernet 802.2

4.3. Ethernet SNAP

4.4. Ethernet II

4.5. Wireless Ethernet: IEEE 802.11b, a, and g



• Ethernet supports four non-compatible frame types– Ethernet 802.3: used by IPX/SPX on Novell NetWare

2.x and 3.x networks– Ethernet 802.2: used by IPX/SPX on Novell NetWare

3.12 and 4.x networks• Supported by default in Microsoft NWLink

– Ethernet SNAP: used in EtherTalk and mainframes– Ethernet II is used by TCP/IP

• All Ethernet frame types support a packet size between 64 and 1518 bytes, and can be used by all network architectures mentioned previously


4.1. Ethernet 802.3


4.2. Ethernet 802.2

• Ethernet 802.2 frames comply completely with the Ethernet 802.3 standard

• The IEEE 802.2 group didn’t address Ethernet, only the LLC sublayer of the OSI model’s layer 2– Since Novell had already decided to use the term

Ethernet 802.3 to describe Ethernet raw, it’s generally accepted that Ethernet 802.2 means a fully 802.3- and 802.2-compliant Ethernet frame

• Ethernet 802.2 frames contain similar fields to 802.3, with three additional LLC fields


4.3. Ethernet SNAP

• Ethernet SubNetwork Address Protocol (SNAP) is generally used on the AppleTalk Phase 2

• It contains enhancements to the 802.2 frame, including a protocol type field, which indicates the network protocol used in the frame’s data section


4.4. Ethernet II


4.5. Wireless Ethernet: IEEE 802.11b, a, and g

• AP serves as the center of a star topology network

• Stations can’t send and receive at the same time– CSMA/CA is used instead of CSMA/CD

• 802.11b/a/g use handshaking before transmission– Station sends AP an RTS and it responds with CTS

• Standards define a maximum transmission rate, but speeds might be dropped to increase reliability

• No fixed segment length– Maximum of 300 feet without obstructions

• Can be extended with large, high-quality antennas

5. The Token Ring Architecture

5.1. Token Ring Function

5.2. Hardware Components

5.3. Cabling in a Token Ring Environment



5.1. Token Ring Function

• A token passes around the ring– If an “in use” token is received from NAUN, and the

computer has data to send, it attaches its data to the token and sends it to its NADN

– If received token is in use, NIC verifies if it is the destination station

• If not, the computer re-creates the token and the data exactly and sends them to its NADN

• If it is, data is sent to the upper-layer protocols– Two bits in data packet are toggled and token is

sent to NADN; when original sender receives it, it frees the token and then passes it along


Beaconing


5.2. Hardware Components

• A hub can be a multistation access unit (MSAU) or smart multistation access unit (SMAU)

• IBM’s token ring implementation is the most popular adaptation of the IEEE 802.5 standard– Minor variations but very similar to IEEE specs

• IBM equipment is most often used– 8228 MSAU has 10 connection ports, eight of which

can be used for connecting computers– The RO port on one hub connects to RI port on the

next hub, and so on, to form a ring among the hubs• IBM allows connecting 33 hubs


5.3. Cabling in a Token Ring Environment

6. The AppleTalk Environment

6.1. LocalTalk

6.2. EtherTalk and TokenTalk



• Designed for use in Macintosh networks (1983)• Can be run over several physical architectures;

commonly run over Ethernet (EtherTalk)• Easy to implement• Dynamic scheme used to determine device’s

address• AppleTalk Phase 1 supported only 32 computers

per network, and only with LocalTalk cabling– With hubs/repeaters, increased the number to 254

• AppleTalk Phase 2, EtherTalk, and TokenTalk (1989) allow more than 16 million computers


6.1. LocalTalk

• LocalTalk uses STP in a bus topology to allow users to share peripherals and data in a small home or office environment– CSMA/CA channel access method

• Avoids more collisions, but cumbersome

– Maximum transmission speed of 230.4 Kbps• Thus, this architecture was used primarily in small,

Macintosh-only environments


6.2. EtherTalk and TokenTalk

• EtherTalk is the AppleTalk protocol running over a 10 Mbps IEEE 802.3 Ethernet network

• TokenTalk is the AppleTalk protocol running over a 4 or 16 Mbps IEEE 802.5 token ring network

• Both implementations require using a different NIC– Since 1996, Apple Computer has offered systems

with built-in Ethernet NICs or with options to add Ethernet or token ring to its systems at a low cost

– Mac OS X with an Ethernet interface can freely participate in a Windows-based network


7. The Fiber Distributed Data Interface (FDDI) Architecture

8. Networking Alternatives

8.1. Broadband Technologies

8.2. Broadcast Technologies

8.3. Asynchronous Transfer Mode (ATM)

8.4. ATM and SONET Signaling Rates

8.5. High Performance Parallel Interface (HIPPI)



• Many other network architectures are available

• Some are good for specialized applications, and others are emerging as new standards

• Topics– Broadband technologies (cable modem and DSL)– Broadcast technologies– ATM– ATM and SONET Signaling Rates– High Performance Parallel Interface (HIPPI)


8.1. Broadband Technologies

• Baseband systems use a digital encoding scheme at a single fixed frequency

• Broadband systems use analog techniques to encode information across a continuous range of values– Signals move across the medium in the form of

continuous electromagnetic or optical waves– Data flows one way only, so two channels are

necessary for computers to send and receive data– E.g., cable TV


8.1.1. Cable Modem Technology


8.1.2. Digital Subscriber Line (DSL)

• Competes with cable modem for Internet access– Broadband technology that uses existing phone lines

to carry voice and data simultaneously– Most prominent variation for home Internet access is

Asymmetric DSL (ADSL)• Splits phone line in two ranges: Frequencies below 4

KHz are used for voice transmission, and frequencies above 4 KHz are used to transmit data

• Typical connection speeds for downloading data range from 256 Kbps to 8 Mbps; upload speeds are in the range of 16 Kbps to 640 Kbps


8.2. Broadcast Technologies

• By definition: one-way transmissions– This changed in Internet access by satellite

television systems

• Work on the principle that most traffic a user generates is to receive files, text, and graphics– The average user’s computer sends very little traffic– User connects to service provider through a modem– Service provider sends data by satellite to the user’s

home at speeds up to 400 Kbps– E.g., service offered by DirectTV, through its

DirectPC add-on products


8.3. Asynchronous Transfer Mode (ATM)

• High-speed network technology for LANs and WANs– Connection-oriented switches

• Dedicated circuits are set up before communicating

– Data travels in fixed-size 53-byte cells (5 byte-header)• Enables ATM to work at extremely high speeds

– Quick switching– Predictable traffic flow

• Enables ATM to guarantee QoS

– Used quite heavily for the backbone and infrastructure in large communications companies

– LAN emulation (LANE) required for LAN applications


8.4. ATM and SONET Signaling Rates


8.5. High Performance Parallel Interface (HIPPI)

• HIPPI (late 1980s): high-speed interface developed for supercomputers and high-end workstations– Serial HIPPI is a fiber-optic version that uses point-

to-point optical links for bandwidth up to 800 Mbps• In early 1990s, it was used as a network backbone

and for interconnecting supercomputers– With the advent of Gigabit Ethernet, interest in HIPPI as

a LAN backbone decreased

– HIPPI-6400 (1998): up to 6.4 Gbps transfer rates• Known as Gigabyte System Network (GSN)

– HIPPI and GSN are now exotic networking products and aren’t often found in typical corporate networks


Summary

• Cable access methods determine how a network architecture gains access to the network medium

• A network architecture defines how data is placed, transmitted, and at what speed, and how problems in the network are handled

• DIX introduced Ethernet, which later became the IEEE 802.3 standard, transmitting data at 10 Mbps– Standards for 10Mbps, 100Mbps, 1000Mbps (Gigabit),

and 10G indicate the supported network mediums• 10 Gigabit Ethernet runs only over fiber-optic cable and

only in full-duplex mode


• Token ring networks are reliable, fast, and efficient– Capable of transmitting at 4 Mbps or 16 Mbps

• Macintosh computers use AppleTalk to communicate

• FDDI is an extremely reliable, fast network architecture that uses dual counter-rotating rings

• Cable modem technology delivers high-speed Internet access to homes and businesses

• ATM, a high-speed network technology designed both for LANs and WANs, uses connection-oriented switches

Chapter 7 Network Architectures

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