Computer Networks 18-1

Chapter 18. Virtual-Circuit Networks:Frame Relay and ATM

18.1 Frame Relay

18.2 ATM

18.3 ATM LANs

Computer Networks 18-2

Wide area network and switching methods

Computer Networks 18-3

Circuit switching

• Create a real circuit (dedicated line) between source and destination

• Physical layer technology

Computer Networks 18-4

Packet Switching: Datagram Approach

• Mostly used in the network layer

• Routing (selecting the best route for a packet) is performed at each router

Computer Networks 18-5

Packet Switching: Virtual Circuit Approach

• Packets (frames) are switched along a pre-determined path from source to destination• Virtual circuit network has two addresses

– Global address which is unique in the WAN– Virtual circuit identifier which is actually used for data transfer

• VCI has switch scope; it is used between two switches• Each switch can use its own unique set of VCIs

Computer Networks 18-6

VCI Phases

• Two approaches for the VC setup– Permanent virtual circuit (PVC): – Switched virtual circuit (SVC): setup, data transfer, teardown

Computer Networks 18-7

Data Transfer Phase

• All switches need to have a table entry for the virtual circuit

Computer Networks 18-8

Data Transfer using VCI

Computer Networks 18-9

SVC Setup: Request and Acknowledgment

Computer Networks 18-10

Frame Relay

• Frame Relay is a virtual circuit wide area network

• VCIs in Frame Relay are called DLCIs(Data Link Connection Identifier)

Computer Networks 18-11

Frame Relay Features

• Frame relay operates at a higher speed. It can easily be used instead of a mesh of T-1 or T-3 lines (1.544 Mbps or 44.376 Mbps)

• Frame relay operates just the physical and data link layers. It is good as a backbone to provide services to protocols that already have a network layer protocol, such as Internet

• It allows bursty data

• It allows a frame size of 9000 bytes accommodating all LAN frame sizes

• It is less expensive than other traditional WANs

• It has error detection at the data link layer only. There is no flow control pr error control

• X.25 Leased Lines Frame Relay

Computer Networks 18-12

Frame Relay vs. T-line Network

Computer Networks 18-13

Frame Relay vs. X.25 Network

Computer Networks 18-14

Frame Relay Layers

• Frame relay operates only at the physical and data link layers

Computer Networks 18-15

Comparing Layers: X.25 & Frame Relay

Computer Networks 18-16

Frame Relay Frame

Computer Networks 18-17

Congestion Control

• Frame relay requires congestion control, because– Frame Relay does not have a network layer– No flow control at the data link layer– Frame Relay allows the user to transmit bursty data

• Congestion avoidance– Two bits in the frame are used– BECN(Backward Explicit Congestion Notification)– FECN(Forward Explicit Congestion Notification)

• Discard eligibility(DE): – Priority level of the frame for traffic control– Discarding frame to avoid the congestion or collapsing

Computer Networks 18-18

BECN

Computer Networks 18-19

FECN

Computer Networks 18-20

Four Cases of Congestion

Computer Networks 18-21

Extended Address: Three Address Formats

FRAD

Computer Networks 18-22

ATM

• Asynchronous Transfer Mode

• ATM is the cell relay protocol designed by ATM forum and adopted by ITU-T

• ATM uses asynchronous TDM

• Cells are transmitted along virtual circuits

• Design Goals– Large bandwidth and less susceptible to noise degradation– Interface with existing systems without lowering their

effectiveness– Inexpensive implementation– Support the existing telecommunications hierarchies– Connection-oriented to ensure accurate and predictable

delivery– Many functions are hardware implementable

Computer Networks 18-23

Multiplexing using Cells

• The variety of packet sizes makes traffic unpredictable

• A cell network uses the cell as the basic unit of data exchange– A cell is defined as a small, fixed sized block of information– Cells are interleaved so that non suffers a long delay– A cell network can handle real-time transmissions– Network operation is more efficient and cheaper

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Synchronous vs. Asynchronous TDM

Computer Networks 18-25

ATM Architecture

• UNI: user-to-network interface

• NNI: network-to-network interface

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Virtual Connection

• Connection between two endpoints is accomplished through– Transmission path (TP)– Virtual path (VP)– Virtual circuit (VC)

• A virtual connection is defined by a pair of numbers: VPI and VCI

Computer Networks 18-27

VPI and VCI: Hierarchical Switching

Computer Networks 18-28

Identifiers and Cells

Computer Networks 18-29

VP Switch and VPC Switch

Computer Networks 18-30

ATM Layers

Computer Networks 18-31

ATM Layer and Headers

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Application Adaptation Layer (AAL)• Convert data from upper-layer into 48-byte data units for the ATM cells• AAL1 – constant bit rate (CBR) video and voice• AAL2 – variable bit rate (VBR) stream low-bit-rate traffic an short-frame traffic such as audio (ex: mobile phone)• AAL3/4 – connection-oriented/connectionless data• AAL5 – SEAL (Simple and Efficient Adaptation Layer) No sequencing and error control mechanisms

Computer Networks 18-33

AAL1

Computer Networks 18-34

AAL2

Computer Networks 18-35

AAL3/4

Computer Networks 18-36

AAL5

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ATM LAN

• ATM is mainly a wide-area network (WAN ATM); however, the technology can be ATM is mainly a wide-area network (WAN ATM); however, the technology can be adapted to local-area networks (ATM LANs). The high data rate of the technology adapted to local-area networks (ATM LANs). The high data rate of the technology has attracted the attention of designers who are looking for greater and greater has attracted the attention of designers who are looking for greater and greater speeds in LANs. speeds in LANs.

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Pure and Legacy ATM LAN

Computer Networks 18-39

Mixed Architecture ATM LAN

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LAN Emulation (LANE)

• Connectionless versus connection-orientedConnectionless versus connection-oriented

• Physical addresses versus virtual-circuit identifiersPhysical addresses versus virtual-circuit identifiers

• Multicasting and broadcasting deliveryMulticasting and broadcasting delivery

• InteroperabilityInteroperability

• Client/Server model in a LANEClient/Server model in a LANE– LANE Configuration Server (LECS), LANE Server (LES), LANE Client (LEC)LANE Configuration Server (LECS), LANE Server (LES), LANE Client (LEC)

– Broadcast/Unknown Server (BUS)Broadcast/Unknown Server (BUS)

Computer Networks 18-41

Mixed Architecture Using LANE