Chapter 18. Virtual-Circuit Networks: Frame Relay and ATM
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Transcript of Chapter 18. Virtual-Circuit Networks: Frame Relay and ATM
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Chapter 18. Virtual-Circuit Networks:Frame Relay and ATM
18.1 Frame Relay
18.2 ATM
18.3 ATM LANs
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Wide area network and switching methods
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Circuit switching
• Create a real circuit (dedicated line) between source and destination
• Physical layer technology
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Packet Switching: Datagram Approach
• Mostly used in the network layer
• Routing (selecting the best route for a packet) is performed at each router
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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
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VCI Phases
• Two approaches for the VC setup– Permanent virtual circuit (PVC): – Switched virtual circuit (SVC): setup, data transfer, teardown
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Data Transfer Phase
• All switches need to have a table entry for the virtual circuit
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Data Transfer using VCI
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SVC Setup: Request and Acknowledgment
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Frame Relay
• Frame Relay is a virtual circuit wide area network
• VCIs in Frame Relay are called DLCIs(Data Link Connection Identifier)
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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
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Frame Relay vs. T-line Network
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Frame Relay vs. X.25 Network
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Frame Relay Layers
• Frame relay operates only at the physical and data link layers
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Comparing Layers: X.25 & Frame Relay
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Frame Relay Frame
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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
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BECN
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FECN
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Four Cases of Congestion
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Extended Address: Three Address Formats
FRAD
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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
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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
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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
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VPI and VCI: Hierarchical Switching
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Identifiers and Cells
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VP Switch and VPC Switch
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ATM Layers
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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
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AAL1
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AAL2
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AAL3/4
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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
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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)
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Mixed Architecture Using LANE