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Overview of Virtual Circuit Networks: X.25, Frame Relay, and ATM Technologies

This text discusses the evolution of virtual circuit networks, starting with X.25 and its limitations, including low data rates and high overhead costs. It explores frame relay's advantages, such as supporting bursty data and error detection, while highlighting its operational aspects like DLCIs and frame formats. Additionally, it examines Asynchronous Transfer Mode (ATM) technology, focusing on its high data rates, interoperability, and architecture, which includes the use of cells for data exchange. It covers ATM's application in LANs, supporting various traffic types.

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Overview of Virtual Circuit Networks: X.25, Frame Relay, and ATM Technologies

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Presentation Transcript

  1. Ch 18. Virtual Circuit Networks

  2. Old WAN Technology • X.25 • Virtual-circuit switching network (designed in 1970s) • Operating at the network layer and below  additional encapsulation of IP packets • Low data rate: 64 Kbps • Large overhead: flow & error controls at multiple layers • Leasing T-1 or T-3 lines • Costly, since it requires n(n-1)/2 lines to connect n branches (mesh networks) • Inefficient and cannot handle “bursty data”

  3. 18.1 Frame Relay • Virtual-circuit WAN • Higher speed: 1.544 Mbps or 44.376 Mbps • No network layer: operates in PHY and data link layers • Allows bursty data • Support large frame up to 9000 bytes • Error detection only (no flow control or error control)

  4. Architecture • Virtual circuits • A VC is identified by data link connection id (DLCI) • Support both permanent and switched virtual circuits • Switch has a table with (incoming-port, DLCI) pairs as we have discussed in Chapter 8 • Data link layer • Addressing (DLCI) • Congestion control • Error detection • Physical layer • No specific protocol is defined

  5. Frame Format • DLCI: address • Command/response (C/R): not used • Extended address (EA) • Forward(destination) explicit congestion notification (FECN) • Backward(sender) explicit congestion notification (BECN) • Discard eligibility (DE): priority level of frame when a frame needs to be discarded

  6. Extended Address • To increase the range of DLCI, the Frame Relay address have been extended from the original 2-byte address to 3- or 4-byte addresses. • Three address formats by setting EA

  7. Other Options • To handle frames arriving from other prototocls .Frame relay uses assembler/disassembler (FRAD) • Mux/demux frames from other protocols • Voice over frame relay (VOFR), voice is digitized (PCM) and the compressed • Quality is not good as a circuit-switched network • Originally Frame relay was designed to provide PVC connections. Local management information (LMI) is added to provide more management features • Keep-alive (check if data is flowing), multicast, switch status check

  8. 18.2 ATM • Goals of Asynchronous Transfer Mode (ATM) • High data rate (e.g., transmission over optical fiber) • Interface with existing systems providing WAN inter-connectivity • Low-cost implementation • Inter-operability with legacy telephone systems • Connection-oriented for accurate and predictable delivery • Move most functions to hardware

  9. Cell Networks • Problem of existing systems • Multiplexing frames of different size can lead to “unfair delay” for small frames • Cell (a small fixed-size data block) network that uses the cell as the basic unit of data exchange can solve the problem

  10. ATM Architecture • Uses asynchronous time-division multiplexing • User-to-network interface (UNI) and network-to-network interface (NNI)

  11. Virtual Connection • Virtual circuit (VC) • A single message flow • Virtual path (VP) • A set of VCs between two switches • Transmission path (TP) • Physical connection between two end points

  12. ATM Cell • A virtual circuit is uniquely identified by a pair of (VPI, VCI) • Cell (the basic data unit) includes (VPI, VCI) • Switching/routing

  13. Layering Structure • PHY can be any physical layer • Originally intended for SONET • ATM layer • Routing, traffic management, switching, multiplexing • Application adaptation layer (AAL) • Data segmentation to fit in a cell

  14. ATM Header • Header format • Generic flow control (GFC): UNI level • VPI and VCI • Payload type (PT) • Cell loss priority (CLP): congestion control • Header error correction (HEC): CRC to correct errors

  15. AAL Layer – AAL1 • Support any type of payload • Four versions: AAL1, AAL2, AAL3/4, AAL5 • AAL1 • Support applications with constant bit rates - Sequence number (SN): 4-bit - Sequence number protection (SNP): 4-bit

  16. AAL Layer – AAL2 • AAL2 • Intended to support variable-data-rate bit stream • Used for low-bit-rate traffic and short-frame traffic

  17. AAL Layer – AAL3/4 • AAL3/4 • For connection-oriented and connectionless services

  18. AAL Layer – AAL5 • AAL5 • Simple and efficient adaptation layer (SEAL)

  19. ATM Layers in Devices and Switches • ATM has a congestion control and quality of service

  20. 18.3 ATM LANs • Adopt ATM technology to LANs • High data rate • Support permanent and temporary connections • Support multimedia traffic with different bandwidths • Pure ATM and Legacy ATM LANs

  21. ATM LANs • Mixed architecture • Issues in LAN Emulation (LANE) • Connectionless versus connection-oriented • Physical addresses versus virtual-circuit identifiers • Multicasting and broadcasting • Interoperability

  22. LAN Emulation (LANE) • Client/Server model • LAN emulation client (LEC): Let upper layers unaware of the existence of the ATM technology • LAN emulation configuration server (LECS): initial connection between the client and LANE • LAN emulation server (LES): create virtual circuit for a request of frame delivery • Broadcast/unknown server (BUS): responsible for multicast and broadcast service

  23. Example of Mixed Architecture

  24. Homework • Exercise • 17 • 20 • 21 • 28

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