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ATM Technologies

ATM Technologies. Asynchronous Transfer Mode (ATM). Designed by phone companies Single technology meant to handle Voice Video Data Intended as LAN or WAN Goal: replacement for Internet. ATM Characteristics. End-to-end (application to application) Connection-oriented interface:

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ATM Technologies

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  1. ATM Technologies

  2. Asynchronous Transfer Mode (ATM) • Designed by phone companies • Single technology meant to handle • Voice • Video • Data • Intended as LAN or WAN • Goal: replacement for Internet

  3. ATM Characteristics • End-to-end (application to application) • Connection-oriented interface: • Establish “connection” • Send data • Close connection • Performance guarantees (statistical) • Uses cell switching

  4. ATM Design Issues • Different traffic has different demands • Variable packet size introduces more jitter (variance in delivery time) • Even sending at a constant rate, contention can result jitter • Small packets incur less jitter and delay, but less efficient • Large packets more efficient, delay and jitter is more serious (packet loss)

  5. ATM Cell • Fixed size packet (for highest speed electronics) • Size chosen as compromise between voice (small) and data (large) • 5 octet header • 48 octet payload • Note: size not optimal for any application

  6. ATM Cell Header

  7. ATM Cell Format • Flow control is used for local flow control (user-to-network only) • Flow control doesn't appear in network-to-network interface • Example: it can be used to provide different priorities of a cell • VPI/VCI together provides identification of the cell connection (more later) • Payload type indicates the type of the cell (e.g. user data cell, segment cell) • PRIO: one bit Cell Loss Priority whether or not the cell can be dropped

  8. ATM Switch • Building block of ATM network • Connections to • Computer • Other ATM switches • Accepts and forwards cells

  9. Cell Forwarding • Performed directly by hardware • Incoming cell sent to an outgoing interface • Uses label in cell • Motivation: highest speed

  10. Label Switching • ATM connection identified with 24-bit binary value • Known as Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI) • VPI -- 8 bit long, specifies the path the VC follows through the network • VCI – 16 bit long, specifies a single VC within a path • VPI/VCI rewritten at each switch

  11. Example Of VPI/VCI Rewriting • Sender uses VPI/VCI 3 • Receiver uses VPI/VCI 6 • Intermediate VPI/VCIs are established within each switch

  12. How To Set Up VPI/VCI Entries • Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC) • PVC • Similar to leased line, an entry is permanently set as long as fee is paid • A permanent path is established between two points of connection • An organization may have multiple connection (VPI/VCI) to the Internet

  13. How To Set Up VPI/VCI Entries • SVC • VCs are established and terminated as needed • To request a connection, the sender send a message to the switch • The switch assigns an available slot for the connection • Each pair of the switches negotiate in turn • The connection reaches the receiver • The connection is established

  14. Example Of Switched Network • Network makes connection on demand

  15. Connection Multiplexing • Typical computer has one physical connection to network • All logical connections multiplexed over physical interconnection • Data transferred must include connection identifier (VPI/VCI)

  16. Illustration Of ATM VC • Switch maps VPI/VCI • 17 to 12 • 96 to 8

  17. ATM Quality Of Service • Specified when connection established • Endpoint specifies • Type of data transfer • Throughput desired • Maximum packet burst size • Maximum delay tolerated

  18. Type Of Data Transfer • Constant bit rate (CBR) • Example: audio • Variable bit rate (VBR) • Example: video with adaptive encoding • Available bit rate (ABR) • Example: data • Unspecified bit rate (UBR) • Each type has detailed parameters (e.g., mean, max, burst duration)

  19. Sending Data Over ATM • Uses ATM Adaptation Layer 5 (AAL5) • Accepts and delivers large, variable-size packets • AAL5 divides into cells for transmission • Called segmentation and reassembly

  20. Issues • More expensive than traditional LAN technology • More connection setup time • Cell tax (header/data ~= 10%) • Need to specify service requirement at the connection, some may not know which to specify • Lack of efficient broadcast • Complexity of QoS (Quality of Service): one can specify the request, but hard to enforce it • Assumption of homogeneity

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