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CS4550: Computer Networks II high speed networks, part 2 : Frame Relay, ATM

CS4550: Computer Networks II high speed networks, part 2 : Frame Relay, ATM. frame relay. background : X.25 VC packet switched network FR also a virtual circuit packet switched network, but with much less overhead cheaper alternative to leased lines (T1); similar service, more economical

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CS4550: Computer Networks II high speed networks, part 2 : Frame Relay, ATM

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  1. CS4550:Computer Networks IIhigh speed networks, part 2 : Frame Relay, ATM

  2. frame relay • background : X.25 VC packet switched network • FR also a virtual circuit packet switched network, but with much less overhead • cheaper alternative to leased lines (T1); similar service, more economical • based on permanent VCs • a standard for high speed service, not a particular implementation

  3. frame relay • operates at layer 2/3; IP can operate over FR, treating it as a layer 2 protocol • access speeds from 56K to a few megabits • packet size 8000 bytes, optimized for data • can be viewed as intermediate between X.25 (traditional PS) and ATM (fast PS) • has some similarities to both ATM and X.25 • probably (?) to be overshadowed by ATM or other higher speed networks

  4. frame relay IP frame format example Flag Addr Ctl Pro IP FCS Flag X7e DLCI x03 xCC Datagram X7e • Based on HDLC • Handles Multi-protocols from layer 3 • Layer 2 service

  5. ATM : asynchronous transfer mode • new standard for optical fiber networks; meant for both voice & other data • layer : 2,3; intended to operate above SONET or equivalent. • cell-based technology; cells are small packets • many view ATM as the “network of the future”

  6. ATM : asynchronous transfer mode • specs developed by 2 orgs : ITU-T : a committee of the ITU ATM forum : an international nonprofit industry consortium • the “ultimate” high-speed packet switching network • can be viewed as the culmination of circuit and packet switched tech., having the advantages of each, and without the problems

  7. ATM : basic ideas • virtual-circuit, packet switched network • all data transmitted in small, fixed size packets called “cells” (53 bytes) • designed for optical fiber transmission : assumes high speed, low error rate of fiber. • intended to carry all types of digital data : voice, computer data, video, etc. • may operate over SONET physical medium or others, such as FDDI.

  8. ATM basic design assumptions 1. Will be designed as a hierarchy, as is current telephone network 2. Connection-oriented service 3. Most physical networks will be fiber - with low error rates, high data rates 4. Must support very low cost attachments -- e.g., tel calls • remains to be seen how pervasive ATM will become

  9. Evolution of transfer modes (switching techniques) 1. circuit switching : tel system 2. multirate circuit switching 3. fast circuit switching 4. packet switching : X.25, LANs, Internet 5. fast packet switching : frame relay, ATM

  10. ATM basic principles • no error protection/flow control on link by link basis -why? • connection-oriented -why? • minimal header functionality -why? • small, fixed cell size -why?

  11. ATM layers • Physical layer: transmission medium, signal encoding. optical fiber based. • ATM layer: switching, data transmission& logical connections. Heart of ATM. • ATM Adaptation layer: interface between ATM and upper layers. • applications: ATM customers apps AAL ATM physical

  12. ATM : connections • ATM a “virtual circuit packet switched network” : connections made before data transmitted, & all packets in a connection follow same route. • virtual channel(VC) : a connection. Has a pair of endpoints, and the route (VP). • virtual path(VP) : set of VCs which share the same endpoints. • connections have “QOS parameters” - not allowed unless net can meet them.

  13. ATM connections: VP,VC Virtual path Virtual channels transmission link

  14. ATM network example y x 8 w 5 z 2 9 v 3 suppose X wants to pass data to y. what must the user and the ATM network do?

  15. ATM network example • the internal nodes (circles) represent ATM switches • the end users represented by squares • the communication links are SONET fiber optic links

  16. ATM network example 1. establish connection : user sends a request for a connection with several parameters: destination, bandwidth needed, quality of service. ATM net then determines whether the request can be accepted. 2. data transfer : user “talks.” ATM monitors, to ensure user does not violate contract. 3. terminate connection : when users are done, ATM must remove this connection from the network tables

  17. ATM network example u y x 8 w 5 z 2 9 v 3 connection established : now all packets on connection (x,y) will pass through 5,9,8; the route (5,9,8) is a “virtual path;” the VC includes x,y.

  18. ATM cell format NNI UNI 0 7 3 4 GFC/VPI VPI VPI VCI VCI VCI pay.type CLP HEC info field : 48 bytes

  19. ATM cell format • UNI : user-network interface • NNI : network-network interface GFC : generic flow control (only at UNI) VPI : virtual path id (routing) VCI : virtual channel id (routing) CLP : cell loss protection (0,1) et to 1 for high priority Pay-Type: payload type parameters HEC : header error control

  20. ATM : AAL, ATM adoption layer • AAL interface between ATM layer and user layers (e.g., IP, PCM voice, etc.) • some AAL services: -handle transmission errors -segmentation and reassembly -lost /misinserted cells -flow control and timing control

  21. ATM : AAL IP wrapping LLC SNAP DSAP SSAP CTL ID Type IP Trailer X’AA X’AA X03 X’0 X’08 DATAGRAM ATM h data h data …. h data

  22. quality of service parameters • with each connection, there is an associated QOS, implied or stated. These are qualities such as: 1. peak cell rate, and peak duration 2. average cell rate 3. allowable loss rate 4. cell delay 5. cell delay variation (jitter), or burstiness

  23. quality of service parameters • differing traffic types have different needs, for example: -voice: small bandwidth, tolerates some losses, but needs small delay and jitter -data : tolerates more delay/jitter, but cannot tolerate losses; varying bandwidth needs -video needs high bandwidth but may tolerate losses and different delays than above • challenge of ATM is to balance these widely varying needs, and make max use of net without overloading

  24. ATM : traffic and congestion • traffic& congestion control attempt to keep the number of packets in the network within the capacity of the network to meet its requirements • more critical for ATM because of its real- time, QOS guarantees • traffic control - attempts to prevent congestion from happening • congestion control - attempts to reduce or control congestion

  25. ATM traffic control tools 1.network resource mgt : allocate NW resources optimally; minimize conflicts 2. CAC : connection admission control - NW should not accept traffic it can’t handle 3. UPC : usage parameter control - monitor traffic flows, control cheating 4. priority control - using CLP field, discard low priority cells first 5. fast resource mgt - “further study”

  26. ATM congestion control tools • selective cell discarding - more aggressive in discarding cells • explicit forward congestion indication - notifies other nodes that congestion occurred; a warning

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