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

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Presentation on theme: "CS4550: Computer Networks II high speed networks, part 2 : Frame Relay, ATM."— Presentation transcript:

1 CS4550: Computer Networks II high 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 AddrCtlPro 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 physical ATM AAL apps

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 channels Virtual path transmission link

14 ATM network example 5 2 3 9 8 x y w v z 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 5 2 3 9 8 x y w v z 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. u

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

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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