CS4550:CS4550:

Computer Networks IIComputer Networks II

high speed networks, high speed networks, part 2 : Frame Relay, ATM part 2 : Frame Relay, ATM

frame relay 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

frame relay 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

frame relay frame relay IP frame format example 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

ATM : ATM : asynchronous transfer modeasynchronous 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”

ATM : asynchronous transfer modeATM : 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

ATM : basic ideas 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.

ATM basic design assumptionsATM basic design assumptions

1. Will be designed as a hierarchy, as is current telephone network

2. Connection-oriented service3. 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

Evolution of transfer modes Evolution of transfer modes (switching techniques) (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

ATM basic principles 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?

ATM layersATM 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

ATM : connections 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.

ATM connections: VP,VCATM connections: VP,VC

Vir

t ual

ch

ann

els

Virtual path

transmission link

ATM network example ATM network example

5

2

3

9

8x y

w

v

z

suppose X wants to pass data to y. whatmust the user and the ATM network do?

ATM network example ATM network example

the internal nodes (circles) represent ATM switches

the end users represented by squares

the communication links are SONET fiber optic links

ATM network example 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

ATM network exampleATM network example

5

2

3

9

8x 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

ATM cell formatATM cell format

GFC/VPI VPI

VPI VCI

VCI

VCI pay.type CLP

HEC

info field : 48 bytes

0 3 74NNIUNI

ATM cell format 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 parametersHEC : header error control

ATM : AAL, ATM adoption layerATM : 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

ATM : AAL IP wrapping 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

quality of service parametersquality 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 duration2. average cell rate3. allowable loss rate 4. cell delay5. cell delay variation (jitter), or burstiness

quality of service parametersquality 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

ATM : traffic and congestionATM : 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

ATM traffic control tools ATM traffic control tools

1. network resource mgt : allocate NW resources optimally; minimize conflicts2. 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”

ATM congestion control toolsATM congestion control tools

selective cell discarding - more aggressive in discarding cells

explicit forward congestion indication - notifies other nodes that congestion occurred; a warning