Brought to you by EarthWeb
IT Library Logo

Click Here!
Click Here!


Search the site:
 
EXPERT SEARCH -----
Programming Languages
Databases
Security
Web Services
Network Services
Middleware
Components
Operating Systems
User Interfaces
Groupware & Collaboration
Content Management
Productivity Applications
Hardware
Fun & Games

EarthWeb Direct EarthWeb Direct Fatbrain Auctions Support Source Answers

EarthWeb sites
Crossnodes
Datamation
Developer.com
DICE
EarthWeb.com
EarthWeb Direct
ERP Hub
Gamelan
GoCertify.com
HTMLGoodies
Intranet Journal
IT Knowledge
IT Library
JavaGoodies
JARS
JavaScripts.com
open source IT
RoadCoders
Y2K Info

Previous Table of Contents Next


4-6
Traffic Control Functions in ATM Networks

BYUNG G. KIM

A synchronous transfer mode (ATM) is the universal transport vehicle for multimedia traffic in the context of the broadband integrated services digital network (B-ISDN) specification. ATM is a connection-oriented technology that transports data in 53-byte cells. To accommodate different types of connections, four service classes are currently defined in ATM networks:

  Constant bit rate (CBR).
  Variable bit rate (VBR).
  Available bit rate (ABR).
  Unspecified bit rate (UBR).

ATM SERVICES

Constant Bit Rate Service

CBR service is designed for connections that require a fixed amount of bandwidth for the entire connection time. The amount of bandwidth is equivalent to the peak rate of the connection. CBR service is intended to support real-time applications with stringent requirements for transfer delays and delay variations (e.g., digitized voice and Px64 video).

Variable Bit Rate Service

VBR service, on the other hand, is designed for those sources that generate cells at time-varying rates. Real-time VBR service provides tight bounds on cell delays and delay variations, whereas non-real-time VBR service provides a low cell loss ratio. Examples of VBR connections are an MPEG full-motion video source producing heavier traffic at scene changes and a bursty file transfer connection, occasionally sending a large amount of data.

VBR service uses statistical multiplexing, by which the available bandwidth not used by light-traffic connections is taken advantage of by connections with temporarily heavy traffic. However, there is always a risk that the given VBR capacity may be saturated when many sources generate heavier traffic than expected. The VBR capacity and the buffer space should be carefully designed and managed so that occurrences of such capacity saturation are strictly controlled. Because of the statistical nature of traffic fluctuation in VBR sources, the management of network resources is usually difficult for VBR service.

Available Bit Rate Service

The ABR service class is designed primarily for LAN traffic across an ATM network. The bandwidth left unallocated to CBR and VBR sources is made available to ABR sources for sharing.

An ABR source is required to regulate its transmission rate according to the prevailing congestion condition in the network. Control cells that carry information about congestion, called resource management (RM) cells, are periodically inserted into the cell stream so that the network condition can be acquired from returning RM cells. A network guarantees a low cell loss ratio and a fair sharing of ABR bandwidth for those end systems that adapt their transmission rate according to the feedback information in RM cells.

Unspecified Bit Rate Service

UBR service demands no specific quality of service from the network and is thus handled only after service requirements of CBR, VBR, and ABR connections are satisfied.

CBR AND VBR TRAFFIC CONTROL

An ATM source declares its expected traffic pattern in the source traffic descriptor. The source traffic descriptor includes the peak cell rate (PCR), the sustainable cell rate (SCR), and burst tolerance (BT). The source then requests a connection by submitting connection traffic parameters to the network, which consist of the source traffic descriptor, the cell delay variation (CDV) tolerance, and the conformance definition.

Conformance Definition Using GCRA

As soon as a connection is established, the ATM network starts the usage parameter control function to monitor the actual traffic generated from the source against the source traffic parameters declared for the connection request. This function enforces the service contract so that network resources are protected from malicious or unintentional misbehavior of the source. For an unambiguous specification of the conformance definition, the generic cell rate algorithm (GCRA) is devised.

For each cell arrival, the GCRA determines whether the cell conforms to the declared source traffic parameters. The GCRA can thus be used as a formal definition of traffic conformance test for a connection. However, the network is not obliged to use this algorithm for usage parameter control (UPC) as long as the operation of the UPC does not violate the quality of service objectives of compliant connections.

The GCRA defines the relationship between the peak cell rate and the cell delay variation tolerance, as well as the relationship between the sustainable cell rate and the burst tolerance. The GCRA is based on two parameters, the increment I and the limit L, and is denoted by GCRA(I,L).

For example, let ta(k) denote the arrival time of the k-th cell in a connection. The basic idea of the GCRA is to have a variable—the theoretical arrival time (TAT)—keep track of the expected arrival time of the next subsequent cell. When TAT is updated for the k-th cell, three cases are handled:

  Case 1: TAT £ ta(k). The cell arrival time is on or after the expected arrival time. This cell is arriving at a slower pace than expected and is thus conforming to the source traffic parameters. TAT is updated to ta(k) + I.
  Case 2: TAT-L £ ta(k) < TAT. The cell is arriving before the expected arrival time but within the limit of L. Although the cell is generated slightly ahead of the schedule, the cell is considered to be conforming. TAT is updated to TAT + I.
  Case 3: ta(k) < TAT - L. The cell arrived too early and is not conforming. TAT remains unchanged. The nonconforming cell may be either discarded or tagged to a lower priority.


Previous Table of Contents Next

footer nav
Use of this site is subject certain Terms & Conditions.
Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.