Chapter 12:Wide Area Networks

Business Data Communications, 4e

Wide Area Networking Issues

Trend towards distributed processing architectures to support applications and organizational needs.Expansion of wide area networking technologies and services available to meet those needs.

WAN Alternatives

SMDS: Switched Mulitmegabit Data Service

Integrated Network AccessUsing Dedicated Channels

(STDM)

Integrated Network AccessUsing Public Switched WAN

Frame Relay Characteristics

Designed to eliminate excessive X.25 overheadControl signaling takes place on a separate logical connection (nodes don’t need state tables for each call)Multiplexing/switching take place at layer 2, eliminating a layer of processingNo hop-by-hop flow/error control

Traditional Packet Switching

Frame Relay Operation

Frame Relay Architecture

Committed Information Rate - CIR

ATM - Asynchronous Transfer Mode

Also known as cell relayFaster than X.25, more streamlined than frame relaySupports data rates several orders of magnitude greater than frame relayData on logical connection is organized into fixed-size packets, called cells. No link-by-link error control or flow control.

Virtual Channels & Virtual Paths

Logical connections in ATM are virtual channels analogous to a virtual circuit in X.25 or a

frame relay logical connection used for connections between two end

users, user-network exchange (control signaling), and network-network exchange (network management and routing)

A virtual path is a bundle of virtual channels that have the same endpoints.

VP vs. VC

Advantages of Virtual Paths

Simplified network architectureIncreased network performance and reliabilityReduced processing and short connection setup timeEnhanced network services

Virtual-Path/Virtual-Channel Characteristics

Quality of service Cell loss ratio, Cell delay variation

Switched and semi-permanent virtual-channel connectionsCell sequence integrityTraffic parameter negotiation and usage monitoring- average rate, peak rate, burstiness, peak duration

ATM Cell Format

UNI NNI

ATM Cell Header GFC (Generic Flow Control): 4 bits

Control of cell flow at local user-network interface

Virtual Path Identifier (VPI) 8 bits at UNI, 12 bits at NNI

Virtual Channel Identifier (VCI) : 16 bits VCI functions much as a service access point.

Payload Type (PT) : 3 bits Type of information

Cell Loss Priority (CLP): 1 bit 0: Relative high priority

Header Error Control (HEC): 8 bits To correct single-bit errors And to detect double-bit

errors

Payload Type (PT)

ATM Bit Rate Services

Constant bit rate (CBR) Fixed data rate

Variable bit rate (VBR) Sustained rate for normal use Faster burst rate at peak periods

Available bit rate (ABR) Guaranteed minimum rate

Unspecified bit rate (UBR) Best-effort service

Leased lineLeased lineLeased lineLeased line

Voice/videoVoice/videoVoice/videoVoice/video

On-line sessionsOn-line sessionsOn-line sessionsOn-line sessions

E-mailE-mailE-mailE-mail

ATM Bit Rate Services