business data communications & networking lecture 8 wide area networks

Business Data Communications & Networking

Lecture 8

Wide Area Networks


Network Types

Wide Area Network Cover large geographical areas, often

crossing public right-of-ways Usually consist of several interconnected

switching pointsLocal Area Network

Small scope, usually a building or cluster Typically owned by the same organization

that owns the equipment

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Circuit-Switching

Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes

Dominant in both voice and data communications today e.g. PSTN is a circuit-switched network

Relatively inefficient (100% dedication even without 100% utilization)




Circuit-Switching Stages

Circuit establishmentTransfer of information

point-to-point from endpoints to node internal switching/multiplexing among

nodesCircuit disconnect



Circuit Establishment

Station requests connection from nodeNode determines best route, sends

message to next linkEach subsequent node continues the

establishment of a pathOnce nodes have established connection,

test message is sent to determine if receiver is ready/able to accept message



Information Transfer

Point-to-point transfer from source to node

Internal switching and multiplexed transfer from node to node

Point-to-point transfer from node to receiver

Usually a full-duplex connection throughout



Circuit Disconnect

When transfer is complete, one station initiates termination

Signals must be propagated to all nodes used in transit in order to free up resources


Public Switched Telephone Network (PSTN)

SubscribersLocal loop

Connects subscriber to local telco exchange

Exchanges Telco switching

centers Also known as end

office >19,000 in US

Trunks Connections

between exchanges Carry multiple voice

circuits using FDM or synchronous TDM

Managed by IXCs (inter-exchange carriers)


Circuit-Switching Node



Circuit Switching Node:Digital Switch

Provides transparent signal path between any pair of attached devices

Typically full-duplex


Circuit-Switching Node:Network Interface

Provides hardware and functions to connect digital devices to switch

Analog devices can be connected if interface includes CODEC functions

Typically full-duplex


Circuit-Switching Node:Control Unit

Establishes on-demand connectionsMaintains connection while neededBreaks down connection on

completion


Blocking/Nonblocking Networks

Blocking: network is unable to connect two stations because all possible paths are already in use

Nonblocking: permits all possible connection requests because any two stations can be connected


Switching Techniques

Space-Division Switching Developed for analog

environment, but has been carried over into digital communication

Requires separate physical paths for each signal connection

Uses metallic or semiconductor ‘gates’

Time-Division Switching Used in digital

transmission Utilizes multiplexing

to place all signals onto a common transmission path

Bus must have higher data rate than individual I/O lines






Routing in Circuit-Switched Networks

Requires balancing efficiency and resiliency

Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks

Newer circuit-switched networks are dynamically routed: all nodes are peer-to-peer, making routing more complex


Alternate Routing

Possible routes between two end offices are predefined

Originating switch selects the best route for each call

Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)



Control Signaling

Manage the establishment, maintenance, and termination of signal paths

Includes signaling from subscriber to network, and signals within network

In-channel signaling uses the same channel for control signals and calls

Common-channel signaling uses independent channels for controls (SS7)


Packet-Switching Networks

Includes X.25, ISDN, ATM and frame-relay technologies

Data is broken into packets, each of which can be routed separately

Advantages: better line efficiency, signals can always be routed, prioritization option

Disadvantages: transmission delay in nodes, variable delays can cause jitter, extra overhead for packet addresses


Packet-Switching Techniques

Datagram each packet treated independently and

referred to as a datagram packets may take different routes, arrive

out of sequenceVirtual Circuit

preplanned route established for all packets similar to circuit switching, but the circuit is

not dedicated [vc]



Packet-Switched Routing

Adaptive routing changes based on network conditions

Factors influencing routing are failure and congestion

Nodes must exchange information on network status

Tradeoff between quality and amount of overhead


Packet-Switched Congestion Control

When line utilization is >80%, queue length grows too quickly

Congestion control limits queue length to avoid throughput problems

Status information exchanged among nodes

Control signals regulate data flow using interface protocols (usually X.25)


X.25 Interface Standard

ITU-T standard for interface between host and packet-switched network

Physical level handles physical connection between host and link to the node Technically X.21, but other standards can be

substituted, including RS-232

Link level provides for reliable data transfer Uses LAPB, which is a subset of HDLC

Packet level provides virtual circuits between subscribers


Virtual-Circuit Service

External virtual circuit: logical connection between two stations on the network

Internal virtual circuit: specific preplanned route through the network

X.25 usually has a 1:1 relationship between external and internal circuits

In some cases, X.25 can be implemented as a packet-switched network


ISDN

First generation is narrowband ISDN uses 64kbps channels circuit-switched

Second generation is broadband ISDN (B-ISDN uses higher data rates (hundreds of mbps) packet-switched network development effort led to ATM/cell relay


ISDN PrinciplesSupport of voice and nonvoice using limited

set of standard facilitiesSupport for switched and nonswitched

applicationsReliance on 64kbps connections Intelligence in the networksLayered protocol architecture (can be

mapped onto OSI model)Variety of configurations


ISDN User Interface

‘ Pipe’ to user’s premises has fixed capacity

Standard physical interface can be used for voice, data, etc

Use of the pipe can be a variable mix of voice and data, up to the capacity

User can be charged based on use rather than time


ISDN Network Architecture

Physical path from user to office subscriber loop, aka local loop full-duplex primarily twisted pair, but fiber’s coming

Central office connecting subscriber loops B channels: 64kbps D channels: 16 or 64kbps H channels: 384, 1536, or 1920 kbps


ISDN B Channel

Basic user channel (aka ‘bearer channel’)Can carry digital voice, data, or mixture

Mixed data must have same destinationFour kinds of connections possible

Circuit-switched Packet-switched Frame mode Semipermanent


ISDN D Channel

Carries signaling information using common-channel signaling call management billing data

Allows B channels to be used more efficiently

Can be used for packet switching


ISDN H Channel

High speed ratesOnly available over primary interface Used in ATM


ISDN Basic Access

Basic Rate Interface (BRI)Two full-duplex 64kbps B channelsOne full-duplex 16kbps D channelFraming, synchronization, and overhead

bring total data rate to 192kbpsCan be supported by existing twisted pair

local loops2B+D most common, but 1B+D available


ISDN Primary Access

Primary Rate Interface (PRI)Used when greater capacity requiredNo international agreement on rates

US, Canada, Japan: 1.544mbps (= to T1) Europe: 2.048mbps

Typically 23 64kbps B + 1 64kbps DFractional use of nB+D possibleCan be used to support H channels


WANs for Voice

Requires very small and nonvariable delays for natural conversation--difficult to provide this with packet-switching

As a result, the preferred method for voice transmission is circuit-switching

Most businesses use public telephone networks, but a few organizations have implemented private voice networks


WANs for Data

Public packet-switched networks (X.25)Private packet-switched networksLeased lines between sites (non-switched)Public circuit-switched networksPrivate circuit-switched networks

(interconnected digital PBXs)ISDN (integrated X.25 and traditional

circuit-switching)


WAN Considerations

Nature of traffic stream generally works best with dedicated

circuits bursty better suited to packet-switching

Strategic and growth control--limited with public networks

Reliability--greater with packet-switchingSecurity--greater with private networks


High-Speed WAN Alternatives

Faster multiplexed lines T-3 SONET

Switched networks Frame relay SMDS ATM


Common Nonswitched Offerings

Analog lines (dedicated modems)Digital data lines (dedicated DSUs)T-1, T-3 leased linesFrame relay over dedicated linesSONET


Common Switched Offerings

Dial-up modemsX.25 packet switchingISDNFrame relaySMDSATM


Changes in WAN Structure

Until recently, most devices were fed through a synchronous TDM multiplexer (MUX), sent to a carrier that split it up into dedicated circuits

Increasingly, organizations now use a switched alternative, allowing better utilization of bandwidth, and removing need for a MUX


Integrated Network withDedicated Channels


Integrated Network withPublic Switched WAN


High-Speed WAN Options

Frame RelayATM (Asynchronous Transfer Mode)SMDS (Switched Multimegabit Data

Service)Broadband ISDN


Frame Relay

Designed to eliminate excessive X.25 overhead

Control 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 processing

No hop-by-hop flow/error control


Packet Switching Diagram


Frame Relay Diagram

business data communications & networking lecture 8 wide area networks

Documents

data communications

dedicated communications

possible connection

digital devices

linkeach subsequent

multiplexed transfer

switchanalog devices

possible paths