Approaches to Networking

7/30/2019 Approaches to Networking

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Approaches to Networking

Business Data Communications, 4e


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LANs, WANs, and MANs

Ownership

WANs can be either public or privateLANs are usually privately owned

Capacity

LANs are usually higher capacity, to carry greater internal

communications load

Coverage

LANs are typically limited to a single location

WANs interconnect locations


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Types of WANs

Circuit-switched (todays lecture)

Packet-switched (Thursdays lecture)


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Circuit-SwitchingDefinition: Communication in which adedicated 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)


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Circuit-Switching Stages

Circuit establishment

Transfer of information

point-to-point from endpoints to node

internal switching/multiplexing among nodes

Circuit disconnect


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Circuit Establishment

Station requests connection from node

Node determines best route, sends message tonext link

Each subsequent node continues the

establishment of a path

Once nodes have established connection, test

message is sent to determine if receiver is

ready/able to accept message


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Information Transfer

Point-to-point transfer from source to node

Internal switching and multiplexed transferfrom node to node

Point-to-point transfer from node to receiver

Usually a full-duplex connection throughout


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Circuit Disconnect

When transfer is complete, one station

initiates terminationSignals must be propagated to all nodes used

in transit in order to free up resources


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Public Switched TelephoneNetwork (PSTN)

Subscribers

Local loopConnects subscriber to

local telco exchange

Exchanges

Telco switching centers

Also known as end

office

>19,000 in US

Trunks

Connections betweenexchanges

Carry multiple voice

circuits using FDM or

synchronous TDMManaged by IXCs

(inter-exchange carriers)


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Digital Circuit-Switching Node


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Circuit Switching Node:Digital Switch

Provides transparent signal path between any

pair of attached devicesTypically full-duplex


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Circuit-Switching Node:Network Interface

Provides hardware and functions to connect

digital devices to switchAnalog devices can be connected if interface

includes CODEC functions

Typically full-duplex


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Circuit-Switching Node:Control Unit

Establishes on-demand connections

Maintains connection while needed

Breaks down connection on completion


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Blocking/Nonblocking Networks

Blocking: network is unable to connect two

stations because all possible paths are alreadyin use

Nonblocking: permits all possible connection

requests because any two stations can beconnected


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Switching Techniques

Space-Division Switching

Developed for analog

environment, but has been

carried over into digital

communication

Requires separate physical

paths for each signalconnection

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


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Routing in Circuit-SwitchedNetworks

Requires balancing efficiency and resiliency

Traditional circuit-switched model ishierarchical, sometimes supplemented with

peer-to-peer trunks

Newer circuit-switched networks aredynamically routed: all nodes are peer-to-

peer, making routing more complex


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Alternate Routing

Possible routes between two end offices are

predefinedOriginating switch selects the best route for

each call

Routing paths can be fixed (1 route) ordynamic (multiple routes, selected based on

current and historical traffic)


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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)


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ISDN 1st generation: narrowband ISDN

Basic Rate Interface (BRI)

two 64Kbps bearer channels + 16Kbps data channel

(2B+D) = 144 Kbps

circuit-switched

2nd generation: broadband ISDN (B-ISDN)

Primary Rate Interface (PRI)twenty-three 64Kbps bearer channels + 64 data channel

(23B+D) = 1.536 Mbps

packet-switched network

development effort led to ATM/cell relay


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Past Criticism of ISDNInnovations Subscribers Dont Need , It

Still Doesnt Network , It Still Does

NothingWhy so much criticism?

overhyping of services before delivery

high price of equipment

delay in implementing infrastructure

incompatibility between providers' equipment.

Didnt live up to early promises


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ISDN Principles

Support of voice and nonvoice using limited set of

standard facilities

Support for switched and nonswitched applications

Reliance on 64kbps connections

Intelligence in the networks

Layered protocol architecture (can be mapped ontoOSI model)

Variety of configurations


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ISDN User Interface

Pipe to users premises has fixed capacity

Standard physical interface can be used forvoice, data, etc

Use of the pipe can be a variable mix of voice

and data, up to the capacityUser can be charged based on use rather than

time


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ISDN Network ArchitecturePhysical path from user to office

subscriber loop, aka local loop

full-duplex

primarily twisted pair, but fiber use growing

Central office connecting subscriber loops

B channels: 64kbps

D channels: 16 or 64kbps

H channels: 384, 1536, or 1920 kbps


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ISDN B ChannelBasic user channel (aka bearer channel)

Can carry digital voice, data, or mixture

Mixed data must have same destination

Four kinds of connections possible

Circuit-switched

Packet-switched

Frame mode

Semipermanent


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ISDN D Channel

Carries signaling information using common-

channel signalingcall management

billing data

Allows B channels to be used moreefficiently

Can be used for packet switching


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ISDN H Channel

Only available over primary interface

High speed rates

Used in ATM


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ISDN Basic Access

Basic Rate Interface (BRI)

Two full-duplex 64kbps B channelsOne full-duplex 16kbps D channel

Framing, synchronization, and overhead bring total

data rate to 192kbpsCan be supported by existing twisted pair local loops

2B+D most common, but 1B+D available


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ISDN Primary Access

Primary Rate Interface (PRI)

Used when greater capacity required

No international agreement on rates

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

Europe: 2.048mbps

Typically 23 64kbps B + 1 64kbps D Fractional use of nB+D possible

Can be used to support H channels


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Packet-Switching Networks

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

technologies

Data is broken into packets, each of which can berouted 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


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Packet-Switching Techniques

Datagram

each packet treated independently and referred to as adatagram

packets may take different routes, arrive out of sequence

Virtual Circuit

preplanned route established for all packetssimilar to circuit switching, but the circuit is not dedicated


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Packet-Switched Routing

Adaptive routing changes based on network

conditionsFactors influencing routing are failure and

congestion

Nodes must exchange information on network status

Tradeoff between quality and amount of overhead


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Packet-Switched CongestionControl

When line utilization is >80%, queue length

grows too quicklyCongestion control limits queue length to

avoid througput problems

Status information exchanged among nodesControl signals regulate data flow using

interface protocols (usually X.25)


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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


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Virtual-Circuit Service

External virtual circuit: logical connection between

two stations on the networkInternal 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


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WANs for Voice

Requires very small and nonvariable delays

for natural conversation--difficult to providethis with packet-switching

As a result, the preferred method for voice

transmission is circuit-switchingMost businesses use public telephone

networks, but a few organizations have

implemented private voice networks


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WANs for Data

Public packet-switched networks (X.25)

Private packet-switched networksLeased lines between sites (non-switched)

Public circuit-switched networks

Private circuit-switched networks (interconnecteddigital PBXs)

ISDN (integrated X.25 and traditional circuit-

switching)


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WAN ConsiderationsNature 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-switching

Security--greater with private networks

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Approaches to Networking

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