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Network Layer and Circuit Switching

Networks provide for communication between devices without direct connections. Circuit Switching is the Oldest Networking

Technology

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Why Networks?

Why we need networks? When point to point link or multipoint link is not available between

devices Why we can’t connect devices directly? This is obvious for two

reasons: The direct connection of every device with every other device would

require N(N-1) full duplex links This would be prohibitively expensive

Each communication link would require a port on the device. The cost of the ports would be a limiting factor and also their physical

size.

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What is a Network?

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Kinds of Networks By Communication Technique

Switched Networks Broadcast Networks e.g. LANs

By Geographical Coverage Wide Area Network

Cover large geographical areas, often crossing public right-of-ways Usually consist of several interconnected switching points

Local Area Network Small scope, usually a building or cluster Typically owned by the same organization that owns the equipment

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Switched Network Types Long distance transmission is typically done over a

network of switched nodes Nodes not concerned with content of data End devices are stations

Computer, terminal, phone, etc.

A collection of nodes and connections is a communications network

Data routed by being switched from node to node Nodes may connect to other nodes only, or to stations and

other nodes

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Switched Network Types

Node to node links usually multiplexed Two types of switched networks

Circuit Switched Networks Packet Switched Networks

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Simple Switched Network

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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

Oldest Networking Technology - more than a hundred years old

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 Dedicated communication path between two stations Three phases

Establish Transfer Disconnect

Must have switching capacity and channel capacity to establish connection

Must have intelligence to work out routing

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Circuit Switching - Disadvantages Inefficient

Channel capacity dedicated for duration of connection If no data, capacity wasted

Set up (connection) takes time Once connected, transfer is transparent Developed for voice traffic (phone)

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Circuit-Switching Stages Circuit establishment

Station requests connection from node Node determines best route, sends message to next 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 Transfer of information

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

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Circuit-Switching Stages 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

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Circuit Switching Application

Circuit switching is well suited for analog voice communications as in the telephone network.

In-efficient for data networks due to its resource allocation nature

Ill-suited to data communication because data traffic is BAD

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Examples of Circuit Switching

Public Switched Telephone Network - PSTN Private Automatic Branch Exchange - PABX Integrated Services Digital Network - ISDN

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Public Switched Telephone Network (PSTN) PSTN (Public Switched Telephone Network), refers

to the international telephone system based on copper wires carrying analog voice

Telephone service carried by the PSTN is often called plain old telephone service (POTS).

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Public Switched Telephone Network (PSTN) Elements Subscribers Local 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)

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Telephone Network Structure

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Telephone Network

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Typical Circuit Route for Medium Distance Calls-

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Circuit Switching Connection

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Circuit Switching Node:Digital Switch

Provides transparent signal path between any pair of attached devices

Typically full-duplex

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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

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Circuit-Switching Node:Control Unit

Control Unit Establish connections

Generally on demand Handle and acknowledge

requests Determine if destination is free construct path

Maintain connection -while needed

Disconnect - Breaks down connection on completion

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Blocking or Non-blocking Blocking

A network is unable to connect stations because all paths are in use

A blocking network allows this Used on voice systems

Short duration calls

Non-blocking Permits all stations to connect (in pairs) at once Used for some data connections

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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 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

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Circuit Switch Design - Cross Bar Switch Crossbar switch

Number of crosspoints grows as square of number of stations

Loss of crosspoint prevents connection

Inefficient use of crosspoints All stations connected,

only a few crosspoints in use

Non-blocking

Cross Bar Switch A cross bar switch connects n inputs to m outputs in a grid,

using electronic micro switches (transistors) at each crosspoint.

The major limitation of this design is the number of crosspoints required.

To connect n inputs to m outputs using a crossbar switch requires nxm crosspoints.

E.g to connect 1000 inputs to 1000 outputs requires a switch with 1,000,000 crosspoints

A crossbar with this number of crosspoints is impractical. Such a switch is also inefficient because only fewer than

25% of the crosspoints are in use at any given time.

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Multistage Switch Reduced number of crosspoints More than one path through network

Increased reliability More complex control May be blocking

Multistage Switch To design a three-stage switch, we follow these

steps:

1) We divide the N input lines into groups, each of n lines. For each group, we use one crossbar of size nxk, where k is the number of crossbars in the middle stage. In other words, the first stage has N/n crossbars of nxk crosspoints

2) We use k crossbars, each of size (N/n) x (N/n) in the middle stage

3) We use N/n crossbars, each of size kxn at the third stage

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N/n(nxk) + k(N/n x N/n) + N/n(k x n) =In a three-stage switch, the total number of crosspoints is

2kN + k(N/n)2

which is much smaller than the number of crosspoints in a single-stage switch (N2).

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Example 8.3Design a three-stage, 200 × 200 switch (N = 200) with k = 4 and n = 20.

SolutionIn the first stage we have N/n or 10 crossbars, each of size

20 × 4. In the second stage, we have 4 crossbars, each of

size 10 × 10. In the third stage, we have 10 crossbars,

each of size 4 × 20. The total number of crosspoints is

2kN + k(N/n)2, or 2000 crosspoints. This is 5 percent of

the number of crosspoints in a single-stage switch (200 ×

200 = 40,000).

Clos criterion Clos investigated the condition of nonblocking in

multistage switches and came up with the following formula

In a nonblocking switch, the number of middle-stage switches must be at least 2n-1. In other words, we need to have k > 2n-1

According to the Clos criterion:

n = (N/2)1/2

k > 2n – 1

Crosspoints ≥ 4N [(2N)1/2 – 1]

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Redesign the previous three-stage, 200 × 200 switch, using the Clos criteria with a minimum number of crosspoints.

SolutionWe let n = (200/2)1/2, or n = 10. We calculate k = 2n − 1 = 19. In the first stage, we have 200/10, or 20, crossbars, each with 10 × 19 crosspoints. In the second stage, we have 19 crossbars, each with 10 × 10 crosspoints. In the third stage, we have 20 crossbars each with 19 × 10 crosspoints. The total number of crosspoints is 20(10 × 19) + 19(10 × 10) + 20(19 ×10) = 9500.

Example 8.4

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Circuit Switch Design - TDM Bus Switch

Partition low speed bit stream into pieces that share higher speed stream

e.g. TDM bus switching based on synchronous time division

multiplexing Each station connects through

controlled gates to high speed bus Time slot allows small amount of data

onto bus Another line’s gate is enabled for

output at the same time

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Issues in Circuit Switched Networks Routing Control Signalling

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Routing Routing in the network tries to determine the path

from a given source to a given destination Many connections will need paths through more

than one switch Need to find a route

Efficiency Resilience

Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing depending

on traffic Uses a peer structure for nodes

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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 - almost like packet switching

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Static Hierarchical Routing Tracing up the tree to the first common node, and then tracing down the tree to the called subscriber

Resiliency to the network, additional high usage trunks were added that cut across the tree structure to connect exchanges.

Minimum switching costs, flexibility only via alternate trunks

Not able to adapt to changing conditions Typically in such designs the result of a failure is

a major local congestion near the site of the failure

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Alternate Routing A dynamic routing approach is one in which routing

decisions are influenced by current traffic conditions In Alternate routing schemes the possible routes

between two end offices are predefined Originating switch selects the best route for each call Routes listed in preference order Different sets of routes may be used at different

times Routing paths can be fixed (1 route) or dynamic

(multiple routes, selected based on current and historical traffic)

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Alternate RoutingA form of the dynamic alternate routing technique is employed by the Bell Operating Companies for providing local and regional telephone service, referred as multialternate routing (MAR)

This approach also used in AT&T in its long distance network which is referred as dynamic non-hierarchal routing (DNHR)

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Adaptive Routing Traffic reporting and analysis with new paths

computed periodically, adapts to net load, events

Need to use algorithms to determine paths dynamically, based on load/congestion vectors

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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 Control Signalling

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

Common Channel Control Signalling Common-channel signaling uses independent channels for control

(SS7)

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Control Signaling Functions

Audible communication with subscriber Transmission of dialed number Call can not be completed indication Call ended indication Signal to ring phone Billing info Equipment and trunk status info Diagnostic info Control of specialist equipment

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Control Signal Sequence Both phones on hook Subscriber lifts receiver (off hook) End office switch signaled Switch responds with dial tone Caller dials number If target not busy, send ringer signal to target subscriber Feedback to caller

Ringing tone, engaged tone, unobtainable Target accepts call by lifting receiver Switch terminates ringing signal and ringing tone Switch establishes connection Connection release when Source subscriber hangs up

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Switch to Switch Signaling

Subscribers connected to different switches Originating switch seizes interswitch trunk Send off hook signal on trunk, requesting

digit register at target switch (for address) Terminating switch sends off hook followed

by on hook (wink) to show register ready Originating switch sends address

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Control Signals

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Location of Signaling Subscriber to network

Depends on subscriber device and switch Within network

Management of subscriber calls and network More complex SS7

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In Channel Signaling Use same channel for signaling and call

Requires no additional transmission facilities Inband

Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech path

Out of band Voice signals do not use full 4kHz bandwidth Narrow signal band within 4kHz used for control Can be sent whether or not voice signals are present Need extra electronics Slower signal rate (narrow bandwidth)

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Drawbacks of In Channel Signaling

Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channel signaling

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Common Channel Signaling Control signals carried over paths independent of voice

channel One control signal channel can carry signals for a number

of subscriber channels Common control channel for these subscriber lines Associated Mode

Common channel closely tracks interswitch trunks

Disassociated Mode Additional nodes (signal transfer points) Effectively two separate networks

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Common v. In Channel Signaling

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Common Channel Signaling Modes

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Signaling System Number 7 SS7 Common channel signaling scheme Used in ISDN and inside PSTN Optimized for 64k digital channel network Call control, remote control management and

maintenance Reliable means of transfer of info in sequence

without loss or duplication Will operate over analog and below 64k Point to point terrestrial and satellite links Every element of the SS7 is replicated for

resilience

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SS7 Benefits SS7 adds intelligence to a network Basis of new end user services e.g.

800 and 900 services Mobile Telephone Service Mobile subscriber authentication Caller identification Charging calls to a credit card Charging calls to a calling card

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SS7 Signaling Network Elements Signaling point (SP)

Any point in the network capable of handling SS7 control message

Signal transfer point (STP) A signaling point capable of routing control

messages Control plane

Responsible for establishing and managing connections

Information plane Once a connection is set up, info is transferred in

the information plane

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Transfer Points

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Signaling Network Structures STP capacities

Number of signaling links that can be handled Message transfer time Throughput capacity

Network performance Number of SPs Signaling delays

Availability and reliability Ability of network to provide services in the face of

STP failures

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Circuit Switching - Summary A type of communications in which a

dedicated channel (or circuit) is established for the duration of a transmission.

The most ubiquitous circuit-switching network is the telephone system, which links together wire segments to create a single unbroken line for each telephone call.

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Circuit Switching - Summary Circuit-switching networks are sometimes

called connection-oriented networks because the devices must first establish a connection before exchanging data

Circuit-switching systems are ideal for communications that require data to be transmitted in real-time. Packet-switching networks are more efficient if some amount of delay is acceptable.