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Data Communications TDC 362 / TDC 460. Circuit Switching and Packet Switching. 8.1 Circuit Switching. Space-Division Switch Time-Division Switch TDM Bus Combinations. Figure 8.1 Circuit-switched network. Figure 8.2 A circuit switch. Blocking or Non-blocking. Blocking

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Data communications tdc 362 tdc 460 l.jpg

Data CommunicationsTDC 362 / TDC 460

Circuit Switching and

Packet Switching

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8.1 Circuit Switching

Space-Division Switch

Time-Division Switch



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Figure 8.1Circuit-switched network

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Figure 8.2A circuit switch

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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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Figure 8.4Crossbar switch

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Figure 8.5Multistage switch

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Figure 8.6Switching path

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Figure 8.7Time-division multiplexing, without and with a Time-slot interchange

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Figure 8.8Time-slot interchange

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Figure 8.9TDM bus

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Figure 8.10TST (Time-space-time) switch

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Circuit-Switched Routing

  • 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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Alternate Routing

  • Possible routes between end offices predefined

  • Originating switch selects appropriate route

  • Routes listed in preference order

  • Different sets of routes may be used at different times

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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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Location of Signaling

  • Subscriber to network

    • Depends on subscriber device and switch

  • Within network

    • Management of subscriber calls and network

    • ore complex

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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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Signaling System Number 7

  • SS7

  • Most widely used common channel signaling scheme

  • Internationally standardized and general purpose

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  • SS7 network and protocol used for:

    • Basic call setup, management, tear down

    • Wireless services such as PCS, roaming, authentication

    • Toll free and toll (900) wireline services

    • Enhanced features such as call forwarding, caller ID, 3-way calling

    • Efficient and secure worldwide telecommunications

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  • SS7 messages are exchanged between central offices and specialized databases via signal transfer points (packet switches).

  • 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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SS7 Signaling Network Elements

  • Service switching point (SSP)

    • SSPs enable central offices to communicate with SS7 databases (the user entry point into SS7)

  • Signal transfer point (STP)

    • A signaling point (packet switch) capable of routing control messages

  • Service control point (SCP)

    • SCPs contain databases with call routing instructions

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SS7 Characteristics

  • SSPs are telephone switches that send signaling messages to other SSPs to setup, manage, and release voice circuits

  • An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g. a toll-free number)

  • Because the SS7 network is critical to call processing, SCPs and STPs are deployed in mated pair configurations in separate physical locations

  • Links between signaling points are also in pairs

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Packet Switching Principles

  • Circuit switching designed for voice

    • Resources dedicated to a particular call

    • Much of the time a data connection is idle

    • Data rate is fixed

      • Both ends must operate at the same rate

    • What if we don’t want a dedicated call, or the data rate is bursty? You want packet switching!

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Basic Operation

  • Data transmitted in small packets

    • Typically 1000 bytes

    • Longer messages split into series of packets

    • Each packet contains a portion of user data plus some control info (such as addressing info or packet type)

  • Packets are received, stored briefly (buffered) and passed on to the next node

    • Store and forward (only ATM does not do this)

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  • Line efficiency

    • Single node to node link can be shared by many packets over time

    • Packets queued and transmitted as fast as possible

  • Data rate conversion

    • Each station connects to the local node at its own speed

    • Nodes buffer data if required to equalize rates

  • Packets are accepted even when network is busy

    • Delivery may slow down

  • Priorities can be used

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Two Basic Forms of Packet Switching

  • Packets handled in two ways

    • Datagram

    • Virtual circuit

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  • Each packet treated independently

  • Packets can take any practical route

  • Packets may arrive out of order

  • Packets may get lost or delayed

  • Up to receiver to re-order packets and recover from missing packets

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Virtual Circuit

  • Preplanned route established before any packets sent

  • Call request and call accept packets establish connection (handshake)

  • Each packet contains a virtual circuit identifier instead of destination address

  • No routing decisions required for each packet

  • Clear request to drop circuit

  • Not a dedicated path

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Figure 18.2Virtual Circuit Identifier (VCI)

VCI is known only between two switches. (It is not a global


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Figure 18.4Switch and table

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Figure 18.5Source-to-destination data transfer

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S(witched)VC vs. P(ermanent)VC setup

A virtual circuit can be either switched or permanent.

If permanent, an outgoing VCI is given to the source,

and an incoming VCI is given to the destination.

The source always uses this VCI to send frames to

this particular destination.

The destination knows that the frame is coming from

that particular source if the frame carries the

corresponding incoming VCI.

If a duplex connection is needed, two virtual circuits

are established.

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S(witched)VC vs. P(ermanent)VC setup

A PVC has several drawbacks:

1. Always connected, so always paying

2. Connection is between two parties only. If

you need a connection to another point, you

need another PVC.

Don’t like these disadvantages? Use an SVC.

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Figure 18.6SVC setup request

1 - Setup frame sent from A to Switch I.

Note how the Outgoing VCI is not yet known.

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Figure 18.7SVC setup acknowledgment

As the acknowledgment frame goes back, the VCI number

is placed into the Outgoing VCI entry in each table.

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Virtual Circuits vs Datagram

  • Virtual circuits

    • Network can provide sequencing and error control

    • Packets are forwarded more quickly

      • No routing decisions to make

    • Less reliable

      • Loss of a node looses all circuits through that node

  • Datagram

    • No call setup phase

      • Better if few packets

    • More flexible

      • Routing can be used to avoid congested parts of the network

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  • Complex, crucial aspect of packet switched networks

  • Characteristics required

    • Correctness

    • Simplicity

    • Robustness

    • Stability

    • Fairness

    • Optimality

    • Efficiency

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Performance Criteria

  • Used for selection of route

  • Minimum hop

  • Least cost

    • Dijkstra’s algorithm most common

    • Finds the least cost path from one starting node to all other nodes

    • Algorithm can be repeated for each starting node

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Decision Time and Place

  • Time

    • Packet or virtual circuit basis

  • Place

    • Distributed

      • Made by each node

    • Centralized - dead

    • Source - dead

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Basic Routing Strategies

  • Adaptive versus Fixed (dead?)

  • Distributed versus Centralized (dead?)

  • Flooding

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Centralizedand DistributedRoutingTables

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  • No network info required

  • Packet sent by node to every neighbor

  • Incoming packets retransmitted on every link except incoming link

  • Eventually a number of copies will arrive at destination

  • Each packet is uniquely numbered so duplicates can be discarded

  • Nodes can remember packets already forwarded to keep network load in bounds

  • Can include a hop count in packets

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Flooding Example

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Properties of Flooding

  • All possible routes are tried

    • Very robust

  • At least one packet will have taken minimum hop count route

    • Can be used to set up virtual circuit

  • All nodes are visited

    • Useful to distribute information (e.g. routing)

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Adaptive Routing

  • Used by almost all packet switching networks

  • Routing decisions change as conditions on the network change

    • Failure

    • Congestion

  • Requires info about network

  • Decisions more complex

  • Tradeoff between quality of network info and overhead

  • Reacting too quickly can cause oscillation

  • Reacts too slow to be relevant

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Adaptive Routing - Advantages

  • Improved performance

  • Aid congestion control

  • Complex system

    • May not realize theoretical benefits

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Where does routing info come from?

  • Local (isolated)

    • Route to outgoing link with shortest queue

    • Can include bias for each destination

    • Rarely used - do not make use of easily available info

  • Adjacent (neighbor) nodes only

  • All nodes in network

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Chapter 21


Routing Overview:

Routing Protocols

(Details in TDC 365/463)

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Figure 21.1Unicasting

In unicast routing, the router forwards the received packet through only one of its ports.

Three basic unicast routing protocols: RIP, OSPF, BGP

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Figure 21.3Autonomous systems

R1, R2, R3 and R4 use an interior and exterior routing

protocol. The other routers use only an interior protocol.

RIP and OSPF are interior, BGP is exterior.

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RIP (Routing Information Protocol) is an interior routing

Protocol based on distance vector routing which uses the

Bellman-Ford algorithm.

Each router shares its routing knowledge with its neighbors,

every 30 seconds.

This shared information is used to update a router’s routing

table. An entry in the routing table consists of the destination

network address, the shortest distance to reach the

destination in hop count, and the next router to which the

packet should be delivered. (see next slide)

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Table 21.1 A distance vector routing table

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RIP Updating Algorithm

Receive: a response RIP message

1. Add one hop to the hop count for each advertised destination.

2. Repeat the following steps for each advertised destination:

1. If (destination not in the routing table)

1. Add the advertised information to the table.

2. Else

1. If (next-hop field is the same)

1. Replace entry in the table with the advertised one.

2. Else

1. If (advertised hop count smaller than one in the table)

1. Replace entry in the routing table.

3. Return.

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Figure 21.4Example of updating a routing table

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OSPF (Open Shortest Path First) protocol is another interior

routing protocol for autonomous systems.

Special routers called autonomous system boundary routers

are responsible for dissipating information about other

autonomous systems into the current system.

To handle routing efficiently and in a timely manner, OSPF

divides an autonomous system into areas.

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Figure 21.7Areas in an autonomous system

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In OSPF, each router sends the state of its neighborhood to

every other router in the area. It does this by flooding.

The state of its neighborhood is only shared when there is

new information. This generates much less traffic than does

distance vector routing (RIP).

OSPF keeps information on its links (the connection between

two routers). There are 4 types of links: point-to-point,

transient, stub, and virtual.

To share information about their neighbors, each entity

distributes link state advertisements (LSAs).

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There are 5 different types of LSAs: router link, network link,

summary link to network, summary link to AS boundary

router, and external link.

Every router in an area receives the router link LSAs and

network link LSAs from every other router and forms a

link state database.

Dijkstra’s least cost algorithm is applied to this link state

database to create the routing table. The routing table shows

the cost of reaching each network in the area.

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RIP and OSPF have shortcomings.

RIP (distance vector routing) is not always optimal because

The smallest hop count is not always the optimal route. Plus,

bad news moves slowly.

OSPF (link state routing) has the shortcoming of a possibly

huge routing table. To use link state routing for the whole

internet would require each router to have a huge database.

What about BGP (Border Gateway Protocol)? It is an inter-

autonomous system routing protocol and is based on a routing

method called path vector routing.