chap6 switching

Switching

G.K. PakleAdopted From: Behrouz Forouzan

Department of Information Technology

SGGS Institute of Engineering & Technology, Naned.

G.K. Pakle Adopted From: Behrouz Forouzan (Department of Information Technology )SwitchingSGGS Institute of Engineering & Technology, Naned. 1

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Outline

1 Switching

2 Circuit Switched NetworksIntroductionExamplesPhasesPerformance

3 Datagram NetworksIntroductionProperties

4 Virtual Circuit NetworksIntroductionThree Phases

5 Structure of Packet Switches


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Switching

Switched Network

Series of interlinked nodes is called switches.

Switches create temporary connections.

Some switches are connected to end systems others only do routing.


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Switching

Switching: Methods

Packet switching combines datagram and virtual circuit networks.


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Circuit Switched Networks Introduction

Outline

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Circuit Switched Networks

Consists of set of switches connected by physical links.

Connection between two stations is a dedicated path of one or morelinks.

Each link is divided into n channels by using FDM/TDM.

End systems directly connected to switch..


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Circuit Switched Networks

Setup Phase: requesting communication.

Channels are reserved, dedicated path is created then data transfer

takes place.

Properties of Circuit Switching:

1 Takes place in physical layer.

2 Resources of channels (time, bu�ers, ports) are dedicated during datatransfer until teardown phase.

3 Data �ow is continuous and not in packets.

4 No addressing involved in data transfer. Data transfer takes place onoccupied band/ time slot.

In circuit switching, the resources need to be reserved during the setupphase; the resources remain dedicated for the entire duration of datatransfer until the teardown phase.


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Circuit Switched Networks Examples

Outline

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Example 1: Circuit Switching

Let us use a circuit-switched network to connect eight telephones in a smallarea. Communication is through 4-kHz voice channels. We assume thateach link uses FDM to connect a maximum of two voice channels. Thebandwidth of each link is then 8 kHz. Figure shows the situation.Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Ofcourse the situation may change when new connections are made. Theswitch controls the connections.


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As another example, consider a circuit-switched network that connectscomputers in two remote o�ces of a private company. The o�ces areconnected using a T-1 line leased from a communication service provider.There are two 4 Ö 8 (4 inputs and 8 outputs) switches in this network. Foreach switch, four output ports are folded into the input ports to allowcommunication between computers in the same o�ce. Four other outputports allow communication between the two o�ces. Figure shows thesituation.


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Circuit Switched Networks Phases

Outline

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Circuit Switched Networks Phases

Circuit Switching: Phases

Setup Phase:

Setup means creating dedicated channels between the switches.

To make a connection, an acknowledgement from receiver needs to besent in opposite direction to sender.

Only after receiving acknowledgement connection is established.

End to end addressing is required to create the connection.

Data Transfer Phase:

After establishment of dedicated circuit, two parties can transfer data.

Teardown Phase:

When one to the parties needs to disconnect, a signal is sent to eachswitch to release the resources.


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Circuit Switched Networks Performance

Outline

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Circuit Switched Networks Performance

Circuit Switching

E�ciency:

Not e�cient as packet switching because resources needs to beallocated for entire duration of connection.When circuit switching used for computers, resources are wasted if nodata transfer.

Delay:

Delay is minimal.Total delay is due to create connection, transfer data anddisconnection of circuit.


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Datagram Networks Introduction

Outline

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Datagram Networks

To send messages between systems, needs to be divided into packetsof variable sizes.

Packet is determined by network and governing protocol.

Resources are allocated on demand.

Lack of resource reservation creates delay at switches. (FIFO)

Each packet treated independently of others. Packets are calleddatagrams.

Switching is done at Network Layer.

Switches in datagram are referred as Routers.


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Datagram Networks


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Datagram Networks Properties

Outline

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Routing Table

Each switch maintains a routing table based on destination address.

Routing tables are dynamic and updated periodically.


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Other Properties

E�ciency:Better than circuit switched networks.

Delay:

Greater delay.

Packets experience wait at each switch before forwarding.

Delay is not uniform.


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Virtual Circuit Networks Introduction

Outline

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Introduction

1 Contains setup, data transfer and teardown phases.

2 Resources are allocated in set up phase or on demand.

3 Data is packetized and each packet carries an address in header.Address has local juridiction only.

4 All packets follow the same path as in circuit switching.

5 Implemented in data link layer.


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Addressing

Global Addressing:

Source/Destination needs to have global addresses.

Global addresses are used to create virtual circuit identi�ers.

Virtual Circuit Identi�ers (VCI):

VCI is actually used for data transfer.

VCI has only switch scope. Used by frame between two switches.


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Virtual Circuit Networks Three Phases

Outline

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Data Transfer Phase

All switches have table entry for virtual circuit.

Table has four columns.

Each switch changes VCI and routes the frame.

Process creates a virtual circuit not a real circuit between source anddestination.


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Setup Phase

Setup Request:

1 Source sends setup frame to switch 1.

2 Switch 1 creates entry in table. Only three columns are �lled.

3 Same happens for switch 2 and 3.

4 Destination assigns VCI to incoming frame to let know that it hascome from source.


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Setup Phase

Acknowledgement: Acknowledgement frame completes the entries inswitching tables.

1 Destination sends acknowledgement to switch 3. ACK carries globalsource and destination addresses.

2 Switch 3 updates incoming VCI as outgoing VCI.

3 Switch 2 and 1 updates accordingly.


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Teardown Phase

Teardown Phase

Source after sending all frames to destination, sends a special frame calledteardown request. Destination responds with teardown con�rmation frame.All switches deletes corresponding entries from their tables.


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Structure of Packet Switches


Components:

Input Ports

Output Ports

Routing Processor

Switching Fabric


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Input Ports

Performs physical and data link functions of packet switch.

Signals converted to bits.

Errors are detected and corrected.

I/P Port has bu�ers to hold packets.


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Output Ports:

Same functions as input port but in reverse order.

Routing Processor:

Performs the functions of network layer.

Destination address used to �nd the next hop and port number.(Tablelookup)

Switching Fabric:

Switching fabric moves the packet from input queue to output queue.

Three types: Crossbar, banyan, batcher banyan


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Banyan Switch

Multistage switch with microswitches at each stage.Output port represented as binary string.For n inputs and n outputs we have log2 n stages with n/2microswitches at each stage.


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chap6 switching

Documents

circuit switched

virtual circuit

data transfer

virtual circuit

datagram networks

switched network

circuit switching

teardown phase