communication networks -...

Communication Networks

Chapter 6 – Switching Technology

Communication Networks - 6. Switching Technologies 201

Overview

1. Switching in General

2. Circuit / Line Switching

a) Space Division Multiplex

b) Time Division Multiplex

3. Store & Forward

a) Message Switching

b) Packet Switching

i. Virtual Circuit / Virtual Connection

ii. Datagram


Example of a Telecommunication Network (Repetition)



LAN 1

Wireless LAN 3

MobileUsers

LAN 2

Backbone

URL Web Page


Task of Switching

• Extension of point-to-point links to multi-hop communication• Provision of communication paths through the network between two end devices• Worldwide reachability different network providers involved• Probably different priorities, e.g. emergency calls• Adaptivity to changing characteristics on the network: occasional failures varying load situations mobile devices

• Globally unique addresses required


Switching and Signaling

Switching: Provides a temporary communication

path through the communication network on demand

Involves both end systems and a chain of switching systems

Can be realized both connection-oriented and connectionless

Signaling: Comprises information for switching

for connection setup

for management purpose

Starts at the requesting end systems, traverses all involved switching systems and ends at the called end system

Is (usually) implemented in a separate protocol stack




Connection-oriented and Connectionless Communication

• Connection-oriented Communication Connection has to be established before data can be transferred End node address only required during connection establishment, later on only the

connection needs to be identified High complexity – especially for the intermediate nodes connection context Quality of service can be guaranteed by reserving resources in the network

• Connectionless Communication No context in the switches / routers No guarantee for quality of service Low complexity (“Shoot and pray”)



Routing

• Relevant for partly meshed networks Probably several paths to the destination Different metrics for the paths Applications and/or users might have different requirements

• Goal: Finding the optimal path to the given destination Proactive or reactive routing Both for connection-oriented and connectionless communication Mainly done in the network layer Basis: given metrics


2. Circuit Switching

Circuit Switching / Line Switching

• Originally developed for public telephone networks• Connection is established over an end-to-end physical channel between end

users• Physical channel dedicated to a single connection• Physical channel is on physical layer only and does not include any buffers• Delay is only depending on the propagation delay of the physical signals and

hence on the length of the channel itself • Delivery is ordered• Number of parallel connections limited by number of channels


Circuit Switching Scheme



Subscriber Line Long Distance Line Subscriber Line

LocalExchange

Long-haul ExchangeEnd Systems

LocalExchange

PSTN Office Classification Hierarchy Source:

http://en.wikipedia.org/wiki/Image:PSTN_office_classification_hierarchy.svg




The Beginning of Circuit Switching

• Caller turned the crank.• In the central office, a flip fell down.• The operator asked who the caller

wanted to talk to.• She plugged the according lines.• When the talk was over, the

operator had to disconnect the phones again.



Evolution of Circuit Switching

• At the beginning, the network was created using analog voice connections through manual switchboards.

• As a next step, automated telephone exchanges replaced most switchboards.• Later digital switch technologies were used. • Most switches now use digital circuits between exchanges, with analog two-wire

circuits still used to connect to most telephones.


2.a Circuit Switching based on Space Division Multiple Access

Circuit Switching based on SDMA

• Crosspoints on crossbars are used for connection establishment

• Example: m incoming lines n outgoing lines Here, incoming line 1

is connected tooutgoing line 2


1

2

...

m

1 2 ... n

Incoming Lines

Outgoing lines

2.b Circuit Switching based on Time Division Multiple Access

Circuit Switching based on TDMA

• Incoming line is connected with the outgoing line only for a short time (time slot).

• A scheduler is responsible for setting the corresponding crosspoint.


12...m

Outgoing 1 2 3 ... nLines

SchedulerIncoming

Lines

73...5

Time-Division Multiplexing (TDM) I



Buffer 1

A1

Buffer 2

A2

Buffer 3

A3

Buffer 4

A4

A2 A4 A1 A3A4 A3 A2 A1

3142

Scheduler

tE4 tE3 tE2 tE1

125µs-Period

tA4 tA3 tA2 tA1

125µs-Period

tE1

tE2

tE3

tE4

tA2

tA4

tA1

tA3

Control addresses( Buffer ID)

IncomingMultiplexedLine

OutgoingMultiplexed

Line

Buffersfor each Slot

CyclicInput Random

output

Time-Division Multiplexing (TDM) II



A B

S1

S2

S3

S4

S1S2

S3 S4

S1

S4

S2

S3

Frame

S1

S2

S3

S4

Time Slott4 t3 t2 t1 t4 t3 t2 t1

t4 t3 t2 t1

Multiplexing Demultiplexing

3. Store and Forward

Store and Forward

• Firstly used in telegram switching.• Switching systems equipped with buffer.• No engaged lines, but data might have to wait or might get lost if there is no

buffer available.• Incoming data is stored in the buffer until the desired outgoing line is free.• Since several data portions from different sources might be waiting for

transmission, they can be re-ordered (e. g. according to priority).• For two data portions sent after each other, there is no fix timing.• Sender and receiver do not have to work at the same speed.


Switching Network



LL = Local LoopIL = Internal Data Link

A

B

C

E

D

SwitchingSystem

End System(User)

LLA

LLB

LLC

LLD

LLE

End System(User)

IL1

IL2

IL3


Data Link

• Communication from buffer to buffer• Data link consists of

Sender buffer Physical link Receiver buffer

• Communication is finished, when the data frame has been completely received, checked against errors and stored in the receiver buffer


Sender withdata frames

in buffer

Receiver withbuffer for

incoming framesNon-buffering

physical channel

Data Link

Model of a Switching Network



A

B

E

D

C

TerminalEquipment

TerminalEquipment

TerminalEquipment

Store and Forward: Advantages and Disadvantages

Advantages• Single link shared by multiple senders and

receivers• No occupied links• Different data rates at sender and receiver

possible• Establishment of packet-priority systems• Charging by the volume of data (number of

packets) transmitted rather than connection time

Disadvantages• Variable, unforeseeable transmission delays

caused by packet processing and packet queues at packet switches

• Variable packet sizes leading to longer packet processing times at packet switches

• Overhead data in packets leading to lower data transmission efficiency and throughput than in circuit-switched networks



3.a Message Switching

Message Switching

• Definition “Message”:

a self-contained object of communication variable length depending of the contained information message might be too long to be transferred in one data frame only



Segmenting Messages

• Partitioning the message (layer 3) into several data frames (layer 2)• According to the layer 2 entity, data frames might have a minimum length, which

has to be achieved by padding


Message

Message BodyMessage Header

Data Frame1

Data Frame2

Data Frame3

Data Frames

DF4 Padding

Layer 3

Layer 2


Segmenting over Several Data Links

• According to the characteristics of the layer 2 entities, data frames may have different sizes on different data links:


1, 2, 3, 4: Data frames on the according data link

1

Entity of Layer 3Entity of Layer 3

Data Link 1Data Link 2221 3 4

MessageMessageLayer 3

Layer 2


Segmenting and Reassembling

• Segmenting and reassembling the layer 3 message to consider the maximum frame size of different data links


Segmenting

Reassembling

Entity of Layer 3

PCI = Protocol Control Information of the according layerSDU = Service Data Unit of the according layer

Entity of Layer 3

Data Link 1Data Link 2 Segmenting

PCI SDU

SDUPCISDUPCI

PCI SDUPCI SDUPCI SDUPCI SDU

Layer 3

Layer 2


Message Switching Principle

• The message hast to be completely received and reassembled, before it can be transmitted over the next data link.

• A message consists of message header

source and destination addressmessage typemessage lengthpriority timing information (e.g. sending time)

message bodyuser information to be transferred


3.b Packet Switching

Packet Switching

• Layer 3 deals with packets that have a maximum length might have a fixed length

• Packets are quite short layer 3 packet completely fits into a layer 2 frame

• Each packet contains control information in a header especially for packet forwarding user data, possibly part of a larger message

• Packets are received, buffered (until the outgoing link is free to send) and forwarded to the next node



Packet Forwarding

• Packets are forwarded independently of each other

• No reassembling needed in the forwarding nodes Most important difference to message

switching• Temporal overlap of the packets

results in shorter throughput times compared to message switching


S R1 R2 D

Data Da

ta 1

Data

2

1 1HDa

taH

Data

H

HH

Data

1Da

ta 2

HH

Data

1Da

ta 2

HH

Tim

e

Link 1 Link 2 Link 3

H2

H3

H4

H5

H

1H

2H

3H

4H

5H

1H

2H

3H

4H

5H

H2H

3H4H

5H6H

7H8H

9HAH

1H2H

3H4H

5H6H

7H8H

9HAH

1H2H

3H4H

5H6H

7H8H

9HAH

According to Stallings (2014)


Packet Switching Alternatives

• Connection-oriented:Virtual Connection / Virtual CircuitPermanent Virtual Circuit (PVC)Switched Virtual Circuit (SVC)

• Connectionless:Datagram Switching


3.b.i Virtual Connection

Characteristics of Virtual Connections

• Bidirectional fixed transmission path (full duplex)• Virtual Connection established between two connection end points in the data terminating

equipment of sender and receiver routing decision only once at connection setup packets carry connection identifiers instead of full addresses

• Virtual Connection can comprise several switches• Permanent Virtual Connection/Circuit

established for a longer time comparable to a leased line

• Switched Virtual Connection/Circuit established on demand establishment must precede data exchange phase comparable to Circuit Switching


3.b.i Virtual Connection

Establishment of a Virtual Connection

• During establishment phase, a path is defined for all packets belonging to the virtual circuit• Thus, all packets are forwarded over the same path• Switches need to store information about all active virtual circuits


AE

SwitchingSystem

End System(User) End System

(User)

3.b.ii Datagram Switching

Datagram Switching

• Datagrams are packets that contain complete addresses source address destination address

• No connection establishment (and release) required• Datagram switches do not need to store information about ongoing communication associations• Routing decision is done separately for each datagram • Advantages

Information transfer is quicker (no connection establishment!) Network resources can be better utilized

• Disadvantages Quality of service cannot be guaranteed Reordering of datagrams is possible


3.b.ii Datagram Switching

Process of Datagram Switching

• Each datagram may have its own path• No reservation of resources possible


AE

SwitchingSystem

End System(User) End System

(User)

Overview of the Different Switching Technologies


4. Conclusion

Switching Technology

Circuit Switching Store and Forward

SpaceMultiplex

TimeMultiplex

MessageSwitching

PacketSwitching

VirtualCircuit

Data-gram

permanent switched

References

References

• Flood, John Edward (2001): Telecommunications Switching, Traffic and Networks. New York: Prentice Hall (A Pearson Education print on demand edition).

• Seitz, Jochen; Debes, Maik (2016): Kommunikationsnetze. Eine umfassende Einführung. Anwendungen – Dienste – Protokolle. Ilmenau: Unicopy Campus Edition.

• Stallings, William (2014): Data and Computer Communications. 10th edition. Upper Saddle River, N.J.: Pearson.

• Tanenbaum, Andrew S.; Wetherall, David J. (2011): Computer Networks. 5th edition. Boston: Pearson Prentice Hall.


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