3331 Ch.10 Circuit Switching And Packet Switching

8/7/2019 3331 Ch.10 Circuit Switching And Packet Switching

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Ch. 10 Circuit Switching and

Packet Switching


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10.1 Switched Communication Networks

Fig. 10.1 Simple switching network.

End stations are attached to the "cloud".

Inside the cloud are communication network nodes

interconnected with transmission lines. The transmission lines often use multiplexing.

The network is generally not fully connected, but

alternate paths exist.

Two technologies for WANs

Circuit Switching

Packet Switching


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10.2 Circuit-Switching Networks

The three phases of a circuit switched

connection are

Circuit establishment

Data transfer

Circuit disconnect


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10.2 Circuit-Switching Networks (p.2)

Four generic architectural components of thepublic telecommunications network:

Subscribers

Subscriber line (or local loop) Exchanges

Trunks

Fig. 10.2 illustrates the public switchedtelephone network (PSTN).

Fig. 10.3 illustrates two possible connections

over the PSTN.


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10.3 Circuit-Switching Concepts

Fig.10.4 Elements of a Circuit-Switch NodeDigital Switch

Provides a transparent signal path between any pair of

attached devices.

Control Unit Establishes connections.

Maintains connections.

Tears down connections.

Network Interface

Functions and hardware needed to connect digital and

analog terminals and trunk lines.


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10.3 Circuit-Switching Concepts (p.2)

Blocking vs. Nonblocking

Relates to the capability of making connections.

A blocking network is one in which blocking ispossible.

A nonblocking network permits all stations to

be connected (in pairs) as long as the stations

are not in use.


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Space-Division Switching Defn: A circuit-switching technique in which

each connection through the switch takes a

physically separate and dedicated path.

Basic building block--a metallic crosspoint or

semiconductor gate.

"Crossbar" Matrix (Fig. 10.5)

Multi-stage space-division switches reduces thetotal number of crosspoints required, but increases

complexity and introduces the possibility of

blocking.(Fig. 10.6)


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T

ime-D

ivision Switching Defn: A circuit-switching technique in which time

slots in a time-multiplexed stream of data are

manipulated to pass data from an input to an output.

All modern circuit switches use digital time divisiontechniques or some combination of space division

switching and time division switching.


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10.4 Control Signaling

Signaling Functions Audible communications with subscriber (dial tone,

busy signals, etc.)

Transmission of number dialed to switches to

attempt a connection. Transmission of information between switches

indicating that a call can or cannot be completed.

Transmission of information between switches that a

call has ended.


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10.4 Control Signaling (p.2)

Signaling Functions (cont.) A signal to make the phone ring.

Transmission of information for billing.

Transmission of information giving status of

equipment or lines.

Transmission of information used in diagnosing and

isolating system failures.

Control of special equipment such as satellitechannel equipment.


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Grouping of Control Signals

Supervisory--binary character (on/off) signals that

are related to control functions such as request for

service, answer, alerting, idle.Address--signals that identify a subscriber.

Call information--audible tones that provide

information about the status of a call.

Network management--signals that are used formaintenance, trouble shooting, and operation of the

network.


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Location of Signaling User to network

Within the network (computer to computer)

Common Channel Signaling Inchannel Signaling: Inband and Out-of-Band--

Table 10.1

Fig. 10.7 Inchannel and Common Channel

Signaling

Fig.10.8 Common Channel Signaling Modes.


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Signaling System Number 7 Designed to support command channel

signaling for ISDN.

Control messages are routed through the

network to perform call management andnetwork management.

Each message is a short block (or packet) and it

is transported over a packet switched network

to control the circuit switch network.


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Signaling System Number 7 (cont.)

Signaling Network Elements

Signaling point (SP)--any point in the signaling

network capable of handling SS7 control messages.

Signal transfer point (STP)--signaling point capableof routing control message.

Signaling link--data link that connectws signaling

points.

Figure 10.9 illustrates the Control plane and the

Information plane.


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10.5 Softswitch Architecture

Specialized software is run on a computer thatturns it into a smart phone switch (Fig.10.10).

Performs traditional circuit-switching functions.

Can convert a stream of digitized voice into packets

(VoIP).

Media Gateway (MG) performs the physical

switching function.

Media Gateway Controller (MGC) performscall processing.

RFC 3015--communications between the two.


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10.6 Packet-Switching Principles Definition: A method of transmitting

messages through a communication

network, in which long messages are

subdivided into short packets. The packets

are then sent through the network to the

destination node. (See Fig. 10-11)


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10.6 Packet-Switching Principles (p.2)

Two Techniques

Datagram (Fig. 10.12)

Each packet contains addressing

information and is routed separately.

Virtual Circuits (Fig. 10.13)

A logical connection is established

before any packets are sent; packetsfollow the same route.


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Packet Size Each packet has overhead.

With a larger packet size

Fewer packets are required (less overhead.) But longer queuing delays exist at each packet

switch.

Figure 10.14 illustrates this issue.


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Delay in Switching Networks Setup Time--connection oriented networks.

Transmission Time

Propagation Delay Nodal Delay--processing time at nodes.

Fig. 10.15 and Table 10.2 compare the

performance of circuit switching, datagrampacket switching, and virtual-circuit packet

switching.


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Delay in Circuit Switched Networks Call setup time.

Message transmission time--occurs once at the

source.

Propagation delay--sum of all links.

Very little node delay.


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Delay in Packet Switching Connection Setup Time

Required for virtual circuit.

None for datagram.

Packet transmission time and propagation

delay occurs on each link.

Processing delay occurs at every node.

Datagram networks may require more than virtualcircuit networks.


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Problem 10.4

Consider the delay across a network. Let B= data rate on every link.

Let N= the number of links.

Let L= the length of the source message.

Let D= the average delay on a link.

Let S= setup time (when required.)

Let P= packet size for packet switched

networks--fixed length packets.

Let H=the number of bits of overhead in each

packet header, for packet switched networks.


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Problem 10.4 (p.2)

Circuit SwitchingDelay Let t0 be the time that the first bit is transmitted at

the source node and t1 be the time that the last bit

is received at the destination node.

Then let T= t1-t0 be the "end-to-end" delay.

Follow the last bit across the network.

No network layer overhead and little nodal delay.

Ignore any data link protocol delay (U=1).

T = S + L/B + N x D


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Problem 10.4 (p.3)

D

atagramP

acket SwitchD

elay Let NoPa= Number of Packets= L/(P-H)

rounded up (ceiling).

Assume no link level related overhead (U=1.)

The last packet waits at the source and then is

transmitted over every link in a store and

forward fashion.

T= (NoP

a-1)P

/B

+ N(P

/B

+D

) Virtual-Circuit Packet Switch Delay

T= S + (NoPa-1)P/B + N(P/B + D)


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10.7 X.25

First approved in 1976 and revised in 1980,1984, 1988, 1992, and 1993.

Specifies an interface between a host system and

a packet-switched networks. Almost universally used and is employed for

packet-switching in ISDN.

Fig. 10.16 illustrates the concept of virtual

circuits over an X.25 network.


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10.7 X.25 (p.2)

Three Layers are defined--Fig. 10.17.

X.21 is the physical layer interface (often

EIA-232 is substituted)LAP-B is the link-level logical interface-

-it is a subset of HDLC.

Layer 3 has a multi-channel interface--sequence numbers are used to

acknowledge packets on each virtual

circuit.


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10.8 Frame Relay

Traditional packet switching has the X.25protocols

Call control packets are carried on the same

channel and the same virtual circuit as datapackets.

Multiplexing of virtual circuits takes place at

layer 3.

Both layer 2 and layer 3 include flow-controland error-control mechanisms.

Considerable overhead is required.


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10.8 Frame Relay (p.2)

Frame Relay Call control signaling is carried on a separate

logical connection; intermediate nodes have less

processing required.

Multiplexing and switching of logical

connections take place at layer 2 instead of layer

3 (eliminating a layer of processing).

No hop-by-hop flow control and error control--(performed at a higher layer if at all).

Less overhead required.


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Frame Relay Protocol Architecture

Fig. 10.18 depicts the protocol architecture.

C-plane protocols are for access control

between the subscriber and the network. U-plane protocols provide end-to-end (user)

functionality.


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Fig. 10.19 --LAPF-Core Formats

Similar to LAPD and LAPB except there is

no control field.

Only one frame type (for user data).

It is not possible to use in-band signaling.

It is not possible to perform flow control and

error control (no sequence numbers). Address Field--data link connection identifier

(DLCI) is similar to virtual circuit numbers in

X.25.

3331 Ch.10 Circuit Switching And Packet Switching

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