28071480 circuit switching and packet switching

8/3/2019 28071480 Circuit Switching and Packet Switching

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CSE 413: Computer Network

Circuit Switching and Packet SwitchingNetworks

Md. Kamrul Hasan

09-03-2010


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The network core:

mesh of interconnected

routers

the fundamental

question: how is datatransferred through net?

circuit switching:

dedicated circuit per call:

telephone net packet-switching: data

sent through net in

discrete chunks

(packets) on shared

media


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The network core:

Circuit SwitchingEnd-to-end

resources reserved

for call link bandwidth, switchcapacity

dedicated resources: no

sharing

circuit-like (guaranteed)

performance

call setup required


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Circuit Switching

DATA

Caller Callee

Boston

SwitchLA

Switch

propagation

delay

between

caller

and Boston

switch

processing delay at switch

Its the method used bythe telephone network

A call has threephases:

1. Establish circuitfrom end-to-end(dialing),

2. Communicate,

3. Close circuit (teardown).

If circuit not available:busy signal

(1)

(2)

(3)


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Switch

Circuit Switching:

Multiplexing/Demultiplexing

Time divided into frames and frames divided into slots

Relative slot position inside a frame determines whichconversation the data belongs to

E.g., slot 0 belongs to the red conversation

Need synchronization between sender and receiver

Frames

0 1 2 3 4 5 0 1 2 3 4 5Slots =

One way for sharing a circuit is TDM:

Lecture notes use the word frame for slot


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The network core:

Circuit Switchingnetwork resources (e.g., bandwidth) divided

into pieces

pieces allocated to calls resource piece idle if not used by owning call

(no sharing)

Consumers are charged on a per-minute basis

2 ways of dividing the link bandwidth intopieces

frequency division multiplexing (FDM)

time division multiplexing (TDM)


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Circuit Switching: FDM and TDM

Frequency Division Multiplexing (FDM)

frequency

time

Time Division Multiplexing. (TDM)

frequency

time

4 users

Example:


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Numerical example

How long does it take to send a file of640,000 bits from host A to host B over a

circuit-switched network?

The links transmission rate = 1.536 Mbps Each link uses TDM with 24 slots/sec

500 msec to establish end-to-end circuit

Figure it out

Solution:

Bandwidth of circuit = 1.536/24 = 64 kbps

Time to send: 640 kbits/64 kbps + 0.5s = 10.5s

What would be different if we use FDM instead ofTDM?


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Common mistake/confusion :Question: A) Express transmission rate of 1Kbits/sec in bits/sec

B) Express the file size of 1KBytes in bits

Answer: A) 1000 bits/sec (in throughput, K = 103=1000)

B) 1024 Bytes = 8192 bits (in data size, K = 2

10

=1024)

Electronic speeds/times: K = 103, M = 106, G = 109

Computer file/memory sizes: K = 210 , M = 220, G = 230

Common computer notation:

b(bits) Kb, Mb, Gb B(Bytes) KB, MB, GB

Better computer notation: b(bits) Kib, Mib, Gib

B(Bytes) KiB, MiB, GiB


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Packet Switching Used in the Internet

Data is sent in Packets(header contains controlinfo, e.g., source anddestination addresses)

Per-packet routing

At each node the entirepacket is received, stored,and then forwarded (store-and-forwardnetworks)

No capacity is allocated

Header Data

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

processing

delay of

Packet 1

at Node 2

propagation

delay

between

Host 1 &

Node 2transmission

time of

Packet 1

at Host 1

Host 1 Host 2

Node 1 Node 2


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Router

Packet Switching:

Multiplexing/Demultiplexing

Multiplex using a queue

Routers need memory/buffer

Demultiplex using information in packet header Header has destination

Router has a routing table that contains informationabout which link to use to reach a destination

Queue


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Packet switching also show reordering

Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Packets in a flow may not follow the same path (depends

on routing as we will see later) packets may bereordered


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The network core:

Packet Switching all streams sharenetwork resources

each packet uses full

link bandwidth resources used as

needed

Resource contention:

aggregate resourcedemand can exceed

amount available

congestion: packets

queue, wait for linkBandwidth division into

pieces

Dedicated allocation

Resource reservation


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The network core:

Packet switching Data transmitted in small, independent

pieces

Source divides outgoing messages intopackets

Destination recovers original data

Each packet travels independently

Includes enough information for delivery

May follow different paths

Can be retransmitted if lost


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The network core:

Functions of packet-switching

networks

Packet construction

encode/package data at source

Packet transmission

send packet from source to destination

Packet interpretation unpack/decode data from packet at destination

acknowledge receipt


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statistical multiplexing Sequence of A & B packets does not havefixed pattern; shared on demand.

Compare: in TDM, each host gets same slot (periodically)

in FDM, each host gets same bandwidth (continuously)

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

The network core:

statistical multiplexing


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Differences Between Circuit & Packet

SwitchingCircuit-switching Packet-Switching

Guaranteed capacity No guarantees (best

effort)

Capacity is wasted if

data is bursty

More efficient

Before sending data

establishes a path

Send data immediately

All data in a single flow

follow one path

Different packets might

follow different paths

No reordering; constant

delay; no pkt drops

Packets may be

reordered, delayed, ordro ed


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End-to-end delay (nodal delay) :

Total time from initiating send (from source) to

completed receive (at destination)

Throughput : Rate (bits/sec) at which bits are actually being

transferred between sender/receiver

instantaneous: rate at given point in time

average: rate over longer period of time

Network performance metrics


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Four sources of packet delay

1. nodal processing:

check bit errors

determine output link

A

B

propagation

transmission

nodalprocessing queueing

2. queueing delay

time waiting at output

link for transmission

depends oncongestion level of

router


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3. Transmission delay:

R=link bandwidth (speed

in bits per second, i.e.

bps)

L=packet length (in bits)

transmission delay = L/R

4. Propagation delay: d = length of physical link (in

meters)

s = propagation speed in

medium (~2.5 x 108 m/sec)

propagation delay = d/s

Note: R and s are verydifferent quantities!

Four sources of packet delay

A

B

propagation

transmission

nodal

processing queueing

28071480 circuit switching and packet switching

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