Computer Networks

Chapter 4: Transmission systems and the telephone network

B B

C C

A A

B

C

A

B

C

A

MUXMUX

(a) (b)

Trunkgroup

Multiplexing

A CBf

Cf

Bf

Af

W

W

W

0

0

0

(a) Individual signals occupy W Hz (b) Combined signal fits into channel bandwidth

FDM

● Original multiplexing systems used it for voice comm.●Group=12*4kHz (60-108kHz), Supergroup=5 groups●Mastergroups=10 supergroups●Currently used in radio and TV comms: ●AM channels(10kHz), FM (200kHz), TV(6MHz)

(b) Combined signal transmits 1 unit every T seconds

tA1 A2

tB1 B2

tC1 C2

3T0T 6T

3T0T 6T

3T0T 6T

tB1 C1 A2 C2B2A1

0T 1T 2T 3T 4T 5T 6T

(a) Each signal transmits 1 signal every 3T

TDM

●Obviously the outgoing line has n times the rate of an incoming line●Practically the outgoing rate is slightly higher: make up sync. defficiencies ●Most existing multiplexing hierarchies based on TDM●Ex: T1= (1+24*8)bits/frame*8000frames/s=1.544Mbps

PrimaryMultiplexEg. DigitalSwitch30 chan PCM

4th orderMultiplex

x4

2nd orderMultiplex

x4

3rd orderMultiplex

x4

34.368 Mbps2.048 Mbps 8.448 Mbps

139.264 Mbps

CEPT 1CEPT 4

North American Digital Hierarchy

European Digital Hierarchy

28

M13Multiplex

M23Multiplex

x7

PrimaryMultiplexEg. DigitalSwitch24 chan PCM

M12Multiplex

x4

1

DS3 44.736 Mbps

DS1 1.544 Mbps DS2 6.312 Mbps DS3 44.736 Mbps

Existing digital multiplexing hierarchies

Low-SpeedMappingFunction

MediumSpeed

MappingFunction

High-Speed

MappingFunction

DS3

44.736

DS1

DS2

CEPT-1

CEPT-4

139.264

ATM

150 Mbps

STS-1

STS-1

STS-1STS-1STS-1

STS-1STS-1STS-1

STS-3c

STS-3c

OC-n

Scrambler E/O

51.84 Mbps

High-Speed

MappingFunction

Mux

STS-n

SONET multiplexing

●Interconnection of cities/networks

MUX DEMUX MUX DEMUX

MUX DEMUX

(a) pre-SONET multiplexing

removetributary

inserttributary

ADM

removetributary

inserttributary

(b) SONET Add-Drop multiplexing

Add/drop in multiplexing

a

b c

logical fully-connected net

(a)

a

b

c

3 ADMs connected inphysical ring topology

OC-3nOC-3n

OC-3n

(b)

Ring topology in SONET

Inter-OfficeRings

MetroRing

RegionalRing

Ring structures: Local Metropolitan, regional networks

B BB 87B

InformationPayload 9 Rows

125 sTransportoverhead

90 bytes

SectionOverhead 3 rows

6 rowsLineOverhead

Sonet ST-1 frame structure

● Data bit rate 8x9x90x8000=51.84Mbps● Information bit rate 8x9x87x8000=50.122Mbps● Efficiency =96.7%

1

2

m

OpticalMUX

1

2

m

OpticaldeMUX

1 2.m

Opticalfiber

Wavelength-division multiplexing: The present and future

(a) WDM chain network

a b c d

(b) WDM ring networka

b

c

3 ADMs

User 1

SwitchLink

User n

User n-1

(a) Network (b) Switch

Control

123

N

12

3

N

Connectionof inputs to outputs

Circuit switching

● Switchs: non-blocking or blocking, complexity is limiting factor

N

1 2

1

N

2

N-1

nxk

nxk

nxk

nxk

N/n x N/n

N/n x N/n

N/n x N/n

kxn1

2

N/n

Ninputs

1

2

3 3

N/n

Noutputs

1

2

k

2(N/n)nk + k (N/n)2 crosspoints

kxn

kxn

kxn

Space-division and multistage switch

Crossbar:Non-blocking

Multistage:If k<n can beblocking

12

24

12

24

FromTDM

DeMUX

ToTDMMUX

24 23 12

2 241 23

Read slots inpermuted order

nxk

nxk

nxk

nxk

N/n x N/n kxn1

2

N/n

Ninputs

1

3

1

12

n

input TDM frame with n slots

output TDM frame with k slots

nxk

nxk

nxk

nxk

N/n x N/nTime-Shared

SpaceSwitch

kxn1

2

N/n

Ninputs

1

2

3 3

N/n

Noutputs

TDMn slots

n slots

n slots

n slots

kxn

kxn

kxn

TDMk slots

TDMk slots

TSI Stage TSI StageSpace Stage

●We will skip the study of the telephone system, and its signaling schemes

N(t)

t

1

2

3

4

5

6

7

all trunks busy

trun

k #

0.0001

0.001

0.01

0.1

11 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

# trunks

Blo

ckin

g Pr

obab

ilit

y

1 2 3 4 5 6 7

8

9

10

offered load

Tandem Switch 2

Tandem Switch 1

B CA

(b) Trunkgroup

10 Erlangs between each pair90 Erlangs

E

F

D

B

C

A

(a)

E FD

Routing control

1%blocking prob., 10Erl=>18trunks18*9=162trunks 106 trunks

High Usage Route B-E

Tandem Switch 1

Alternate Routes for B-E, C-F

High Usage Route C-F

Switch ASwitch B

Switch C

Switch E

Switch D

Switch F

Tandem Switch 2

High Usage Route

Alternate Routes

Tandem Switch 3

Tandem Switch 1

Tandem Switch 2

Switch A Switch B

Car

ried

Loa

d

Offered Load

Network Capacity

We will skip cellular telephone networks and satellite cellular networks