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Chapter 4Circuit-Switching
NetworksMultiplexing
SONETTransport NetworksCircuit Switches
The Telephone NetworkSignaling
Traffic and Overload Control in Telephone NetworksCellular Telephone Networks
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Circuit Switching Networksz End-to-end dedicated circuits between clients
z Client can be a person or equipment (router or switch)z Circuit can take different forms
z Dedicated path for the transfer of electrical currentz Dedicated time slots for transfer of voice samplesz Dedicated frames for transfer of Nx51.84 Mbps signalsz Dedicated wavelengths for transfer of optical signals
z Circuit switching networks require:z Multiplexing & switching of circuitsz Signaling & control for establishing circuits
z These are the subjects covered in this chapter
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(a) A switch provides the network to a cluster of users, e.g.a telephone switch connects a local community
(b) A multiplexer connects two access networks, e.g. a highspeed line connects two switches
Accessnetwork
Network
How a network grows
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Metropolitan network Aviewed as Network A of Access Subnetworks
National network viewedas Network of RegionalSubnetworks (including A)
A
National &International
Network of RegionalSubnetworks
(a)
(b)
A
Network of
AccessSubnetworks
dc
ba
A
Metropolitan
1*
a
c
b
d
2
34
A Network Keeps Growing
z Very high-speed lines
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Chapter 4Circuit-Switching
NetworksMultiplexing
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z Multiplexing involves the sharing of a transmission channel(resource) by several connections or information flows
z
Channel = 1 wire, 1 optical fiber, or 1 frequency bandz Significant economies of scale can be achieved by combining
many signals into onez Fewer wires/pole; fiber replaces thousands of cables
z Implicit or explicit information is required to demultiplex theinformation flows.
Multiplexing
B B
C C
A A
B
C
A
B
C
A(a) (b)
MUX MUX
SharedChannel
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(b) Combinedsignal fits into
channelbandwidth
Frequency-Division Multiplexingz Channel divided into frequency slots
z
Guard bandsrequiredz AM or FM radio
stationsz TV stations in
air or cablez Analog
telephonesystems
Cf
B f
Af
W u
W u
0
0
0 W u
A CB
f W 0
(a) Individual
signals occupyWu Hz
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(a) Each signaltransmits 1 unit
every 3 T seconds
(b) Combinedsignal transmits1 unit every T
seconds
Time-Division Multiplexing
t A1 A23T 0T 6T
t B
1B
2
3T 0T 6T
t C 1 C2
3T 0T 6T
B1 C1 A2 C2B2 A1 t 0T 1T 2T 3T 4T 5T 6T
z High-speed digital channel divided into time slots
z Framingrequired
z Telephonedigitaltransmission
z Digitaltransmission inbackbonenetwork
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T-Carrier Systemz Digital telephone system uses TDM.z PCM voice channel is basic unit for TDM
z 1 channel = 8 bits/sample x 8000 samples/sec. = 64 kbpsz T-1 carrier carries Digital Signal 1 (DS-1) that
combines 24 voice channels into a digital stream:
Bit Rate = 8000 frames/sec. x (1 + 8 x 24) bits/frame= 1.544 Mbps
2
24
1 1
2
24
24 b1 2 . . .b2322
Frame
24 . . .
. . .
MUX MUX
Framing bit
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North American Digital
Multiplexing Hierarchy
z DS0, 64 Kbps channelz DS1, 1.544 Mbps channelz DS2, 6.312 Mbps channelz DS3, 44.736 Mbps channelz DS4, 274.176 Mbps channel
1
24
1
4
1
7
1
6
..
..
.
.
.
.
Mux
Mux
Mux
Mux
DS1 signal, 1.544Mbps
DS2 signal, 6.312Mbps
DS3 signal, 44.736Mpbs
DS4 signal
274.176Mbps
24 DS04 DS1
7 DS2
6 DS3
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CCITT Digital Hierarchy
1
30
1
4
1
1
4
..
..
.
.
.
.
Mux
Mux
Mux
Mux
2.048 Mbps
8.448 Mbps
34.368 Mpbs
139.264 Mbps
64 Kbps
z CCITT digital hierarchy based on 30 PCM channels
z E1, 2.048 Mbps channelz E2, 8.448 Mbps channelz E3, 34.368 Mbps channelz E4, 139.264 Mbps channel
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12345 12345
t MUX
Clock Synch & Bit Slipsz Digital streams cannot be kept perfectly synchronizedz Bit slips can occur in multiplexers
Slow clock results in late bitarrival and bit slip
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Pulse Stuffingz Pulse Stuffing: synchronization to avoid data loss due to slipsz Output rate > R1+R2
z i.e. DS2, 6.312Mbps=4x1.544Mbps + 136 Kbpsz Pulse stuffing format
z Fixed-length master frames with each channel allowed to stuff or not to stuff a single bit in the master frame.
z Redundant stuffing specificationsz signaling or specification bits (other than data bits) are distributed
across a master frame.
Muxing of equal-rate signals Pulse stuffing
requires perfect synch
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Wavelength-Division Multiplexingz Optical fiber link carries several wavelengths
z From few (4-8) to many (64-160) wavelengths per fiber z Imagine prism combining different colors into single beamz Each wavelength carries a high-speed stream
z Each wavelength can carry different format signalz e.g. 1 Gbps, 2.5 Gbps, or 10 Gbps
1
2
m
OpticalMUX 1
2
m
OpticaldeMUX
1 2. m
Opticalfiber
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Example: WDM with 16
wavelengths
1 5 5 0 nm
1 5 6 0 nm
1 5 4
0 nm
30 dB
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Typical U.S. Optical Long-Haul
Network
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Chapter 4Circuit-Switching
NetworksSONET
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SONET: Overviewz S ynchronous Optical NET workz North American TDM physical layer standard for
optical fiber communicationsz 8000 frames/sec. (T frame = 125 sec)
z compatible with North American digital hierarchyz SDH (Synchronous Digital Hierarchy) elsewhere
z Needs to carry E1 and E3 signalsz Compatible with SONET at higher speeds
z Greatly simplifies multiplexing in network backbonez OA&M support to facilitate network managementz Protection & restoration
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Pre-SONET multiplexing: Pulse stuffing required demultiplexingall channels
SONET Add-Drop Multiplexing: Allows taking individual channels inand out without full demultiplexing
Removetributary
Inserttributary
DEMUX MUXMUX DEMUX
ADM
Removetributary
Inserttributary
MUX DEMUX
SONET simplifies multiplexing
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SONET Specificationsz Defines electrical & optical signal interfacesz
Electricalz Multiplexing, Regeneration performed in electrical
domainz STS Synchronous Transport Signals definedz Very short range (e.g., within a switch)
z Opticalz Transmission carried out in optical domainz Optical transmitter & receiver z OC Optical Carrier
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SONET & SDH Hierarchy
STM: SynchronousTransfer Module
OC: Optical Channel STS: SynchronousTransport Signal
STM-649953.28OC-192STS-192
STM-162488.32OC-48STS-48
STM-121866.24OC-36STS-36
STM-81244.16OC-24STS-24STM-6933.12OC-18STS-18
STM-4622.08OC-12STS-12
STM-3466.56OC-9STS-9
STM-1155.52OC-3STS-3N/A51.84OC-1STS-1
SDHElectrical Signal
Bit Rate (Mbps)Optical SignalSONET Electr icalSignal
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SONET Equipmentz By Functionality
z ADMs: dropping & inserting tributariesz Regenerators: digital signal regenerationz Cross-Connects: interconnecting SONET streams
z By Signaling between elementsz
Section Terminating Equipment (STE): span of fiber between adjacent devices, e.g. regeneratorsz Line Terminating Equipment (LTE): span between adjacent
multiplexers, encompasses multiple sectionsz
Path Terminating Equipment (PTE): span between SONETterminals at end of network, encompasses multiple lines
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Section, Line, & Path in SONET
z Often, PTE and LTE equipment are the samez Difference is based on function and locationz PTE is at the ends, e.g., STS-1 multiplexer.z LTE in the middle, e.g., STS-3 to STS-1 multiplexer.
PTELTE
STE
STS-1 Path
STS Line
Section Section
STE = Section Terminating Equipment, e.g., a repeater/regenerator LTE = Line Terminating Equipment, e.g., a STS-1 to STS-3 multiplexer PTE = Path Terminating Equipment, e.g., an STS-1 multiplexer
MUX MUXReg Reg Reg
SONETterminal
STE STELTE
PTE
SONETterminal
Section Section
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Optical
Section
Optical
Section
Optical
Section
Optical
SectionLine
Optical
SectionLine
Optical
SectionLine
Path
Optical
SectionLine
Path
Section, Line, & Path Layers in
SONET
z SONET has four layersz Optical, section, line, pathz Each layer is concerned with the integrity of its own signals
z
Each layer has its own protocolsz SONET provides signaling channels for elements within a
layer
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SONET STS Framez SONET streams carry two types of overheadz
Path overhead (POH) :z inserted & removed at the endsz Synchronous Payload Envelope (SPE) consisting
of Data + POH traverses network as a single unitz Transport Overhead (TOH):
z processed at every SONET nodez TOH occupies a portion of each SONET framez TOH carries management & link integrity
information
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Special OH octets :
A1, A2 Frame Synch
B1 Parity on Previous Frame(BER monitoring)J0 Section trace
(Connection Alive?)H1, H2, H3 Pointer Action
K1, K2 Automatic ProtectionSwitching
810 Octets per frame @ 8000 frames/sec
9 rows
90 columns
1
2Order of transmission
A1 A2 J0 J1
B1 E1 F1 B3D1 D2 D3 C2
H1 H2 H3 G1
B2 K1 K2 F2
D4 D5 D6 H4D7 D8 D9 Z3
D10 D11 D12 Z4
S1 M0/1 E2 N1
3 Columns of Transport OH
Section Overhead
Line Overhead
Synchronous Payload Envelope (SPE)1 column of Path OH + 8 data columns
Path Overhead
Data
STS-1 Framez 810x64kbps=51.84 Mbps
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SPE Can Span Consecutive Frames
z Pointer indicates where SPE begins within a framez Pointer enables add/drop capability
Pointer 87 Columns
9 Rows
First column is path overhead
Synchronouspayload
envelope
Frame
k
Framek +1
Pointer
First octet
Last octet
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Stuffing in SONETz Consider system with different clocks (faster out than in)z Use buffer (e.g., 8 bit FIFO) to manage differencez Buffer empties eventuallyz One solution: send stuffz Problem:
z Need to signal stuff to receiver
FIFO1,000,000 bps 1,000,001 bps
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STS-1
STS-1 STS-1
STS-1
STS-1 STS-1
Map
Map
Map
STS-1 STS-1
STS-1 STS-1
STS-1 STS-1
ByteInterleave
STS-3
IncomingSTS-1 frames
Synchronized new
STS-1 frames
Synchronous Multiplexingz Synchronize each incoming STS-1 to local clock
z Terminate section & line OH and map incoming SPE into a new STS-1 synchronized to the local clock
z This can be done on-the-fly by adjusting the pointer z All STS-1s are synched to local clock so bytes can be
interleaved to produce STS-n
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Concatenated Payloadsz Needed if payloads of interleaved
frames are locked into a bigger unit
z Data systems send big blocks of information grouped together, e.g.,a router operating at 622 Mbps
z SONET/SDH needs to handlethese as a single unit
z H1,H2,H3 tell us if there isconcatenation
z STS-3c has more payload than 3STS-1s
z STS-Nc payload = Nx780 bytesz OC-3c = 149.760 Mb/sz OC-12c = 599.040 Mb/sz OC-48c = 2.3961 Gb/sz OC-192c = 9.5846 Gb/s
Concatenated Payload OC-Nc
J1B3C2G1
F2H4Z3Z4N1
(N /3) 1columns of
fixed stuff
z N x 87 columns
87N - ( N /3)columns of
payload
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Chapter 4Circuit-Switching
NetworksTransport Networks
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TelephoneSwitch
Transport Network
Router Router
Router
TelephoneSwitch
TelephoneSwitch
Transport Networksz Backbone of modern networksz Provide high-speed connections: Typically STS-1 up to OC-192z Clients: large routers, telephone switches, regional networksz Very high reliability required because of consequences of failure
z 1 STS-1 = 783 voice calls; 1 OC-48 = 32000 voice calls;
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ADM
Removetributary
Inserttributary
MUX DEMUX
SONET ADM Networks
z SONET ADMs: the heart of existingtransport networks
z ADMs interconnected in linear and ring
topologiesz SONET signaling enables fast restoration
(within 50 ms) of transport connections
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1 2 43
1
2
3
4
Linear ADM Topologyz ADMs connected in linear fashionz Tributaries inserted and dropped to connect clients
z Tributaries traverse ADMs transparentlyz Connections create a logical topology seen by clientsz Tributaries from right to left are not shown
1+1 Li A i P i
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T = Transmitter W = Working lineR = Receiver P = Protection line
Bridge
T
T R
RW
P
Selector
1+1 Linear Automatic Protection
Switching
Simultaneous transmission over diverse routes Monitoring of signal quality Fast switching in response to signal degradation
100% redundant bandwidth
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Switch
T
T R
RW
P
Switch
APS signaling
1:1 Linear APS
Transmission on working fiber Signal for switch to protection route in response to
signal degradation
Can carry extra (preemptible traffic) on protection line
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Switch
T RW
T RP
Switch
T RW
1
T RW n
APS signaling
1:N Linear APS
Transmission on diverse routes; protect for 1 fault Reverts to original working channel after repair More bandwidth efficient
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a
b
c
OC-3 nOC-3 n
OC-3 n
(a) (b)
Three ADMs connected in
physical ring topology
Logical fully connected
topology
a
b c
SONET Ringsz ADMs can be connected in ring topologyz Clients see logical topology created by tributaries
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SONET Ring Optionsz 2 vs. 4 Fiber Ring Networkz
Unidirectional vs. bidirectional transmissionz Path vs. Link protection
z Spatial capacity re-use & bandwidthefficiency
z
Signalling requirements
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W = Working Paths
W
P
1
2
3
4
UPSR
P = Protection Pathsz No spatial re-useEach path uses 2x bw
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UPSR Propertiesz Low complexityz
Fast path protectionz 2 TX, 2 RXz No spatial re-use; ok for hub traffic patternz Suitable for lower-speed access networksz Different delay between W and P path
Four-Fiber Bidirectional Line
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Four Fiber Bidirectional Line
Switched Ringz 1 working fiber pair; 1 protection fiber pair z Bidirectional
z Working traffic & protection traffic use same routein working pair
z 1:N likez Line restoration provided by either:
z Restoring a failed spanz Switching the line around the ring
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P
WEqualdelay
SpatialReuse
1
2
3
4
4-BLSR
Standby
bandwidthis shared
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P
WEqualdelay
1
2
3
4
Fault onworking
links
BLSR Span Switching
z SpanSwitchingrestores
failed line
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P
WEqualdelay
1
2
3
4
Fault onworking and
protectionlinks
BLSR Span Switching
z LineSwitchingrestores
failed lines
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4-BLSR Propertiesz High complexity: signalling requiredz Fast line protection for restricted distance
(1200 km) and number of nodes (16)z 4 TX, 4 RXz Spatial re-use; higher bandwidth efficiencyz Good for uniform traffic patternz Suitable for high-speed backbone networksz Multiple simultaneous faults can be handled
Backbone Networks consist of
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Interofficerings
Metroring
Regionalring
Interconnected Rings
UPSR
OC-12
BLSR
OC-48,OC-192
UPSR or BLSR
OC-12,OC-48
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Mesh Topology Networks using
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BC
DF
A
G E
Router
Router
Router
Router
p gy g
SONET Cross-Connectsz Cross-Connects are nxn switchesz Interconnects SONET streamsz More flexible and efficient than ringsz Need mesh protection & restoration
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From SONET to WDMSONETz combines multiple SPEs
into high speed digitalstream
z ADMs andcrossconnectsinterconnected to form
networksz SPE paths between
clients from logicaltopology
z
High reliability throughprotection switching
WDMz combines multiple wavelengths into a
common fiber z Optical ADMs can be built to insert and
drop wavelengths in same manner asin SONET ADMS
z Optical crossconnects can also be builtz All-optical backbone networks will
provide end-to-end wavelengthconnections
z Protection schemes for recovering
from failures are being developed toprovide high reliability in all-opticalnetworks
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Opticalfiber switch
Wavelengthcross-connect
W D M
W D M W
D M
Output Input M U X
D
e M U X
Addedwavelengths
Droppedwavelengths
W D M
Optical Switching
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Chapter 4Circuit-Switching
NetworksCircuit Switches
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User 1
SwitchLink
User n
User n 1
Control
123
N
123
N
Connectionof inputsto outputs
Network: Links & switchesz Circuit consists of dedicated resources in sequence
of links & switches across networkz Circuit switch connects input links to output links
z Network
z Switch
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Circuit Switch Typesz Space-Division switches
z Provide separate physical connection betweeninputs and outputs
z Crossbar switchesz Multistage switches
z Time-Division switchesz Time-slot interchange techniquez Time-space-time switches
z Hybrids combine Time & Space switching
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N
1 2
1
N
2
N 1
Crossbar Space Switch
z N x N array of crosspoints
z Connect an input toan output by closinga crosspoint
z Nonblocking: Anyinput can connect toidle output
z Complexity: N 2
crosspoints
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Clos Non-Blocking Condition:
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nxk
nxk
nxk
N/n x N/n
N/n x N/n
N/n x N/n
k xn1
N/n
Desiredinput
1
j m
N/n
Desiredoutput
1
2n -1
k xn
k xn
n-1
N/n x N/nn+1
N/n x N/n2n-2
Free path Free path
n-1busy
n-1busy
k=2n-1z Request connection from last input to input switch j to last output in output switch m
z Worst Case: All other inputs have seized top n-1 middle switches AND all other outputs have seized next n-1 middle switches
z If k=2n-1 , there is another path left to connect desired input to desired output
# internal links =2x # external links
l l h
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C(n) = number of crosspoints in Clos switch
= 2Nk + k ( )2 = 2 N (2n 1)+(2 n 1)( ) 2
Differentiate with respect to n:
0 = = 4 N + 4N ==> n
The minimized number of crosspoints is then:
C* = (2N + )(2( )1/2
1) 4N 2N = 4 2N 1.5
This is lower than N 2 for large N
Minimum Complexity Clos Switch
N 2
N /2
2N 2
n2
2N 2
n3
2N 2
n2
N 2
C
n
N
n
N
n
N
2
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Time-Slot Interchange (TSI)S i hi
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1
2
3
22
zzz
23
24
Writeslots inorder of arrival
Read slots
according toconnectionpermutation
24 23 12
Time-slot interchange
24 23 12abcd b a d c
a
b
c
d
Switchingz Write bytes from arriving TDM stream into memoryz Read bytes in permuted order into outgoing TDM streamz Max # slots = 125 sec / (2 x memory cycle time)
z Incoming TDM
stream
z Outgoing TDM
stream
Ti S Ti H b id S it h
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nxk
nxk
nxk
nxk
N/n x N/n k xn1
2
N/n
N inputs
1
3
1
12
zzz
n
Time-slot interchange
Input TDMframe withn slots
Output TDMframe with k slots
n 2 1 k 2 1
Time-Space-Time Hybrid Switchz Use TSI in first & third stage; Use crossbar in middle
z Replace n input x k output space switch by TSI switch that takes n-slotinput frame and switches it to k-slot output frame
Flow of time slots betweenit h
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n k N/n N/n
N/n N/n
N/n N/n
k n1 1
2
N/n
1
2
k
k n
k n
n k 2
n k N/n
First slot
k th slot
First slot
k th slot
switches
z Only one space switch active in each time slot
Time Share the Crossbar Switch
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nxk
nxk
nxk
nxk
N/n x N/nTime-sharedspace switch
k xn
1
2
N/n
N inputs
1
2
3 3
N/n
N outputs
TDM
n slots
n slots
n slots
n slots
k xn
k xn
k xn
TDM
k slots
TDM
k slots
TSI stage TSI stageSpace stage
Time-Share the Crossbar Switch
z Interconnection pattern of space switch isreconfigured every time slot
z Very compact design: fewer lines because of TDM& less space because of time-shared crossbar
Example: A 3 B 4 C 1 D 3
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2x3
2x3
3x21
2
1
2
3x2D1
B1 A1B2 A2
C1D2 C2
B1 A1
C1D1
A1
B1
C 1
D1
A1 C1
B1 D1
(b)
AB
CD
(a)C
A
D
B
Example: A 3, B 4, C 1, D 3
z 3-stageSpaceSwitch
z Equivalent TST Switch
Example: T S T Switch Design
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Example: T-S-T Switch Design
For N = 960z Single stage space switch ~ 1 million crosspointsz T-S-T
z Let n = 120 N/n = 8 TSIsz k = 2 n 1 = 239 for non-blockingz Pick k = 240 time slotsz Need 8x8 time-multiplexed space switch
For N = 96,000z T-S-T
z Let n = 120 k = 239z N / n = 800z Need 800x800 space switch
Available TSI Chips circa 2002
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Available TSI Chips circa 2002
z OC-192 SONET Framer Chipsz Decompose 192 STS1s and perform (restricted)
TSI
z Single-chip TSTz 64 inputs x 64 outputsz Each line @ STS-12 (622 Mbps)z
Equivalent to 768x768 STS-1 switch
Pure Optical Switching
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Pure Optical Switching
z Pure Optical switching: light-in, light-out,without optical-to-electronic conversion
z Space switching theory can be used todesign optical switchesz Multistage designs using small optical switchesz Typically 2x2 or 4x4z MEMs and Electro-optic switching devices
z Wavelength switchesz Very interesting designs when space switching is
combined with wavelength conversion devices
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Chapter 4Circuit-Switching
NetworksThe Telephone Network
Telephone Call
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Telephone Call
z User requests connectionz Network signaling
establishes connectionz Speakers conversez User(s) hang upz Network releases
connection resources
Signal
Source
Signal
Release
Signal
Destination
Goahead Message
Call Routing
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(b)
LATA 1 LATA 2
Net 1
Net 2
(a)
1
2 3
4
5
A B
C D
Call Routingz Local calls routed
through local network(In U.S. Local Access &Transport Area)
z Long distance calls
routed to long distanceservice provider
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Fiber-to-the-Home or Fiber-to-the-Curve?
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Fiber to the Curve?
z Fiber connection to thehome provides huge
amount of bandwidth,but cost of opticalmodems still high
z
Fiber to the curve(pedestal) with shorter distance from pedestalto home can providehigh speeds over copper pairs
Table 3.5 Data rates of 24-gauge twisted pair
1000 feet, 300 m51.840Mbps
STS-1
3000 feet, 0.9 km25.920Mbps
1/2 STS-1
4500 feet, 1.4 km12.960Mbps1/4 STS-1
12,000 feet, 3.7 km6.312 MbpsDS2
18,000 feet, 5.5 km1.544 MbpsT-1
DistanceData RateStandard
Two- & Four-wire connections
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Originalsignal
Hybridtransformer
Received signal
Echoedsignal
Receive pair
Transmit pair
z From telephone to CO, two wires carry signals in both directionsz Inside network, 1 wire pair per directionz Conversion from 2-wire to 4-wire occurs at hybrid transformer in
the COz Signal reflections can occur causing speech echoz Echo cancellers used to subtract the echo from the voice signals
z Two Wires
z Four Wires
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Setting Up Connections
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g p
Manuallyz Human Interventionz Telephone
z Voice commands &switchboard operators
z Transport Networksz Order forms &
dispatching of craftpersons
Automaticallyz Management Interface
z Operator at console setsup connections atvarious switches
z Automatic signalingz Request for connection
generates signalingmessages that control
connection setup inswitches
Stored-Program Control Switches
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SPC
ControlSignaling Message
z SPC switches (1960s)z Crossbar switches with crossbars built from relays
that open/close mechanically through electrical controlz Computer program controls set up opening/closing of crosspoints to establish connections between switchinputs and outputs
z Signaling required to coordinate path set up acrossnetwork
Message Signaling
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Switch
Processor
Office B
Switch
Office A
Processor Signaling
ModemModem
Trunks
z Processors that control switches exchange signalingmessages
z Protocols defining messages & actions definedz Modems developed to communicate digitally over
converted voice trunks
Signaling Network
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Access SignalingDial tone
Internodal SignalingSignaling System 7
STP
STP
STP
STP
SSP SSP
Transport Network
Signaling Network
SSP = service switching point (signal to message)STP = signal transfer point (packet switch)
SCP = service control point (processing)
SCP
z Common Channel Signaling (CCS) #7 deployed in 1970s to control call setupz Protocol stack developed to support signalingz Signaling network based on highly reliable packet switching networkz
Processors & databases attached to signaling network enabled many newservices: caller id, call forwarding, call waiting, user mobility
Signaling System Protocol Stack
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Application layer
Transport layer
Network layer
Data link layer
Physical layer
Presentation layer
Session layer
SCCP
MTP level 3
MTP level 2
MTP level 1
ISUPTCAPTUP
ISUP = ISDN user part MTP = message transfer partSSCP = signaling connection control part TCAP = transaction capabilities part
TUP = telephone user part
z Lower 3 layers ensuredelivery of messages tosignaling nodes
z SCCP allowsmessages to bedirected to applications
z TCAP defines
messages & protocolsbetween applications
z ISUP performs basiccall setup & release
z TUP instead of ISUP insome countries
Future Signaling: Calls, Sessions,& Connections
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Call/Sessionz An agreement by two end
parties to communicatez Answering a ringing
phone (after looking atcaller ID)
z TCP three-wayhandshake
z Applies in connection-less &connection-oriented
networksz Session Initiation Protocol
(SIP) provides for establishment of sessions inmany Internet applications
Connectionz Allocation of resources to
enable information transfer between communicatingpartiesz Path establishment in
telephone callz Does not apply in
connectionless networksz ReSerVation Protocol
(RSVP) provides for resourcereservation along paths inInternet
Network Intelligence
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SSPSSP
Transport Network
Signaling
Network
Intelligent
Peripheral
ExternalDatabase
z Intelligent Peripherals provide additional service capabilitiesz Voice Recognition & Voice Synthesis systems allow users to access
applications via speech commandsz Voice browsers currently under development (See: www.voicexml.org)z Long-term trend is for IP network to replace signaling system and provideequivalent servicesz Services can then be provided by telephone companies as well as new
types of service companies
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Traffic Management & OverloadControl
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z Telephone calls come and goz People activity follow patterns
z Mid-morning & mid-afternoon at officez Evening at homez Summer vacation
z Outlier Days are extra busyz Mothers Day, Christmas,
z Disasters & other events cause surges in trafficz Need traffic management & overload control
Traffic concentration
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z Traffic fluctuates as calls initiated & terminatedz Driven by human activity
z Providing resources soz Call requests always met is too expensivez Call requests met most of the time cost-effective
z Switches concentrate traffic onto shared trunksz Blocking of requests will occur from time to time
z Traffic engineering provisions resources to meet blockingperformance targets
Fewer
trunks
Manylines
Fluctuation in Trunk Occupancy
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1
2
3
45
6
7
T r u n k n u m
b e r
N(t)
t
All trunks busy, new call requests blocked
z Number of busy trunks
z activez active
z active
z activez active
z active
z activez active
z active
z active
Modeling Traffic Processes
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z Find the statistics of N(t) the number of calls in the system
Modelz Call request arrival rate : requests per secondz In a very small time interval ,
z Prob[ new request ] = z Prob[no new request] = 1 -
z
The resulting random process is a Poisson arrival process:
z Holding time : Time a user maintains a connectionz X a random variable with mean E(X)
z Offered load : rate at which work is offered by users:z a = calls/sec * E( X ) seconds/call (Erlangs)
( T)k e T k!
Prob( k arrivals in time T ) =
Blocking Probability & Utilization
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z c = Number of Trunksz Blocking occurs if all trunks are busy, i.e. N(t)=c z If call requests are Poisson, then blocking probability
P b is given by Erlang B Formula
z The utilization is the average # of trunks in use
P b =
a c
c!
k !a k
k =0
c
Utilization = (1 P b) E [ X ]/c = (1 P b ) a /c
Blocking Performance
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z a
To achieve 1% blocking probability:a = 5 Erlangs requires 11 trunksa = 10 Erlangs requires 18 trunks
Multiplexing Gain
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0.85117100
0.8510690
0.8075600.786450
0.714230
0.561810
0.53179
0.53158
0.50147
0.46136
0.45115
0.401040.3883
0.2972
0.2051
UtilizationTrunks@1%Loadz At a given P b, the
system becomes more
efficient in utilizingtrunks with increasingsystem size
z Aggregating trafficflows to share centrallyallocated resources ismore efficient
z This effect is calledMultiplexing Gain
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Alternative Routing
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z Deploy trunks between switches with significant traffic volumez Allocate trunks with high blocking, say 10%, so utilization is highz Meet 1% end-to-end blocking requirement by overflowing to
longer paths over tandem switchz Tandem switch handles overflow traffic from other switches so it
can operate efficientlyz Typical scenario shown in next slide
Switch SwitchHigh-usage route
Tandemswitch
Alternative route
Typical Routing Scenario
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High-usage route B-E
Tandemswitch 1
Alternative routesfor B-E, C-F
High-usage route C-F
Switch B
Switch C
Switch E
Switch D
Switch F
Tandem
switch 2
Switch A
Dynamic Routing
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High-usage route
Alternative routes
Switch A Switch B
Tandemswitch 3
Tandemswitch 1
Tandemswitch 2
z Traffic varies according to time of day, day of weekz East coast of North America busy while West coast idle
z Network can use idle resources by adapting route selectiondynamicallyz Route some intra-East-coast calls through West-coast switches
z Try high-usage route and overflow to alternative routes
Overload Control
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C a r r
i e d l o a
d
Offered load
Network capacity
Overload Situationsz Mothers Day, Xmasz Catastrophesz Network Faults
Strategiesz Direct routes firstz Outbound firstz Code blockingz Call request pacing
Chapter 4Ci i S i hi
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Circuit-SwitchingNetworks
Cellular Telephone Networks
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Cellular Communications
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Two basic concepts:z Frequency Reuse
z A region is partitioned into cellsz Each cell is covered by base stationz Power transmission levels controlled to minimize inter-cell
interferencez Spectrum can be reused in other cells
z Handoff z Procedures to ensure continuity of call as user moves from
cell to another z Involves setting up call in new cell and tearing down old one
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Signaling & Connection Control
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z Setup channels set aside for call setup & handoff z Mobile unit selects setup channel with strongest signal &
monitors this channelz Incoming call to mobile unit
z MSC sends call request to all BSSsz BSSs broadcast request on all setup channelsz Mobile unit replies on reverse setup channelz BSS forwards reply to MSCz BSS assigns forward & reverse voice channelsz BSS informs mobile to use thesez Mobile phone rings
Mobile Originated Call
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z Mobile sends request in reverse setup channelz Message from mobile includes serial # and possibly
authentication informationz BSS forwards message to MSCz MSC consults Home Location Register for
information about the subscriber z MSC may consult Authentication center z MSC establishes call to PSTNz BSS assigns forward & reverse channel
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Roaming
U b ib i i i
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z Users subscribe to roaming service to use serviceoutside their home region
z Signaling network used for message exchangebetween home & visited network
z Roamer uses setup channels to register in new area
z MSC in visited areas requests authorization fromusers Home Location Register z Visitor Location Register informed of new user z
User can now receive & place calls
GSM Signaling Standardz Base station
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z Base stationz Base Transceiver Station (BTS)
z Antenna + Transceiver to mobilez Monitoring signal strength
z Base Station Controller z Manages radio resources or 1 or more BTSsz Set up of channels & handoff z Interposed between BTS & MSC
z Mobile & MSC Applicationsz Call Management (CM)z Mobility Management (MM)
z Radio Resources Management (RRM) concernsmobile, BTS, BSC, and MSC
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CM Um
Cellular Network Protocol Stack
R di Ai I t f (U )
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LAPD m
Radio Radio
LAPD m
RRM
MM
C
RRM
LAPD
64kbps
m
Mobile station Basetransceiver
station
Radio Air Interface (U m)z LAPD m is data link control
adapted to mobilez RRM deals with setting up of
radio channels & handover
Abis
Cellular Network Protocol Stack
A I t f
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Radio
LAPD m
64kbps
LAPD
RRMRRM
LAPD
64kbps
64kbps
MTPLevel 3
MTPLevel 2
SCCP
Basetransceiver
station
Basestation
controller
Abis Interfacez 64 kbps link physical layer z LAPD mz BSC RRM can handle
handover for cells within its
control
CM A
Cellular Network Protocol Stack
Signaling Network (A)CM
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SCCP
MTPLevel 3
MTPLevel 2
64kbps
LAPD
MM
RRMRRM
64kbps
64kbps
MTPLevel 3
MTPLevel 2
SCCP
Basestation
controller
MSC
Signaling Network (A)Interface
z RRM deals handover involving cells withdifferent BSCs
z
MM deals withmobile user location,authentication
z CM deals with callsetup & releaseusing modified ISUP
LAPD m
Radio
RRM
MM
Mobile station
Whats Next for Cellular Networks?
z Mobility makes cellular phone compelling
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z Mobility makes cellular phone compellingz Cell phone use increasing at expense of telephone
z Short Message Service (SMS) transfers text usingsignaling infrastructurez Growing very rapidly
z
Multimedia cell phonesz Digital camera to stimulate more usagez Higher speed data capabilities
z GPRS & EDGE for data transfer from laptops & PDAsz WiFi (802.11 wireless LAN) a major competitor