Chapter 4
Circuit-Switching
Networks
Multiplexing
SONET
Circuit Switches
The Telephone Network
Signaling
Traffic and Overload Control in Telephone Networks
Cellular Telephone Networks
How a network grows
(a) A switch provides the network to a cluster of users, e.g. a
telephone switch connects a local community
Network
Access
network
(b) A multiplexer connects two access networks, e.g. a high
speed line connects two switches
A Network Keeps Growing
1*
b
a
a
2
(a)
(b)
Metropolitan network A
viewed as Network A of
Access Subnetworks
b
4
3
A
A
c
d
Metropolitan
National network viewed
as Network of Regional
Subnetworks (including A)
d
c
Network of
Access
Subnetworks
A
Very
highspeed lines

Network of Regional
Subnetworks
National &
International
Chapter 4
Circuit-Switching
Networks
Multiplexing
Multiplexing

Multiplexing involves the sharing of a transmission channel
(resource) by several connections or information flows


Significant economies of scale can be achieved by combining
many signals into one


Channel = 1 wire, 1 optical fiber, or 1 frequency band
Fewer wires/pole; fiber replaces thousands of cables
Implicit or explicit information is required to demultiplex the
information flows.
(a)
Shared
Channel
(b)
A
A
A
B
B
B
C
C
C
MUX
MUX
A
B
C
Frequency-Division Multiplexing

Channel divided into frequency slots

A
0
(a) Individual
signals occupy
Wu Hz
f
Wu

B
0
f
Wu

C
0
(b) Combined
signal fits into
channel
bandwidth

Wu
A
0
f
B
C
W
f
Guard bands
required
AM or FM radio
stations
TV stations in
air or cable
Analog
telephone
systems
Time-Division Multiplexing

High-speed digital channel divided into time slots
A1
0T
…
A2
t
6T
3T

(a) Each signal
transmits 1 unit
every 3T
seconds
B1
C1
0T
(b) Combined
signal transmits
1 unit every T
seconds
0T
1T 2T
C1 A2
3T 4T

…
C2
t
6T
3T
A1 B1
t
6T
3T
0T
…
B2
B2
C2
5T 6T
…
t

Framing
required
Telephone
digital
transmission
Digital
transmission in
backbone
network
T-Carrier System

Digital telephone system uses TDM.
PCM voice channel is basic unit for TDM


1 channel = 8 bits/sample x 8000 samples/sec. = 64 kbps
T-1 carrier carries Digital Signal 1 (DS-1) that
combines 24 voice channels into a digital stream:
1
...
2
24
1
MUX
MUX
22
23
24
b
1
2
...
24
b
Frame
2
...

24
Framing bit
Bit Rate = 8000 frames/sec. x (1 + 8 x 24) bits/frame
= 1.544 Mbps
North American Digital
Multiplexing Hierarchy
1
24
.
.
DS1 signal, 1.544Mbps
Mux
1
24 DS0
4 DS1
4
.
.
DS2 signal, 6.312Mbps
Mux
1
7 DS2
7
.
.
DS3 signal, 44.736Mpbs
Mux
1





DS0,
DS1,
DS2,
DS3,
DS4,
64 Kbps channel
1.544 Mbps channel
6.312 Mbps channel
44.736 Mbps channel
274.176 Mbps channel
6 DS3
6
.
.
Mux
DS4 signal
274.176Mbps
Clock Synch & Bit Slips


Digital streams cannot be kept perfectly synchronized
Bit slips can occur in multiplexers
Slow clock results in late bit
arrival and bit slip
MUX
5
4
3
2
1
t
5
4
3
2
1
Pulse Stuffing


Pulse Stuffing: synchronization to avoid data loss due to slips
Output rate > R1+R2


i.e. DS2, 6.312Mbps=4x1.544Mbps + 136 Kbps
Pulse stuffing format



Fixed-length master frames with each channel allowed to stuff or
not to stuff a single bit in the master frame.
Redundant stuffing specifications
signaling or specification bits (other than data bits) are distributed
across a master frame.
Muxing of equal-rate signals
requires perfect synch
Pulse stuffing
Wavelength-Division Multiplexing

Optical fiber link carries several wavelengths



From few (4-8) to many (64-160) wavelengths per fiber
Imagine prism combining different colors into single beam
Each wavelength carries a high-speed stream


Each wavelength can carry different format signal
e.g. 1 Gbps, 2.5 Gbps, or 10 Gbps
1
2
m
Optical
deMUX
Optical
MUX
1 2 .
m
Optical
fiber
1
2
m
Chapter 4
Circuit-Switching
Networks
SONET
SONET: Overview



Synchronous Optical NETwork
North American TDM physical layer standard for
optical fiber communications
8000 frames/sec. (Tframe = 125 sec)


SDH (Synchronous Digital Hierarchy) elsewhere





compatible with North American digital hierarchy
Needs to carry E1 and E3 signals
Compatible with SONET at higher speeds
Greatly simplifies multiplexing in network backbone
OA&M support to facilitate network management
Protection & restoration
SONET Specifications


Defines electrical & optical signal interfaces
Electrical




Multiplexing, Regeneration performed in electrical
domain
STS – Synchronous Transport Signals defined
Very short range (e.g., within a switch)
Optical



Transmission carried out in optical domain
Optical transmitter & receiver
OC – Optical Carrier
SONET & SDH Hierarchy
SONET Electrical
Signal
Optical Signal
Bit Rate (Mbps)
SDH
Electrical Signal
STS-1
OC-1
51.84
N/A
STS-3
OC-3
155.52
STM-1
STS-9
OC-9
466.56
STM-3
STS-12
OC-12
622.08
STM-4
STS-18
OC-18
933.12
STM-6
STS-24
OC-24
1244.16
STM-8
STS-36
OC-36
1866.24
STM-12
STS-48
OC-48
2488.32
STM-16
STS-192
OC-192
9953.28
STM-64
STS: Synchronous
Transport Signal
OC: Optical Channel
STM: Synchronous
Transfer Module
SONET Multiplexing
DS2
E1
DS3
...
44.736
E4
139.264
ATM or
POS
Low-speed
mapping
function
Medium
speed
mapping
function
Highspeed
mapping
function
Highspeed
mapping
function
STS-1
51.84 Mbps
STS-1
STS-1
STS-1
STS-1
STS-1
STS-1
STS-1
OC-n
STS-n
...
DS1
STS-3c
STS-3c
Scrambler
MUX
E/O
SONET Equipment

By Functionality




ADMs: dropping & inserting tributaries
Regenerators: digital signal regeneration
Cross-Connects: interconnecting SONET streams
By Signaling between elements



Section Terminating Equipment (STE): span of fiber
between adjacent devices, e.g. regenerators
Line Terminating Equipment (LTE): span between adjacent
multiplexers, encompasses multiple sections
Path Terminating Equipment (PTE): span between SONET
terminals at end of network, encompasses multiple lines
Section, Line, & Path in SONET
PTE
PTE
LTE
LTE
SONET
terminal
MUX
Section
STE
STE
STE
Reg
Reg
Reg
Section
Section
MUX
Section
STS Line
STS-1 Path
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

Often, PTE and LTE equipment are the same
 Difference is based on function and location
 PTE is at the ends, e.g., STS-1 multiplexer.
 LTE in the middle, e.g., STS-3 to STS-1 multiplexer.
SONET
terminal
SONET STS Frame


SONET streams carry two types of overhead
Path overhead (POH):



inserted & removed at the ends
Synchronous Payload Envelope (SPE) consisting
of Data + POH traverses network as a single unit
Transport Overhead (TOH):



processed at every SONET node
TOH occupies a portion of each SONET frame
TOH carries management & link integrity
information
STS-1 Frame
810x64kbps=51.84
Mbps
810 Octets per frame @ 8000 frames/sec
90 columns
A1 A2 J0
J1
B1 E1 F1 B3
1
Order of
2 transmission
D1 D2 D3 C2
H1 H2 H3 G1
9 rows
B2 K1 K2 F2
D4 D5 D6 H4
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 Protection
Switching
D7 D8 D9 Z3
D10 D11 D12 Z4
S1 M0/1 E2 N1
3 Columns of
Transport OH
Synchronous Payload Envelope (SPE)
1 column of Path OH + 8 data columns
Section Overhead
Path Overhead
Line Overhead
Data
SPE Can Span Consecutive Frames
Pointer
Frame
k
First octet
87 Columns
Synchronous
payload
envelope
Pointer
9 Rows
Last octet
Frame
k+1
First column is path overhead


Pointer indicates where SPE begins within a frame
Pointer enables add/drop capability
Stuffing in SONET





Consider system with different clocks (faster out than in)
Use buffer (e.g., 8 bit FIFO) to manage difference
Buffer empties eventually
One solution: send “stuff”
Problem:
 Need to signal “stuff” to receiver
FIFO
1,000,000 bps
1,000,001 bps
Synchronous Multiplexing

Synchronize each incoming STS-1 to local clock



Terminate section & line OH and map incoming SPE into a new
STS-1 synchronized to the local clock
This can be done on-the-fly by adjusting the pointer
All STS-1s are synched to local clock so bytes can be
interleaved to produce STS-n
STS-1 STS-1
STS-1 STS-1
STS-1 STS-1
Incoming
STS-1 frames
Map
Map
Map
STS-1 STS-1
STS-1 STS-1
STS-1 STS-1
Synchronized new
STS-1 frames
Byte
STS-3
Interleave
Octet Interleaving
Order of
transmission
1
A1
2
3
A2
J0
J1
J0
A1
A2
J1
B1
E1
F1
B3
J0
A1
A2
J1
B3
B1
E1
F1
D1 D2
D3 C2
B3
B1
E1
F1
D1H1 D2
D3 C2
H2
H3 G1
D1H1 D2
D3 C2
H2
H3 G1
B2
K1
K2
F2
H1
H2
H3 G1
B2
K1
K2
F2
D6
D4 D5
H4
B2
K1
K2
F2
D6
D4 D5
H4
D9
Z3
D7 D8
D6 H4
D4 D5
D9
Z3
D7 D8
D10 D11 D12 Z4
D9
Z3
D7 D8
D10 D11 D12 Z4
N1
S1 M0/1 E2
D10 D11 D12
Z4
N1
S1 M0/1 E2
N1
S1 M0/1 E2
Concatenated Payloads
Concatenated Payload OC-Nc
N

x 87 columns

J1
B3
C2
G1
F2
H4
Z3
Z4
N1






(N/3) – 1
columns of
fixed stuff
Needed if payloads of interleaved
frames are “locked” into a bigger
unit
Data systems send big blocks of
information grouped together, e.g.,
a router operating at 622 Mbps
87N - (N/3)
columns of
payload


SONET/SDH needs to handle
these as a single unit
H1,H2,H3 tell us if there is
concatenation
STS-3c has more payload than 3
STS-1s
STS-Nc payload = Nx780 bytes
OC-3c = 149.760 Mb/s
OC-12c = 599.040 Mb/s
OC-48c = 2.3961 Gb/s
OC-192c = 9.5846 Gb/s
Chapter 4
Circuit-Switching
Networks
Circuit Switches
Network: Links & switches


Circuit consists of dedicated resources in sequence
of links & switches across network
Circuit switch connects input links to output links
Switch
Network
Switch
User n
1
2
3
…
User n – 1
User 1
N
Connection
of inputs
to outputs
1
2
3
…
Link
Control
N
Circuit Switching


Dedicated communication path between two
stations
Three phases





Establish
Transfer
Disconnect
Must have switching capacity and channel
capacity to establish connection
Must have intelligence to work out routing
Circuit Switching - Applications

Inefficient





Channel capacity dedicated for duration of
connection
If no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent
Developed for voice traffic (phone)
Public Circuit Switched Network
Telecomms Components

Subscriber


Local Loop



Subscriber loop
Connection to network
Exchange



Devices attached to network
Switching centers
End office - supports subscribers
Trunks


Branches between exchanges
Multiplexed
Circuit Switch Elements
Circuit Switching Concepts

Digital Switch



Provide transparent signal path between devices
Network Interface
Control Unit

Establish connections






Generally on demand
Handle and acknowledge requests
Determine if destination is free
construct path
Maintain connection
Disconnect
Blocking or Non-blocking

Blocking



A network is unable to connect stations because
all paths are in use
A blocking network allows this
Used on voice systems


Short duration calls
Non-blocking


Permits all stations to connect (in pairs) at once
Used for some data connections
Circuit Switch Types

Space-Division switches




Time-Division switches



Provide separate physical connection between
inputs and outputs
Crossbar switches
Multistage switches
Time-slot interchange technique
Time-space-time switches
Hybrids combine Time & Space switching
Space Division Switching



Developed for analog environment
Separate physical paths
Crossbar switch



Number of crosspoints grows as square of
number of stations
Loss of crosspoint prevents connection
Inefficient use of crosspoints


All stations connected, only a few crosspoints in use
Non-blocking
Crossbar Space Switch



N x N array of
crosspoints
Connect an input to
an output by closing
a crosspoint
Nonblocking: Any
input can connect to
idle output
Complexity: N2
crosspoints
1
2
…

N
…
1
2
N –1 N
Multistage Switch


Reduced number of crosspoints
More than one path through network



Increased reliability
More complex control
May be blocking
Three Stage Space Division
Switch
Chapter 4
Circuit-Switching
Networks
Packet Switches
Packet Switching Principles

Circuit switching designed for voice



Resources dedicated to a particular call
Much of the time a data connection is idle
Data rate is fixed

Both ends must operate at the same rate
Basic Operation

Data transmitted in small packets




Control info


Typically 1000 octets
Longer messages split into series of packets
Each packet contains a portion of user data plus
some control info
Routing (addressing) info
Packets are received, stored briefly (buffered)
and past on to the next node

Store and forward
Use of Packets
Advantages

Line efficiency



Data rate conversion



Each station connects to the local node at its own speed
Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy


Single node to node link can be shared by many packets over
time
Packets queued and transmitted as fast as possible
Delivery may slow down
Priorities can be used
Switching Technique



Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways


Datagram
Virtual circuit
Datagram





Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may go missing
Up to receiver to re-order packets and
recover from missing packets
Datagram
Diagram
Virtual Circuit






Preplanned route established before any
packets sent
Call request and call accept packets establish
connection (handshake)
Each packet contains a virtual circuit identifier
instead of destination address
No routing decisions required for each packet
Clear request to drop circuit
Not a dedicated path
Virtual
Circuit
Diagram
Virtual Circuits v Datagram

Virtual circuits


Network can provide sequencing and error control
Packets are forwarded more quickly


Less reliable


No routing decisions to make
Loss of a node looses all circuits through that node
Datagram

No call setup phase


Better if few packets
More flexible

Routing can be used to avoid congested parts of the
network
Packet Size
Circuit v Packet Switching

Performance



Propagation delay
Transmission time
Node delay
Event Timing