Transcript Chapter 4 Lecture Presentation

Comparison of the three switching
External virtual circuit, Internal virtual
circuit

In this case a user requests a virtual circuit and
a dedicated route through the network is
constructed. All packets follow the same route
External virtual circuit, Internal
datagram

In this case, the network handles each packet
separately. Different packets for the same external
virtual circuit may take different routes as shown in
Fig. The network buffers packets, if necessary, so that
they are delivered to the destination in the proper
order.
Chapter 4
Circuit-Switching
Networks
The Telephone Network
Telephone Call





User requests connection
Network signaling
establishes connection
Speakers converse
User(s) hang up
Network releases
connection resources
Source
Signal
Signal
Destination
Go
ahead
Signal
Message
Release
Call Routing
(a)
C
2
A
4
D
3
1

5
Local calls routed
through local network
B

(b)
Net 1
Net 2
LATA 1
LATA 2
Long distance calls
routed to long distance
service provider
Telephone Local Loop
Local Loop: “Last Mile”
 Copper pair from telephone to CO
 Pedestal to SAI to Main
Distribution Frame (MDF)
 2700 cable pairs in a feeder cable
 MDF connects
Pedestal


Serving
area
interface
Distribution frame
Local telephone office
Distribution
cable
Serving
area
interface
voice signal to telephone switch
DSL signal to routers
Switch
Feeder
cable
For
interesting pictures of switches & MDF, see
web.mit.edu/is/is/delivery/5ess/photos.html
www.museumofcommunications.org/coe.html
Integrated Services Digital Network
(ISDN)
ISDN - Integrated Services Digital Network
 Telephone services -> Telecommunication
services
 Used for voice, image and data

ISDN protocols


E - series for Telephone network and ISDN
I - series for ISDN concepts, aspects and
interfaces
8
Fundamentals

Types of channels


Bearer channel (B-channel=64 kb/s) clear pipe for
data
Delta channel (D-channel, 16 kb/s or 64 kb/s) call
signaling information:




who is calling
type of call
calling what number
Service types


Basic Rate Interface (2 B channels + 1 D channel (16
kb/s))
Primary Rate Interface (30 B channels + 1 D channel
(64 kb/s))
9
Integrated Services Digital
Network (ISDN)




First effort to provide end-to-end digital connections
B channel = 64 kbps, D channel = 16 kbps
ISDN defined interface to network
Network consisted of separate networks for voice, data, signaling
Circuitswitched
network
BRI
PRI
Private
channelswitched
network
Packetswitched
networks
Signaling
network
Basic rate interface
(BRI): 2B+D
BRI
PRI
Primary rate interface
(PRI): 23B+D
Advantages of ISDN

Digital


Speed



2 seconds
Bandwidth on Demand


128 kb/s (160 kb/s) for BRI
1920 kb/s (2048 kb/s) for PRI
Fast call setup


reliable connection
adding new channels to the bundle of channels
Multiple devices

phone, fax, PC, videoconferencing system, router, terminal
adapter,.. each with its own sub-address
11
Example
A basic rate ISDN transmission system uses TDM.
Frames are transmitted at a rate of 4000 frames/
second. Sketch a possible frame structure. Recall that
basic rate ISDN provides two 64 kbps channels and
one 16 kbps channel. Assume that one-fourth of the
frame consists of overhead bits.
Solution

Assuming that the 16 kbps “D” channel is followed
by the two 64 kbps “B” channels and then the
overhead (which is not necessarily the case), in the
time domain, the frame could look as follows:
Chapter 4
Circuit-Switching
Networks
Signaling
Setting Up Connections
Manually
 Human Intervention
 Telephone



Voice commands &
switchboard operators
Transport Networks

Automatically
 Management Interface
Order forms &
dispatching of
craftpersons

Operator at console sets
up connections at
various switches
Automatic signaling

Request for connection
generates signaling
messages that control
connection setup in
switches
Stored-Program Control Switches


SPC switches (1960s)
 Crossbar switches with crossbars built from relays
that open/close mechanically through electrical control
 Computer program controls set up opening/closing of
crosspoints to establish connections between switch
inputs and outputs
Signaling required to coordinate path set up across
network
SPC
Control
Signaling Message
Message Signaling



Processors that control switches exchange signaling
messages
Protocols defining messages & actions defined
Modems developed to communicate digitally over
converted voice trunks
Office A
Office B
Trunks
Switch
Processor
Switch
Modem
Modem
Signaling
Processor
Signaling Network




Common Channel Signaling (CCS) #7 deployed in 1970s to control call setup
Protocol stack developed to support signaling
Signaling network based on highly reliable packet switching network
Processors & databases attached to signaling network enabled many new
services: caller id, call forwarding, call waiting, user mobility
Access Signaling
Dial tone
SSP
Internodal Signaling
Signaling System 7
STP
STP
STP
STP
Signaling Network
Transport Network
SSP = service switching point (signal to message)
STP = signal transfer point (packet switch)
SCP = service control point (processing)
SCP
SSP
Signaling System Protocol Stack

Application layer
Presentation layer
TUP
TCAP
ISUP

Session layer
Transport layer
Network layer
SCCP

MTP level 3

Data link layer
MTP level 2

Physical layer
ISUP = ISDN user part
SSCP = signaling connection control part
TUP = telephone user part
MTP level 1
Lower 3 layers ensure
delivery of messages to
signaling nodes
SCCP allows
messages to be
directed to applications
TCAP defines
messages & protocols
between applications
ISUP performs basic
call setup & release
TUP instead of ISUP in
some countries
MTP = message transfer part
TCAP = transaction capabilities part
Calls, Sessions, & Connections
Call/Session
 An agreement by two end
parties to communicate




Answering a ringing phone
(after looking at caller ID)
TCP three-way handshake
Applies in connection-less &
connection-oriented
networks
Session Initiation Protocol
(SIP) provides for
establishment of sessions in
many Internet applications
Connection
 Allocation of resources to
enable information transfer
between communicating
parties



Path establishment in
telephone call
Does not apply in
connectionless networks
ReSerVation Protocol
(RSVP) provides for resource
reservation along paths in
Internet
Network Intelligence





Intelligent Peripherals provide additional service capabilities
Voice Recognition & Voice Synthesis systems allow users to access
applications via speech commands
“Voice browsers” currently under development (See: www.voicexml.org)
Long-term trend is for IP network to replace signaling system and provide
equivalent services
Services can then be provided by telephone companies as well as new
types of service companies
External
Database
SSP
Signaling
Network
Intelligent
Peripheral
SSP
Transport Network
Chapter 4
Circuit-Switching
Networks
Traffic and Overload Control in
Telephone Networks
Traffic Management & Overload
Control


Telephone calls come and go
People activity follow patterns



Outlier Days/ festivals are extra busy



Mid-morning & mid-afternoon at office
Evening at home
Hari Raya, CNY, Mother’s Day, Christmas, …
Disasters & other events cause surges in traffic
Need traffic management & overload control
Traffic concentration
Many
lines

Traffic fluctuates as calls initiated & terminated



Call requests always met is too expensive
Call requests met most of the time cost-effective
Switches concentrate traffic onto shared trunks


Driven by human activity
Providing resources so


Fewer
trunks
Blocking of requests will occur from time to time
Traffic engineering provisions resources to meet blocking
performance targets
Fluctuation in Trunk Occupancy
Number
of busy trunks
N(t)
All trunks busy, new call requests blocked
t
active
Trunk number
1
active
2
active
3
active
4
5
6
7
active
active
active
active
active
active
Modeling Traffic Processes

Find the statistics of N(t) the number of calls in the system
Model
 Call request arrival rate: l requests per second
 In a very small time interval D,
 Prob[ new request ] = lD
 Prob[no new request] = 1 - lD
 The resulting random process is a Poisson arrival process:
(λT)ke–λT
Prob(k arrivals in time T) =
k!


Holding time: Time a user maintains a connection
 X a random variable with mean E(X)
Offered load: rate at which work is offered by users:
 a = l calls/sec * E(X) seconds/call (Erlangs)
Blocking Probability & Utilization



c = Number of Trunks
Blocking occurs if all trunks are busy, i.e. N(t)=c
If call requests are Poisson, then blocking probability
Pb is given by Erlang B Formula
ac
Pb =
c
∑ ak
k=0

c!
k!
The utilization is the average # of trunks in use
Utilization = λ(1 – Pb) E[X]/c = (1 – Pb) a/c
Blocking Performance
a
To achieve 1% blocking probability:
a = 5 Erlangs requires 11 trunks
a = 10 Erlangs requires 18 trunks
Multiplexing Gain
Load
Trunks@1%
Utilization
1
5
0.20
2
7
0.29
3
8
0.38
4
10
0.40
5
11
0.45
6
13
0.46
7
14
0.50
8
15
0.53
9
17
0.53
10
18
0.56
30
42
0.71
50
64
0.78
60
75
0.80
90
106
0.85
100
117
0.85



At a given Pb, the
system becomes more
efficient in utilizing
trunks with increasing
system size
Aggregating traffic
flows to share centrally
allocated resources is
more efficient
This effect is called
Multiplexing Gain
Routing Control





Routing control: selection of connection paths
Large traffic flows should follow direct route because they are
efficient in use of resources
Useful to combine smaller flows to share resources
Example: 3 close CO’s & 3 other close COs
10 Erlangs between each pair of COs
(a)
(b)
A
D
B
E
C
F
10 Erlangs between each pair
17 trunks for 10 Erlangs
9x17=153 trunks
Efficiency = 90/153=53%
Trunk
group
Tandem
switch 1
A
B
C
Tandem
switch 2
D
E
F
90 Erlangs when combined
106 trunks for 90 Erlangs
Efficiency = 85%
Alternative Routing
Tandem
switch
Alternative route
Switch
Switch
High-usage route





Deploy trunks between switches with significant traffic volume
Allocate trunks with high blocking, say 10%, so utilization is high
Meet 1% end-to-end blocking requirement by overflowing to
longer paths over tandem switch
Tandem switch handles overflow traffic from other switches so it
can operate efficiently
Typical scenario shown in next slide
Typical Routing Scenario
Tandem
switch 2
Tandem
switch 1
Alternative routes
for B-E, C-F
Switch D
Switch A
Switch E
Switch B
High-usage route B-E
Switch C
Switch F
High-usage route C-F
Dynamic Routing
Tandem
switch 1
Tandem
switch 2
Tandem
switch 3
Alternative routes
Switch B
Switch A
High-usage route



Traffic varies according to time of day, day of week
 East coast of North America busy while West coast idle
Network can use idle resources by adapting route selection
dynamically
 Route some intra-East-coast calls through West-coast switches
Try high-usage route and overflow to alternative routes
Overload Control
Carried load
Network capacity
Offered load
Overload Situations
 Holidays/festival
celebration
 Catastrophes
 Network Faults
Strategies
 Direct routes first
 Outbound first
 Code blocking
 Call request pacing
Chapter 4
Circuit-Switching
Networks
Cellular Telephone Networks
Radio Communications




1900s: Radio telephony demonstrated
1920s: Commercial radio broadcast service
1930s: Spectrum regulation introduced to deal with
interference
1940s: Mobile Telephone Service




Police & ambulance radio service
Single antenna covers transmission to mobile users in city
Less powerful car antennas transmit to network of
antennas around a city
Very limited number of users can be supported
Cellular Communications
Two basic concepts:
 Frequency Reuse




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A region is partitioned into cells
Each cell is covered by base station
Power transmission levels controlled to minimize inter-cell
interference
Spectrum can be reused in other cells
Handoff


Procedures to ensure continuity of call as user moves from
cell to another
Involves setting up call in new cell and tearing down old one
Frequency Reuse

2
7
3

4

1
6
2
5
2
7
7
3
4
5

1
6
1
6
3
4
5

Adjacent cells may not
use same band of
frequencies
Frequency Reuse
Pattern specifies how
frequencies are reused
Figure shows 7-cell
reuse: frequencies
divided into 7 groups &
reused as shown
Also 4-cell & 12-cell
reuse possible
Note: CDMA allows
adjacent cells to use
same frequencies
(Chapter 6)
Cellular Network
Base station


BSS
Mobile Switching
Center
BSS
MSC
HLR
VLR
EIR
AC
AC = authentication center
BSS = base station subsystem
EIR = equipment identity register
HLR = home location register
STP
SS7
Wireline
terminal
PSTN
Transmits to users
on forward channels
Receives from users
on reverse channels

Controls connection
setup within cells &
to telephone network
MSC = mobile switching center
PSTN = public switched telephone network
STP = signal transfer point
VLR = visitor location register
Signaling & Connection Control

Setup channels set aside for call setup & handoff


Mobile unit selects setup channel with strongest signal &
monitors this channel
Incoming call to mobile unit







MSC sends call request to all BSSs
BSSs broadcast request on all setup channels
Mobile unit replies on reverse setup channel
BSS forwards reply to MSC
BSS assigns forward & reverse voice channels
BSS informs mobile to use these
Mobile phone rings
Mobile Originated Call







Mobile sends request in reverse setup channel
Message from mobile includes serial # and possibly
authentication information
BSS forwards message to MSC
MSC consults Home Location Register for
information about the subscriber
MSC may consult Authentication center
MSC establishes call to PSTN
BSS assigns forward & reverse channel
Handoff








Base station monitors signal levels from its mobiles
If signal level drops below threshold, MSC notified &
mobile instructed to transmit on setup channel
Base stations in vicinity of mobile instructed to
monitor signal from mobile on setup channel
Results forward to MSC, which selects new cell
Current BSS & mobile instructed to prepare for
handoff
MSC releases connection to first BSS and sets up
connection to new BSS
Mobile changes to new channels in new cell
Brief interruption in connection (except for CDMA)
Roaming






Users subscribe to roaming service to use service
outside their home region
Signaling network used for message exchange
between home & visited network
Roamer uses setup channels to register in new area
MSC in visited areas requests authorization from
users Home Location Register
Visitor Location Register informed of new user
User can now receive & place calls
GSM Signaling Standard

Base station



Mobile & MSC Applications



Base Transceiver Station (BTS)
 Antenna + Transceiver to mobile
 Monitoring signal strength
Base Station Controller
 Manages radio resources or 1 or more BTSs
 Set up of channels & handoff
 Interposed between BTS & MSC
Call Management (CM)
Mobility Management (MM)
Radio Resources Management (RRM) concerns
mobile, BTS, BSC, and MSC
Example 4.1
Consider a cellular telephone system with the following
parameters:
B is the total bandwidth available for the system for
communications in both directions;
b is the bandwidth required by each channel, including guard
bands;
R is the re-use factor;
a is the fraction of channels used for set up.
a. Find an expression for the number of channels available in each cell.
b. Evaluate the number of channels in each cell for the AMPS system
if no.of channel available are 416 total channels and 21 of which
are used for call setup. The reuse factor is 7.
a)
Channel available= B/b;
No. of fraction channel left = (1-a)
Number of channels available in each cell =
channels/cell
b) Thus the number of channels per cell is:
(416 –21)/7 = 56 channels per cell
Example 4.2
Consider the AMPS system in example 4.1.
a. How many Erlangs of traffic can be supported
by the channels in a cell with a 1% blocking
probability?
b.Explain why requests for channels from handoffs
should receive priority over requests for
channels from new calls. How does this change
the Erlang load calculations?

The probability of blocking is given by the equation:

where in the AMPS system c = 56 and a is the number
of Erlangs of traffic in a cell. If the blocking probability is
10% and 1%, the maximum number of Erlangs that can
be handled is:
Pb
1%
10%
Load [Erlangs]
43.342
56.059
b) In telephone conversations, service interruption
is less acceptable than denial of service due to a
busy network.
The calculations in part (a) assumed equal
priority for all calls. If priority is given to current
calls that are being handed off, the Erlang load
calculations are more complicated.
Newly attempted calls in this scenario have a
lower priority and, thus, a higher Pb.
formula
i) Total number of channel per cell (NC)
NC = (Allocated spectrum) / (channel BW x Frequency reuse
factor)
unit = channel / cell
ii) No.of cell in the service area = Total coverage area / area of the cell.
Unit = cell
Traffic intensity of each cell can be found from table or Erlang B
chart. Depend on NC and GOS
Traffic capacity = # of cell x traffic intensity /cell ( Erlang )
Iii) Total no.of user = total traffic / traffic per user
Iv) number of call that can be made at any time = NC x no.of cell
Example 4.3
A city has an area of 3000 sq.km and is covered by a cellular
system using 7 cell per cluster. The area of a cell is 100 sq.km.
The cellular system is allocated total bandwidth of 40 MHz of
spectrum with full duplex channel bandwidth of 60 KHz. For the
GOS of 2 % and the offered traffic per user is 0.03 Erlangs,
calculate;
a)
b)
c)
d)
e)
f)
The number of cell in the city
The number of channels per cell
Traffic intensity of each cell
Traffic intensity for the city
The total number of users that can be served in the city
Number of call that can be made at any time in the city