1. Comparison of the three switching

2. 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

3. 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.

4. Chapter 4 Circuit-Switching Networks
The Telephone Network

5. Telephone Call User requests connection
Network signaling establishes connection
Speakers converse
User(s) hang up
Network releases connection resources
Signal
Source
Release
Destination Go
ahead
Message

6. Call Routing Local calls routed through local network4 C D
Local calls routed through local network 2 3 5 A B 1
Long distance calls routed to long distance service provider
(b)
LATA 1
LATA 2
Net 1
Net 2

7. 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
voice signal to telephone switch
DSL signal to routers
Local telephone office
Distribution frame
Pedestal
Feeder cable
Switch
Distribution cable
Serving area interface
For interesting pictures of switches & MDF, see
web.mit.edu/is/is/delivery/5ess/photos.html

8. Integrated Services Digital Network (ISDN)
ISDN - Integrated Services Digital Network
Telephone services -> Telecommunication services
Used for voice, image and data
E - series for Telephone network and ISDN
I - series for ISDN concepts, aspects and interfaces
ISDN protocols
used for studio quality sound and moving images
E series - international numbering plan, international ISDN addressing
I series - concepts, structures, terminology, User-network interfaces

9. Fundamentals Types of channels Service types
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))

10. 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
BRI
PRI
Circuit-
switched
network
Private
channel-
Signaling
Packet-
networks
Basic rate interface (BRI): 2B+D
Primary rate interface (PRI): 23B+D

11. Advantages of ISDN Digital Speed Fast call setup Bandwidth on Demand
reliable connection
Speed
128 kb/s (160 kb/s) for BRI
1920 kb/s (2048 kb/s) for PRI
Fast call setup
2 seconds
Bandwidth on Demand
adding new channels to the bundle of channels
Multiple devices
phone, fax, PC, videoconferencing system, router, terminal adapter,.. each with its own sub-address

12. 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.

13. 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:

14. Chapter 4 Circuit-Switching Networks
Signaling

15. Setting Up Connections
Manually
Human Intervention
Telephone
Voice commands & switchboard operators
Transport Networks
Order forms & dispatching of craftpersons
Automatically
Management Interface
Operator at console sets up connections at various switches
Automatic signaling
Request for connection generates signaling messages that control connection setup in switches

16. 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

17. Message Signaling
Processors that control switches exchange signaling messages
Protocols defining messages & actions defined
Modems developed to communicate digitally over converted voice trunks
Switch
Processor
Office B
Office A
Signaling
Modem
Trunks

18. 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
Internodal Signaling Signaling System 7
SCP
Access Signaling Dial tone
STP
STP
STP
STP
SSP
Signaling Network
SSP
Transport Network
SSP = service switching point (signal to message)
STP = signal transfer point (packet switch)
SCP = service control point (processing)

19. Signaling System Protocol Stack
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
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
ISUP
TCAP
TUP
ISUP = ISDN user part MTP = message transfer part
SSCP = signaling connection control part TCAP = transaction capabilities part
TUP = telephone user part

20. 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

21. 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:
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
SSP
Transport Network
Signaling
Network
Intelligent
Peripheral
External
Database

22. Chapter 4 Circuit-Switching Networks
Traffic and Overload Control in Telephone Networks

23. Traffic Management & Overload Control
Telephone calls come and go
People activity follow patterns
Mid-morning & mid-afternoon at office
Evening at home
Outlier Days/ festivals are extra busy
Hari Raya, CNY, Mother’s Day, Christmas, …
Disasters & other events cause surges in traffic
Need traffic management & overload control

24. Traffic concentration
Fewer
trunks
Many
lines
Traffic fluctuates as calls initiated & terminated
Driven by human activity
Providing resources so
Call requests always met is too expensive
Call requests met most of the time cost-effective
Switches concentrate traffic onto shared trunks
Blocking of requests will occur from time to time
Traffic engineering provisions resources to meet blocking performance targets

25. Fluctuation in Trunk Occupancy
Number of busy trunks
N(t) t
All trunks busy, new call requests blocked 1 2 3 4 5 6 7
Trunk number
active
active
active
active
active
active
active
active
active
active

26. 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 k!
Prob(k arrivals in time T) =
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)

27. 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
Pb = ac c! k!
∑ ak
k=0 c
The utilization is the average # of trunks in use
Utilization = λ(1 – Pb) E[X]/c = (1 – Pb) a/c

28. Blocking Performance a = 5 Erlangs requires 11 trunks
To achieve 1% blocking probability:
a = 5 Erlangs requires 11 trunks
a = 10 Erlangs requires 18 trunks

29. Multiplexing Gain
Load
Utilization 1 5
0.20 2 7
0.29 3 8
0.38 4 10
0.40 11
0.45 6 13
0.46 14
0.50 15
0.53 9 17 18
0.56 30 42
0.71 50 64
0.78 60 75
0.80 90
106
0.85
100
117
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

30. 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
Tandem switch 2
Tandem switch 1 B C A
(b)
Trunk
group E F D
90 Erlangs when combined E F D B C A
(a)
10 Erlangs between each pair
17 trunks for 10 Erlangs
9x17=153 trunks
Efficiency = 90/153=53%
106 trunks for 90 Erlangs
Efficiency = 85%

31. Alternative Routing
Switch
High-usage route
Tandem switch
Alternative 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

32. Typical Routing Scenario
High-usage route B-E
Tandem switch 1
Alternative routes for B-E, C-F
High-usage route C-F
Switch B
Switch C
Switch E
Switch D
Switch F
Tandem switch 2
Switch A

33. Dynamic Routing Traffic varies according to time of day, day of week
High-usage route
Alternative routes
Switch A
Switch B
Tandem switch 3
Tandem switch 1
Tandem switch 2
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

34. Overload Control Overload Situations Holidays/festival celebration
Catastrophes
Network Faults
Strategies
Direct routes first
Outbound first
Code blocking
Call request pacing
Carried load
Offered load
Network capacity

35. Chapter 4 Circuit-Switching Networks
Cellular Telephone Networks

36. 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

37. Cellular Communications
Two basic concepts:
Frequency Reuse
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

38. Frequency Reuse 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) 6 1 2 5 4 3 7

39. Cellular Network Base station Mobile Switching Center
Transmits to users on forward channels
Receives from users on reverse channels
Mobile Switching Center
Controls connection setup within cells & to telephone network
AC = authentication center
BSS = base station subsystem
EIR = equipment identity register
HLR = home location register
Wireline
terminal
MSC
PSTN
BSS
STP
SS7
HLR
VLR
EIR AC
MSC = mobile switching center
PSTN = public switched telephone network
STP = signal transfer point
VLR = visitor location register

40. 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

41. 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

42. 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)

43. 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

44. GSM Signaling Standard
Base station
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
Mobile & MSC Applications
Call Management (CM)
Mobility Management (MM)
Radio Resources Management (RRM) concerns mobile, BTS, BSC, and MSC

45. 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.

46. 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

47. 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?

48. 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 Load [Erlangs] 1%
10%

49. 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.

51. 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

52. 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;
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