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Transmission Systems and the Telephone Networks. Transmission Systems and the Telephone Networks. Multiplexing. Multiplexing. Multiplexing involves the sharing of network resources by several connections or information flows. The primary shared network resources is bandwidth.

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Transmission systems and the telephone networks

Transmission Systems and the Telephone Networks


Transmission systems and the telephone networks1

Transmission Systems and the Telephone Networks

Multiplexing


Multiplexing

Multiplexing

  • Multiplexing involves the sharing of network resources by several connections or information flows.

    • The primary shared network resources is bandwidth.

      • measured in Hz for analog transmission.

      • measured in bps for digital transmission.

(a)

(b)

A

A

A

A

Trunk

group

B

B

B

B

MUX

DEMUX

C

C

C

C


Frequency division multiplexing

A

f

W

0

B

f

0

W

C

f

0

W

Frequency-division multiplexing

(a) Individual signals occupy W Hz

  • Combined signal fits into

  • channel bandwidth

B

A

C

f


Uses of fdm

Uses of FDM

  • In FDM the user information can be analog or digital form and that the information from all users flows simultaneously.

  • Applications:

    • Broadcast radio

      • FM: 200 kHz band

      • AM: 10 kHz band

    • Broadcast and cable TV

    • Cellular telephony

      • 25 to 30 kHz frequency slot


The l carrier

The L-carrier


Time division multiplexing

A1

A2

t

0T

6T

3T

B1

B2

t

6T

3T

0T

C1

C2

t

0T

6T

3T

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

A2

B2

B1

C1

C2

A1

t

Time-division multiplexing

Each signal transmits 1 unit

every 3T seconds

  • Combined signal transmits 1 unit

    every T seconds


T 1 carrier system

T-1 carrier system

1

1

2

DEMUX

MUX

2

. . .

. . .

b

24

1

22

24

2

23

b

. . .

24

frame

24

(1+248) bits/frame8000 frames/second = 1.544 Mbps


North american digital hierarchies

Primary

Multiplex

Eg. Digital

Switch

24 chan PCM

M23

Multiplex

x7

M12

Multiplex

x4

DS2 6.312 Mbps

DS3 44.736 Mbps

DS1 1.544 Mbps

1

M13

Multiplex

DS3 44.736 Mbps

28

North American digital hierarchies

DS 1, which corresponds to the output of a T-1 multiplexer,

became the basic building block.


European digital hierarchy

CEPT 1

Primary

Multiplex

Eg. Digital

Switch

30 chan PCM

3rd order

Multiplex

x4

CEPT 4

2nd order

Multiplex

x4

4th order

Multiplex

x4

34.368 Mbps

8.448 Mbps

2.048 Mbps

139.264 Mbps

European Digital Hierarchy


Timing in a tdm multiplexer

Timing in a TDM multiplexer

t

5

4

3

2

1

5

4

3

2

1


Bit slip

Bit slip

  • Bit slip: the slow input will fail to produce its input bit.

  • The late bit will be viewed as an early arrival in the next interval.

  • The slow stream will alternate between being late, undergoing a bit slip, and then being early.

  • Bits that are arriving faster than they can be sent out, will accumulate at the multiplexer and eventually be dropped.


Asynchronous multiplexer

Asynchronous multiplexer

  • Time-division multiplexers have traditionally been designed to operate at a speed higher than the combined speed of the inputs.

    • The frame structure of the multiplexer output signal contains bits that are used to indicate to the receiving multiplexer that a slip has occurred.

    • The introduction of these extra bits implies that the frame structure of the output stream is not exactly synchronized to the frame structure of all the input streams.

  • Before we extract an individual input stream, we need to demultiplex the entire combined signal and make the adjustment for slips.


Transmission systems and the telephone networks2

Transmission Systems and the Telephone Networks

SONET


Sonet sdh

SONET/SDH

  • To meet the need for standards to interconnect optical transmission systems, the Synchronous Optical Network (SONET) standard was developed in North America.

  • The CCITT has developed a corresponding set of standards called Synchronous Digital Hierarchy (SDH).

  • SONET and SDH form the basis for current high-speed backbone networks.


Sonet

SONET

  • The basic building block of the SONET hierarchy is the synchronous transport signal level-1 (STS-1) and has a bit rate of 51.84 Mbps.

  • Each STS-n electrical signal has a corresponding optical carrier level-n (OC-n) signal.

    • The bit format of STS-n and OC-n signal is the same except for the use of scrambling in the optical signal.

  • A higher-level signal is obtained through the interleaving of bytes from the lower-level component signals.


Transmission systems and the telephone networks

SDH

  • The SDH standard refers to synchronous transfer modules-n (STM-n) signals and begins at a bit rate of 155.52 Mbps.

  • The SDH STM-1 signal is equivalent to the SONET STS-3 signal.

  • The STM-1 signal accommodates the CEPT-4 signal in the CCITT digital hierarchy.


Sonet digital hierarchy

SONET digital hierarchy


Sonet multiplexing

SONET multiplexing

DS1

Low-Speed

Mapping

Function

DS2

STS-1

CEPT-1

51.84 Mbps

Medium

Speed

Mapping

Function

STS-1

DS3

44.736

OC-n

STS-n

STS-3c

MUX

Scrambler

E/O

STS-1

High-

Speed

Mapping

Function

CEPT-4

STS-1

STS-1

139.264

STS-3c

STS-1

High-

Speed

Mapping

Function

STS-1

Tributary : the component streams

that are multiplexed together.

ATM

STS-1

150 Mbps


Sonet add drop multiplexing

SONET add-drop multiplexing

(a) pre-SONET multiplexing

DEMUX

DEMUX

MUX

MUX

insert

tributary

remove

tributary

(b) SONET Add-Drop multiplexing

ADM

DEMUX

MUX

insert

tributary

remove

tributary


Sonet ring network

SONET ring network

a

ADM

OC-3n

OC-3n

b

c

STS-n

STS-n

ADM

ADM

OC-3n

physical loop net


Configuration of logical networks

a

c

b

logical fully-connected net

Configuration of logical networks

a

OC-3n

OC-3n

b

c

OC-3n

3 ADMs connected in

physical ring topology


Survivability in a sonet ring

Survivability in a SONET ring

a

a

b

d

b

d

c

c

Loop-around in response to fault

Dual ring

In normal operation, one fiber is in working mode,

while another is in a protect mode.


Sonet ring structure

SONET ring structure

The capability to manage bandwidth flexibly and to respond quickly to faults has altered the topology of long-distance and metropolitan area networks from a mesh of point-to-point links to interconnected ring networks.

Regional

Ring

Metro

Ring

Inter-Office

Rings


Section line and path layers

STS

PTE

STS

PTE

LTE

LTE

SONET

Terminal

SONET

Terminal

STE

STE

STE

Mux

Mux

reg

reg

reg

Section

Section

STS Line

STS-1 Path

section

optical

Section, line, and path layers

(a)

STE: Section Terminating Equipment, e.g. a repeater

LTE: Line Terminating Equipment, e.g. a STS-1 to STS-3 multiplexer

PTE: Path Terminating Equipment, e.g. an STS-1 multiplexer

(b)

path

path

line

line

line

line

section

section

section

section

section

section

optical

optical

optical

optical

optical

optical


Sonet sts 1 frame format

SONET STS-1 frame format

90 bytes

87B

B

B

B

Section

Overhead

3rows

Information

Payload

9 Rows

Line

Overhead

6rows

125 s

Transport

overhead

8  9  90  8000 = 51.84 Mbps


The synchronous payload envelope

The synchronous payload envelope

first octet

Pointer

Frame

k

87 columns

Synchronous

Payload

Envelope

9 rows

Pointer

last octet

Frame

k+1

first column is path overhead


Synchronous multiplexing in sonet

Byte

Interleave

Synchronous multiplexing in SONET

STS-1

STS-1

STS-1

STS-1

Map

STS-1

STS-1

STS-3

STS-1

STS-1

Map

STS-1

STS-1

STS-1

STS-1

Map

Incoming

STS-1 Frames

Synchronized New

STS-1 Frames


Virtual tributary

Virtual tributary

  • Various mapping have been defined to combine lower-speed tributaries of various formats into standard SONET stream.

  • A STS-1 signal can be divided into virtual tributary signals.

    • In each SPE, 84 columns are set aside and divided into seven groups of 12 columns.

    • Each group constitutes a virtual tributary and has a bit rate of 12988000=6.912 Mbps.

    • A virtual tributary can accommodate four T-1 carrier signals, or three CEPT-1 signals.


Concatenation

Concatenation

  • Several STS-1 frames can be concatenated to accommodate signals with bit rates that cannot be handled by a single STS-1.

  • The suffix c is appended to the signal designation when concatenation is used.

  • Concatenated STS frames carry only one column of path overhead.

    • The SPE in an STS-3 frames has 86  3 = 258 columns of user data, whereas the SPE in an STS-3c frame carries 87  3 – 1 = 260 columns of user data.


Transmission systems and the telephone networks3

Transmission Systems and the Telephone Networks

Wavelength-Division Multiplexing


Wavelength division multiplexing

Wavelength-division multiplexing

  • Multiple information signals modulate optical signals at different optical wavelengths.

  • The resulting signals are combined and transmitted simultaneously over the same optical fiber.

  • Prisms and diffraction gratings can be used to combine and split color signals.

1

1

2

2

1

2 ,

m

Optical

fiber

Optical

MUX

Optical

deMUX

m

m


Optical signal in a wdm system

Optical signal in a WDM system


Wdm network configuration

WDM network configuration

  • Optical add-drop multiplexers have been designed for WDM systems.

  • The assignments in various multiplexer configuration can then be used to create networks with various logical topologies.

  • In these topologies a light path between two nodes is created by inserting information at an assigned wavelength at the source node, bypassing intermediate nodes, and removing the information at the destination node.


Wdm chain network

WDM chain network

c

b

d

a


Wdm ring network

WDM ring network

Through the assignment of wavelengths, it is possible to obtain logical topologies that differ from the physical topology.

a

3 ADMs

b

c


Transmission systems and the telephone networks4

Transmission Systems and the Telephone Networks

Circuit Switches


Networks links switches

Link

Switch

User n

User n-1

User 1

Networks = links + switches

Control

1

1

Network

2

2

Connection

of inputs

to outputs

3

3

N

N


Crossbar switch

1

2

N

N-1

2

N



1

Crossbar switch

  • The crossbar switch is nonblocking.

    • Connection requests are never denied because of lack of connectivity of resources.


Multistage switch

Multistage switch

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

kxn

nxk

N/n x N/n

1

1

1

kxn

nxk

N

inputs

2

2

N

outputs

N/n x N/n

kxn

nxk

2

3

3

kxn

nxk

N/n

N/n

N/n x N/n

k


It is nonblocking if k 2 n 1

It is nonblocking if k = 2n - 1

kxn

nxk

N/n x N/n

1

1

1

n-1

busy

N/n x N/n

Desired

input

n-1

Desired

output

nxk

kxn

j

n-1

busy

m

N/n x N/n

n+1

N/n x N/n

2n-2

nxk

kxn

N/n x N/n

N/n

free path

free

path

N/n

2n-1


Time slot interchange

Time-slot interchange

1

From

TDM

DeMUX

2

24

24

1

23

2

Read slots in

permuted order

2

1

24

23

1

To

TDM

MUX

2

24

125 sec

Maximum number of slots =

2 x memory cycle time


Hybrid switches

Hybrid switches

kxn

nxk

N/n x N/n

1

1

1

nxk

input TDM frame with n slots

N

inputs

2

nxk

n

3

2

1

nxk

N/n

output TDM frame with k slots


Flow of slots between switches

Flow of slots between switches

first slot

first slot

kxn

nxk

N/n x N/n

1

1

1

kxn

nxk

2

2

N/n x N/n

2

kxn

nxk

N/n x N/n

N/n

N/n

k

kth slot

kth slot


Time space time switches

Time-space-time switches

Space Stage

TSI Stage

TSI Stage

kxn

TDM

n slots

nxk

TDM

k slots

TDM

k slots

1

1

kxn

nxk

n slots

N/n x N/n

Time-Shared

Space

Switch

2

2

N

outputs

N

inputs

kxn

nxk

n slots

3

3

kxn

nxk

n slots

N/n

N/n


Example of a tst switch

Example of a TST switch

B2

A2

B1

A1

B1

A1

A1

A1

C1

C1

3x2

2x3

1

1

B1

D1

D1

B1

3x2

C2

D1

C1

C1

D2

D1

2x3

2

2

(A, B, C, D) to (C, A, D, B)


Transmission systems and the telephone networks5

Transmission Systems and the Telephone Networks

The Telephone Network


Circuit switching

Circuit switching


Telephone call setup

Telephone call setup

  • Three phases of connection-oriented communications

  • call setup

  • message transfer

  • call release

Source

Signal

Go

Ahead

Signal

Message

Release

Signal

Destination


Routing in a local area

Routing in a local area

4

C

D

2

3

5

B

A

1


U s system

U.S. system

  • The telephone network is divided into local access and transport areas (LATAs) that are served by the local exchange carriers (LECs).

  • The LECs consists of

    • Regional bell operating companies (RBOCs)

      • Such as BellSouth, Southwestern Bell, Bell Atlantic, …

    • Independent carriers

      • Such as GTE

  • Communication between LATAs is provided by separate independent interexchange carriers (IXCs).

    • Such as AT&T, MCI


Routing between two latas

Routing between two LATAs

net 1

net 2

LATA 1

LATA 2

LATA: local access and transport area


Access transmission facilities

Access transmission facilities

Pedestal

local telephone office

Serving Area I/f

distribution cable

Distribution Frame

Switch

Serving Area I/f

feeder cable


Two and four wire connection

Two and four-wire connection

Transmit pair

Original signal

Received signal

Hybrid transformer

Echoed signal

Receive pair


Digital cross connect system

Digital cross-connect system

Digital

cross-connect

System

Channel-switched traffic

(digital leased lines)

Local

analog

Tie lines

Foreign exchange

Local

digital

Local

Switch

Digital

trunks

Circuit-switched traffic

A DCC system is digital time-division switch that is used to manage the longer-term flow in a network.


Dcc and sonet

DCC and SONET

ADM

ADM

ADM

Physical SONET

Topology using

ADMs and DCCs

ADM

ADM

ADM

DCC

Logical Topology

Switches see this

topology


End to end digital services

End-to-end digital services

  • In the early 1980s the CCITT developed the ISDN set of standards for providing end-to-end digital connectivity.

  • The ISDN standards defines two interfaces between the user and the network.

  • The interfaces is to provide access to various services that are possibly supported by different networks.


Transmission systems and the telephone networks

ISDN

Circuit

Switched

Network

Channel

Switched

Network

Private

BRI

BRI

Packet

Switched

Networks

PRI

PRI

Signaling

Network

Basic Rate Interface (BRI): 2B+D

Primary Rate Interface (PRI): 23B+D


Control plane and user plane

Control plane and user plane

  • The telephone network requires two basic functions:

    • Signaling to establish and release the call

    • End-to-end transmission to transfer the information between the users

  • ISDN views these two function as separate.

  • The set of protocols that implement signaling in the telephone network constitute the control plane.

    • The signaling function involved the exchange of signaling messages between users and the network and between switches in the network.

    • This process uses all the layers in the OSI reference model.

  • The set of protocols that implement the transfer of information between users is called the user plane.


Transmission systems and the telephone networks6

Transmission Systems and the Telephone Networks

Signaling


Stored program control switch

Stored-program control switch

SPC

Signaling Message

Control


Common channel signaling

Common channel signaling

Office A

Office B

Trunks

Switch

Switch

Modem

Modem

Processor

Processor

Signaling


Signaling network

SCP

Signaling network

STP

STP

STP

STP

SSP

SSP

Signaling Network

Transport Network

SSP = Service switching point (signal to message)

STP = Signal transfer point (message transfer)

SCP = Service control point (processing)


Intelligent network

External

Database

Signaling

Network

Intelligent

Peripheral

SSP

SSP

Transport Network

Intelligent Network

  • Intelligent network: an enhanced signaling network that provides a broad array of services:

    • Identification of the calling person

    • Screening out of certain callers

    • Callback of previous callers

    • Voice mail

    • Personal mobility


Ss7 network architecture

Application Layer

Presentation Layer

TUP

TCAP

ISUP

Session Layer

SCCP

Transport Layer

Network Layer

MTP Level 3

Data Link Layer

MTP Level 2

Physical Layer

MTP Level 1

SS7 network architecture


Transmission systems and the telephone networks

SS7

  • The signaling network #7 network is a packet switching network that controls the setup, managing, and releasing of telephone calls.

  • SS7 architecture uses “parts” instead of “layers.”

  • The message transfer part (MTP) corresponding to the lower three layers of the OSI model.

    • MTP level 1 corresponds to the physical layer of the signaling links.

    • MTP level 2 ensures that messages are delivered reliably across a signaling link.

    • MTP level 3 ensures that messages are delivered between signaling points across the SS7 network.


Transmission systems and the telephone networks

SS7

  • The ISDN user part (ISUP) perform the basic setup, management, and release of telephone calls.

  • The signaling connection control part (SCCP) allows applications to be addressed by building on the MTP to provide connectionless and connection-oriented service.

  • The transaction capabilities part (TCAP) defines the messages and protocols that are used to communicate between applications.


Transmission systems and the telephone networks7

Transmission Systems and the Telephone Networks

Traffic and Overload Control in Telephone Networks


Concentration

Many

Lines

Fewer

Trunks

Concentration

  • The number of trunks in use varies randomly over time but is typically much smaller than the total number of lines.

  • A multiplexer is introduced to concentrate the requests for connections over a smaller number of trunks.

  • While the objective is to maximize the use of the trunks, typically a maximum acceptable probability of blocking is specified.


Number of trunks in use

N(t)

all trunks busy

t

1

2

trunk #

3

4

5

6

7

Number of trunks in use


Traffic analysis

Traffic analysis

  • It has been found that user requests for connections take place according to a Poisson process with connection request rate  calls/second.

  • The time that a user maintain a connection is called the holding time X.

  • The offered loada is defined as the total rate at which work is offered:

    a =  calls/second  E[X] seconds/call (Erlang)

  • One Erlang corresponds to an offered load that would occupy a single trunk 100% of the time.


Erlang b formula

Erlang B formula

  • The blocking probability Pb for a system with c trunks and offered load a is given by

  • The utilization is defined by as the number of trunks in use divided by the total number of trunks.

  • The sharing of network resources becomes more efficient as the size of the system increases.

  • The improvement in system performance that result from aggregating traffic flow is called multiplexing gain.


Blocking prob vs of trunks

# trunks

Blocking Probability

10

9

8

7

1

6

2

3

4

5

offered load

Blocking prob. vs. # of trunks

For a typical 1% blocking probability, 16 trunks for 9 Erlang is more efficient than 4 trunks for 1 Erlang.


Routing control

Routing control

  • Routing control refers to the procedures for assigning paths in a network to connections.

  • Economics considerations lead to

    • providing direct trunks between switches that have large traffic flows between them and

    • providing indirect paths through tandem switches for smaller flows.

  • A hierarchical approach to routing is desirable when the volume of traffic between switches is small.


Hierarchical routing control

Hierarchical routing control

Trunk

group

Tandem Switch 2

A

Tandem Switch 1

D

B

E

C

F

F

E

D

C

A

B

10 Erlangs between each pair

90 Erlangs

18 trunks is needed for 10 Erlang, 1%

No.of total trunks = 18  9 = 162

106 trunks is needed for 90 Erlang, 1%

No.of total trunks = 106


Sensitivity problem

Sensitivity problem

  • The increase in efficiency of trunk utilization that result from the increased offered load introduces a sensitivity problem.

    • The higher efficiency implies that a smaller number of spare circuits is required to meet the same blocking probability.

    • The smaller number of spare circuits makes the system more sensitive to traffic overload condition.

    • That is, the blocking probability for large systems is quite sensitive to traffic overloads,

    • Hence, the selection of the trunk groups must provide a margin for some percentage of overload.


Alternative routing

Switch

Switch

Alternative routing

  • A connection request first attempts to engage in a trunk in the direct path.

  • If no trunk is available in the direct path, then it attempts to secure an alternative path through the tandem switch.

  • The number of trunks in the direct route is selected to have a high usage.

  • The number of trunks in the alternative route needs to be selected so that the overall blocking probability meets the requirement.

Tandem Switch

Alternate Route

High Usage Route


Routing scenario

Routing scenario

Tandem Switch 2

Tandem Switch 1

Alternate Routes for B-E, C-F

Switch D

Switch A

Switch E

Switch B

High Usage Route B-E

Switch F

Switch C

High Usage Route C-F

The traffic from A to D must receive a certain degree of preferential access to the trunks between the tandem switches.


Dynamic nonhierarchical routing

Dynamic nonhierarchical routing

  • The first route attempt consists of a direct route.

  • A certain number of tandem switches provide two-hop alternative routes.

  • The order in which tandem switches are attempted is determined dynamically according to the state of the network.

Tandem Switch 3

Tandem Switch 2

Tandem Switch 1

Alternate Routes

Switch A

Switch B

High Usage Route


Traffic overload

Traffic overload

Network Capacity

Carried Load

Offered Load

As network resources become scarce, many call attempts manages to seize only some of the resources they need and end up uncompleted.


Overload control

Overload control

  • Traffic and routing control are concerned with the handling of traffic flows during normal predictable network conditions.

  • Overload control addresses the handling of traffic flows during unexpected or unusual conditions.

    • Such as holidays, catastrophes, equipment failure

  • Several types of actions can be taken to control overload.

    • Allocation of additional resources

    • Utilize the available resources efficiently

      • Disallowing alternative routes

  • Overload controls make extensive use of the signaling system.

    • This dependence is another potential source of serious problem.

    • Overload control mechanisms are also essential for the signaling system.


Transmission systems and the telephone networks8

Transmission Systems and the Telephone Networks

Cellular Telephone Networks


Cellular telephone

Cellular telephone

  • Cellular telephone networks extend the basic telephone service to mobile users with portable telephones.

  • By reducing the power level, the coverage area can be reduced and the frequency band can then be reused in adjacent areas.

    • The frequency-reuse principle forms the basis for cellular radio communications.

  • A region is divided into a number of geographical areas called cells.

  • Cell areas are established based on the density of subscribers.

    • Large cells are used in rural areas and small cells are used in urban areas.

  • As a mobile user moves from one cell to another, a handoff procedure is carried out that transfer the connection from one base station to the other, allowing the call to continue w/o interruption.


Cellular network

2

7

3

1

6

4

5

2

2

7

3

7

3

1

1

6

4

6

4

5

5

Cellular network

  • Placed near the center of each cell, the base station has an antenna that is used to communicate with mobile users in its vicinity.

  • Base stations are connected by a wireline transmission link or by point-to-point microwave radio to a telephone switch, called the mobile switching center (MSC) or the mobile telephone switching office (MTSO).

  • The MSC handles connections between cells and to the PSTN.

  • The set of available radio channels are reused following the frequency-reuse pattern.

    • Seven disjoint sets of frequency channels are reused as shown.


Transmission systems and the telephone networks

AMPS

  • AMPS allocates 824 to 849 MHz band to transmissions from the mobile to the base station (reverse channels), and the band 869 to 894 MHz to transmission from the base station to the mobile (forward channels).

  • A 30 kHz channel carries one voice signal use.

  • The total number of channels available in each direction is 25 MHz / 30 kHz = 832 channels.

  • The bands are divided equally between two independent service provider, so each cellular network has 416 bidirectional channels.

  • Each forward and reverse channel pair is separated by 45MHz.


Setup channels

Setup channels

  • A small number of channels within each cell have been designated to function as setupchannels.

    • The AMPS allocates 21 channels for this purpose.

    • They are used in the setting up and handling off of calls.

  • When a mobile user turn on his/her unit, the unit scans the setup channels and selects the one with the strongest signal.

  • It then monitors this setup channel as long as the signal remains above a certain threshold.


Calls to the mobile

Calls to the mobile

  • The MSC sends the call request to all of its base stations, which in turn broadcast the request in all the forward setup channels, specifying the mobile user’s telephone number.

  • When the desired mobile unit receive the request message, it replies by identifying itself on a reverse setup channel.

  • The corresponding base station forwards the reply to the MSC and assigns a forward and reverse voice channel.

  • The base station instructs the mobile station to begin using these channels, and the mobile telephone is rung.


Call from the mobile

Call from the mobile

  • The mobile unit sends a request in the reverse setup channel.

  • In addition to its phone number and the destination phone number, the mobile station also transmits a serial number and possible password information used by the MSC to validate the request.

  • The call setup involves consulting the home location register, a database that contains information about subscribers.

  • The validation involves the authentication center, which contains authentication information about subscribers.

  • The MSC then establishes the call to the PSTN by using conventional telephone signaling.


Handoff

Handoff

  • As the call proceeds, the signal level is monitored by the base station.

  • If the signal level falls below a specified threshold, the MSC is notified and the mobile station is instructed to transmit on the setup channel.

  • All base stations in the vicinity are instructed to monitor the strength of the signal level in the prescribed setup channel.

  • The MSC uses this information to determine the best cell to which the call should be handed off.

  • The MSC then releases its connection to the first base station and establishes a connection to the new base station.

  • The connection is interrupted for the brief period that is required to execute the handoff.


Roaming

Roaming

  • Business arrangements must be in place between the home and visited cellular service providers.

  • When the roamer enters a new area, the roamer registers in the area by using the setup channels.

  • The MSC in the new area uses the information provided by the roamer to request authorization from the roamer’s home location register.

  • The visitor location register contains information about visiting subscribers.


Components of a cellular network

Components of a cellular network

BSS

BSS

MSC

SS#7

STP

HLR

VLR

wireline

terminal

EIR

PSTN

AC

MSC= mobile switching center

PSTN= public switched telephone network

STP= signal transfer point

VLR= visitor location register

AC= authentication center

BSS= base station subsystem

EIR= equipment identity register

HLR= home location register


Gsm system

GSM system

  • The base station subsystem (BSS) consists of

    • the base transceiver station (BTS) and

      • The BTS consists of the antenna and transceiver to communicate with the mobile telephone.

      • The BTS is also concerned with the measurement of signal strength.

    • the base station controller (BSC).

      • The BSC manages the radio resources of one or more BTSs.

      • The BSC is concerned with the setup of frequency channels as well as with the handling of handoffs.

  • Each BTS communicates with the MSC thru the BSC


Gsm signaling protocol stacks

GSM signaling protocol stacks

Um

Abis

A

CM

CM

MM

MM

RRM

RRM

RRM

RRM

SCCP

SCCP

MTP Level 3

MTP Level 3

MTP Level 2

MTP Level 2

LAPDm

LAPDm

LAPD

LAPD

64 kbps

64 kbps

64 kbps

64 kbps

radio

radio

mobile station

MSC

base station controller

base transceiver station


Transmission systems and the telephone networks9

Transmission Systems and the Telephone Networks

Satellite Cellular Networks


Satellite communications

Satellite communications

  • The period of time T that it takes a satellite to rotate around the earth is given by 2[A3/g]1/2 seconds.

    • A is the earth’s radius plus the altitude of the satellite.

    • g is the gravitational constant.

  • The rotation of the geostationary earth orbit satellite (GEOS) is synchronized to that of the earth, so the satellite appears stationary with respect to the earth.

  • Two separate frequency bands are used for communications in the uplink and downlink direction to minimize interference.


Spot beam

Spot beam

  • Advances in antenna directionality allowed the introduction of spot beam transmission where the signals can be focused in smaller areas.

  • The use of spot beams allows the frequency band to be reused in geographically separate areas.

  • The introduction of an onboard switch in the satellite allows information to be switched between different spot beams.

  • However, the number of spot beams that can be implemented in a single satellite is limited.


Cellular architecture

Cellular architecture

  • Low-earth orbit satellites (LEOS) uses a cellular architecture to provide global coverage.

  • The altitude of LEOS is typically 750~2000 km, corresponding to rotation periods of about 2 hrs.

  • Two basic approaches to how the cells in a slice of area are defined :

    • Satellite-fixed approach

      • The cells are defined with respect to the satellite.

      • The satellite beam directions are fixed, and the earth stations must adjust to the passing satellite.

    • Earth-fixed approach

      • The cells are fixed with respect to the earth.

      • The satellite closest to the center of a cell steers its beam to a fixed location in the cell.


Cellular structure in leos network

Cellular structure in LEOS network

satellite

motion


Leo constellation

LEO constellation

  • Each LEOS has multiple spot beams so it covers multiple cells.

  • Frequencies can then be reused in nonadjacent cells.

  • Complete coverage of the world is provided by deploying a sufficient number of satellite groups in different orbital planes.

  • Each LEOS acts a switching node, and each satellite is connected to nearby satellites by intersatellite links (ISLs).

  • Examples of LEOS cellular network:

    • Iridium system

    • Teledesic Network


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