Download TN-Lecture8

Chapter 9
Digital Switching and Networks
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1 Introduction
• Philosophically, Data Communication
and Digital Telephony are very different
specially from Signaling aspects.
1.Data Communication
• The service often used is
connectionless.
• Each data frame or packet repeats
address signaling over and over again.
• The frame or packet is an independent
entity.
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• Frame or packet is delivered to the
network and it is on its own to find its way
to the destination.
• Router is the key device, It examines the
header of a data frame or packet where
the address and control information may
be found.
• Based on the destination address in the
header, it routes the message directly to
its destination or via one or more routers
thence to the destination.
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2.Digital Telephony
• Also uses a frame concept, but address
information is not repeated after the first
frame.
• It is sent just once to set up a circuit.
• Some form of supervisory signaling is
required to maintain that circuit so set up
in a “busy condition”, until one or the other
end of the connectivity goes “on hook”.
• Switch is the key device in Digital
Telephony Networks.
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1.1 New Direction
• The radical new direction of a Digital
Telecommunication Network is to have just
one service, that is, the Data Network.
• Where, Digital Voice samples are placed in
the Payload of a Data Packet as any other
form of data.
• There will be just one, singular network
handling Voice and Data as though they
were just one form or another of information.
• This new approach is referred to as Voice
Over IP (VoIP) or Voice over Packet.
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2 Introduction To Switching
•
Switch: is a device that connects inlets to
outlets.
• Switching: is the process of connecting X to Y
rather than Z.
• We can distinguish three types of switching in
telecommunication networks:
1. Circuit Switching.
2. Packet Switching.
3. ATM Switching.
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2.1 Circuit Switching
• Circuit switching: in which a dedicated
channel path (circuit) between two stations
through a node(s) is established prior to
information transfer phase which is
terminated by releasing the path on
demand.
• The circuit guarantees the full bandwidth
of the channel and remains connected for
the duration of the communication
session.
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• The circuit functions as if the stations
were physically connected as with an
electrical circuit.
• Circuit switching is developed for
voice traffic.
• PSTN and ISDN are examples of
Circuit Switched Networks.
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2.2 Packet Switching
• Packet Switching: is a digital networking
communications method that groups all
transmitted data, regardless of content,
type, or structure, into suitably sized
blocks (variable length) with considerable
amount of overhead to compensate for
errors; these blocks are called Packets
which are transmitted independently over
shared network.
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• Each packet is passed through the
network from node to node along some
path leading from source to destination.
• At the each node, the entire packet is
received, stored briefly, and then
transmitted to the next node.
• It is used for Terminal-to-Computer and
Computer-to-Computer communication.
• LAN and WAN are examples of Packet
Switched Networks.
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2.3 ATM Switching
• ATM: It is a culmination of all development
of Circuit and Packet Switching.
• It uses fixed length packets (rather than
variable length) called Cells with little
amount of overhead.
• It uses a connection-oriented model in
which a Virtual Circuit must be
established between two endpoints before
the actual data exchange begins.
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• Developed for carriage of a complete
range of user traffic, including voice,
data, and video signals.
• ATM is a core protocol used over the
SDH/SONET backbone of the PSTN
and ISDN, but its use is declining in
favor of all IP.
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3 Digital Switching
• Switch is the key device in PSTN .
• PSTN is an example of Circuit Switched
Network.
• A Digital Switch in PSTN is divided into
two parts:
1. Space-Division Switch.
2. Time-Division Switch.
• Combination of Space-Division Switch
and Time-Division Switch construct the
Digital Switch.
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• Crossbar Switch is also known as
Space-Division Switch.
• Space Division refers to the fact that
speech paths are physically separated
in space.
• In Space-Division Switching, a metallic
path is set up between calling and called
subscriber.
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A space-division switch showing connectivity from user C to user G
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• Time-Division Switch is also known
as Time-Slot Interchanger (TSI).
• It permits a single common metallic
path to be used by many calls
separated one from the other in the
time domain.
• With Time-Division Switching, the
speech to be switched is digital in
nature (PCM).
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• Where, samples of each telephone call are
assigned time-slots, and PCM switching
involves the distribution of these slots in
sequence to the desired destination port(s) of
the switch.
• Internal functional connectivities in the switch
are carried out by digital highways.
• A highway consists of sequential speech
path time-slots.
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A time-division switch which is a time-slot interchanger (TSI). Connectivity is
from user C (in incoming times slot C) to user G (in outgoing time slot G)
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3.1 Approaches To Digital
Switching
•
A classical Digital Switch is made up of two
functional elements:
1. A Time Switch called “T”.
2. A Space-Switch called “S”.
• The architecture of a digital switch is described
in sequences of Ts and Ss.
• For example, the 4ESS is a TSSSST switch.
• Where, the input stage is a time switch,
followed by four space switches in sequence
and the last stage is a time stage.
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• Another example, the Northern Telecom
DMS-100 is a TSTS switch that is folded
back on itself.
• Many of the new switches or enhanced
versions of the switches just mentioned
have very large capacities (e.g.,100,000
lines) and are simply TST or STS
switches.
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Lucent 5ESS TSSSST Switch
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Northern Telecom DMS-100 Line Card Drawer showing line cards
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3.2 Time Switch
• Time-Division Switch or simply, TimeSwitch is a Time-Slot Interchanger (TSI).
• We know that E1 consists of 32 time-slots in
125 µs, with time slot duration of 3.906 µs,
and each time-slot contain 8-bits.
• TSI involves moving the data contained in
each time-slot from the incoming bit stream
at the switch inlet ports, to an outgoing bit
stream at the switch outlet ports, but with a
different time-slot arrangement in
accordance with the destination of each timeslot.
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• To accomplish this, at least one time-slot
must be stored in memory (Write) and then
called out of memory in a changed position
(Read).
• The operations must be controlled in some
manner, and some of these control actions
must be kept in memory together with the
software managing such actions.
• Typical control functions are time-slot
“idle” or “busy”.
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•
1.
2.
3.
•
1.
2.
The three basic functional blocks of a time
switch are:
Memory for speech.
Memory for control.
Time-slot counter or processor.
There are two choices in handling the time
switch:
Sequential write, random read
Random write, sequential read.
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Time-slot interchange: time switch (T). Sequential write, random read.
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Time-switch, time-slot interchange (T). Random write, sequential read.
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• With sequential write, the time-slots are
written into the speech memory as they
appear in the incoming bit stream.
• With random write, the incoming time-slots
are written into memory in the order of
appearance in the outgoing bit stream (the
desired output order).
• The writing of incoming time-slots into the
speech memory can be controlled by a
simple time-slot counter and can be
sequential (e.g., in the order in which they
appear in the incoming bit stream).
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• If the readout of the speech memory
is controlled by the control memory,
• In this case the readout is random
where the time-slots are read out in
the desired output order.
• If the write is of the speech memory
is controlled by the control memory,
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• In this case, the writing process is
random.
• The memory has as many cells as there
are time-slots (e.g. E1 = 32 time-slots,
DS1 = 24 time-slots).
• This time switch, works well for a single
multiplexed inlet – outlet switch,
which we denote by single inlet –
outlet trunk .
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• How can we increase a switch’s capacity?
• Enter the space switch (S). (see the
figure in the next slide)
• For example, time-slot B1 on the B trunk
is moved to the Z trunk into time-slot Z1,
and time-slot Cn is moved to trunk W into
time-slot Wn.
• However, we see that there is no change
in time-slot position.
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Space switch connects time slots in a spatial configuration.
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3.3 Space Switch
• Figure in the next slide illustrates a typical
time-division space switch.
• It consists of a Cross-Point Matrix made
up of Logic Gates that allow the switching
of time-slots in the spatial domain.
• These PCM time-slot bit streams are
organized by the switch into a pattern
determined by the required network
connectivity.
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Time-division space switch cross-point array showing enabling gates.
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• The matrix consists of a number of
input horizontals and a number of
output verticals with a Logic Gate
at each cross-point.
• The array, as shown in the figure, has
M input horizontals and N output
verticals, and we call it an M × N
array.
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•
•
•
•
If M = N, the switch is Non-blocking.
If M > N, the switch Concentrates;
If N > M, the switch Expands.
For a given time-slot, the appropriate
Logic Gate is enabled and the timeslot passes from the input horizontal
to the desired output vertical.
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• The other horizontals, each serving a
different serial stream of time-slots, can
have the same time-slot (e.g. a time-slot
from time-slots number 1–30, or 1–n; for
instance, time-slot 7 on each stream)
switched into other verticals enabling their
gates.
• In the next time-slot position (e.g. time-slot
8), a completely different path configuration
could occur, again allowing time-slots from
horizontals to be switched to selected
verticals.
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• The selection, of course, is a function of how
the traffic is to be routed at that moment for
calls in progress or being set up.
• The space array (cross-point matrix) does
not switch time-slots as does a time switch
(time-slot interchanger).
• This is because the occurrences of timeslots are identical on the horizontal and on
the vertical.
• It switches in the space domain, not in the
time domain.
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• The control memory in the figure enables
gates in accordance with its stored
information.
• If it is desired to transmit a signal from input
1 (horizontal) to output 2 (vertical), the
gate at the intersection would be activated by
placing an enable signal on S12 during the
desired time-slot period.
• Then the eight bits of that time-slot would
pass through the logic gate onto the vertical.
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• In the same time-slot, an enable signal
on SM1 on the Mth horizontal would
permit that particular time-slot to pass
to vertical 1.
• From this we can see that the
maximum capacity of the array during
any one time-slot interval measured in
simultaneous call connections is the
smaller value of M or N.
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• Example, if the array is 20 × 20 and a
time-slot interchanger is placed on
each input horizontal line and the
interchanger handles 30 time-slots,
the array then can serve 20 × 30 = 600
different time-slots.
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3.4 Time-Space-Time Switch
A time–space–time (TST) switch. TSI, time-slot interchanger.
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• The first stage of the TST switch is the
time-slot interchanger (TSI) or time
stages, that interchange time slots (in the
time domain) between external incoming
digital channels and the subsequent space
stage.
• The space stage provides connectivity
between time stages at the input and
output.
• It is a multiplier of call-handling capacity.
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• The multiplier is either the value for M or
value for N , whichever is smaller.
• We also saw earlier that space-stage timeslots need not have any relation to either
external incoming or outgoing time-slots
regarding number, numbering, or position.
• For instance, incoming time-slot 4 can be
connected to outgoing time-slot 19 via
space network time-slot 8.
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3.5 Space-Time-Space Switch
A space–time–space (STS) switch.
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• STS switch reverses the architecture of a
TST switch.
• The STS switch consists of a space
cross-point matrix at the input followed
by an array of time-slot interchangers
whose ports feed another cross-point
matrix at the output.
• Example: Consider this operational
example with an STS switch.
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• Suppose that an incoming time-slot 5 on port
No. 1 must be connected to an output slot 12 at
outgoing port 4.
• This can be accomplished by time-slot
interchanger No. 1 which would switch it to
time-slot 12, then the outgoing space stage
would place that on outgoing trunk No. 4.
• Alternatively, time-slot 5 could be placed at the
input of TSI No. 4 by the incoming space
switch where it would be switched to time-slot
12, thence out port No. 4.
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3.6 TST Compared to STS
• The architecture of TST switching is more
complex than STS switching with space
concentration.
• For large switches, TST switch becomes
more cost-effective because time
expansion can be achieved at less cost
than space expansion.
• For small switches, STS is favored due to
reduced implementation complexities.
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4 Digital Switching Concepts
• A single switch is manufactured rather
than two distinct switches, to handle both
North American DS1 and European E1
rate.
• This switch has different input ports and a
common internal switching network,
consisting of time and space arrays.
• All digital switches have a common
internal digital format and bit rate.
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• The common internal digital format of a
switch might or might not use 8-bit timeslots, even though the outside world (e.g.
DS1 or E1) required an 8-bit octet
interface and frame of 125 µs duration.
• Examples:
• The Lucent 4ESS, uses the number
“120”.
• It maps 120 8-bit time-slots into 128
time-slots.
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• The 8 time-slots of the remainder are
used for diagnostic and
maintenance purposes.
• The Northern Telecom DMS-100 maps
the external 8-bit time-slot into an internal
10-bit time-slot as illustrated in the figure
(see next slide).
• The example used in the figure is the DS1.
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The make-up of the 16-bit internal time slot Lucent 5ESS.
8-bits of DS1
Bit mapping in the DMS-100
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4.1 Remote Switching
• Remote Switch: is a module taken
from the principal switch and
displaced to a remote location.
• This location may be just hundreds
of meters or kilometers from that
of the principal or “mother” switch.
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Local Exchange Structure with Remote Concentrator Units
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•
1.
2.
3.
4.
The functions carried out in the remote
module as minimum:
Interface with a subscriber.
Battery supply, often −48 volts DC.
Signaling: supervisory and address
signaling.
Alerting the subscriber, some form of
“ring-down”.
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• On the other side of the module there
must be some way of communicating
with the principal switch or “mother”.
• Among the most common methods we
find E1 or DS1 configuration on one or
better yet, two wire pairs.
• Depending on the type of signaling used,
there may be one or two time-slot voice
channels dedicated to signaling.
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• The advantages of Remote Switching:
1. It can serve as a community dial office
(CDO) where a full-blown switch would
not be justified.
2. It can dramatically extend the operational
area of a switch.
3. It can serve as an ADC and DAC point of
conversion providing analog interface with
a subscriber and the digital interface with
the network.
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4. It can serve as a concentrator. In this case
it may provide a capability of switching calls
inside its own serving area.
• When we say “concentration,” we mean a
device that serves, say, 120 subscribers
and has trunk connectivity with the mother
switch with only E1 capacity.
• Therefore it has a concentration capacity
of 120-to-30.
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4.2 Digital Cross-Connect
• DXC has been with us virtually since the
advent of the digital network.
• DXC: is a device that handles the
connections between two or more
telecommunication transmission facilities.
• The types of network cross connects
handled by a DXS can range from nearly
terabit data rates of fiber-optic cable to
relatively low-speed data rates of copper
pairs used to provide access to a group of
residences.
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• But, what is the difference between
DXC and PSTN Digital Switch?
• A PSTN Digital Switch:
1.Whether serving the local area, tandem, or
toll, sets up a short-term virtual circuit
where a connection may last just seconds,
minutes, or several hours.
2.The bandwidth of the switched signals is
in the range 64 kbps to 140 Mbps.
3.It is controlled by embedded control
information in the transmitted signals
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• A DXC:
1.Has more permanency where the
duration of a connection may be
minutes, hours, days, weeks, or
years.
2.The bandwidth of the switched
signals is in the range 155 Mbps to
10 Gbps or higher.
3.It is controlled by commands from an
operating system or an operator.
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4.2.1 DXC Strategies
• There are two strategies for DXC:
1. Centralized:
• In which some central node in the
network gets the entire information about
the network topology, about the traffic
and about other nodes. This then
transmits this information to the
respective nodes.
• The advantage of this is that only one
node is required to keep the information.
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•
The disadvantage is that, if the central node
goes down the entire network is down, i.e.
single point of failure.
2. Distributed.
• In which the node receives information from
its neighboring nodes and then takes the
decision about which way to send the data.
• Delay is the major disadvantage of this
strategy.
• It is reliable due to redundant routs.
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Centralized
Decentralized
Distributed
Centralized, Decentralized, and Distributed Networks
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• For traffic that both originates and
terminates in a Metropolitan Area
Network (MAN), the distributed
DXC strategy makes sense as it
eliminates the need to backhaul the
traffic to and from a Tandem Switch or
large Metropolitan Hub site.
• This will save both bit rate capacity
and equipment costs in the form of
DXC ports and ADM equipment.
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• However, where traffic originates in a
metropolitan network and terminates in
some other network, the distributed model
does not work so well.
• This traffic is usually a mix of PSTN voice,
data, and other long-distance services.
• Such traffic must first be passed through a
gateway at a tandem switch or metro core
site in order to be compatibly routed to
other service provider networks whether
metropolitan or longhaul.
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• Backhaul: defined as transmitting from
a remote site or network to a central or
main site.
• It implies a high-capacity line; for
example, to backhaul from a wireless
mesh network to the wired network
means aggregating all the traffic on the
wireless mesh over one or more highspeed lines to a private network or the
Internet.
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Fiber tower backhaul Network Architecture
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4.3 A New Direction—
Programmable Switching
•
A typical programmable-switching
system consists of three major
elements:
1. Switching System.
2. Host Computer.
3. Real-time Application
Programming Interface (API)
messaging.
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A typical programmable switch in a client-server configuration with an API
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4.3.1 Host Computer
• Host computer and the programmable
switch are in a client-server relationship.
• The function of the host is to store and
execute varying amounts of call control
application software that manages the
switching matrix.
• It controls the matrix switch with API
structures and other subroutines.
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4.3.2 The API
• Advantage of programmable switching
is that it allows developers to implement
new and unique services.
• Because, the manufacturers design the
switches with open strategies allowing
developers access to the software
environment of the switch at one or
more levels.
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• These open strategies involve:
1. Application Programming Interfaces
(APIs),
2. Industry-standard interconnection devices,
3. High-level host-resident development
tools.
• The switch matrix or switching device is
controlled by a host computer via the API.
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• Host instructions through the API are
transmitted to the switch via messaging.
• The larger, richer, and more open the
message set of the API, the greater the
flexibility in programming the switch.
• The key here is open programming even
down to the functional elements of the switch
such as DSPs , common channel signaling
modules (e.g., SSN#7), and network data
protocols.
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4.3.3 Programmable Switching
Device
• The switch consists of a combination of
space switch matrix and time-slot
interchangers, line interface units, and
interconnects with one or more networks.
• Modern switches have a high level of
redundancy to improve availability and
survivability.
• There are many different types of lines
and trunks with which a switch may
require an interface.
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• They may be analog and/or digital,
single channel or multichannel, for
example, E0s and/or E1s.
• These connectivities are usually
routed through interface cards.
• There may be Frame Relay
interfaces, ATM and Ethernet
interfaces.
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• We would also expect to find
service interfaces, especially
signaling, ISDN basic and primary
rate.
• API switches are scalable where
the user can start out “small” and
grow to meet demand.
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• The new API switches will be uniservice devices, handling voice, data,
and image indistinguishably.
• The forerunner of this is VoIP.
• It will find equal applicability whether
in the PSTN or with an Enterprise
Network.
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1 Digital Local Exchange
Local-access network and digital local exchange site
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• All subscriber lines are wired to the MDF, as
shown in the figure.
• MDF is a large construction with huge
number of connectors.
• Ordinary subscriber pairs are used for analog
telephone subscribers, analog and digital
PBX/PABX connections, ISDN basic rate
connections and ADSL.
• ADSL and ordinary analog telephone circuits
use the same 2W subscriber loop.
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• Data and speech connections may be used
simultaneously and they are separated in the
exchange where speech is connected to an
ordinary analog exchange interface and data
are routed to the Internet.
• A digital exchange may include both analog
and digital subscriber interfaces.
• For digital PABX exchange applications, 1.5
or 2 Mbps digital interfaces are available.
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• If the local switch has ISDN capability,
basic rate and primary rate interfaces are
available.
• Ordinary subscriber pairs are used for
ISDN basic rate connections (160 Kbps
bidirectional) and a network terminal (NT)
is required on customer premises.
• The primary rate interface of ISDN (1.5 or
2 Mbps) is used for PABX connections.
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• It requires two pairs, one for each
transmission direction, and supports many
simultaneous external calls.
• In addition to MDF, optical distribution
frame (ODF) are also found for optical
communication services.
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2 Digital Exchange Structure
Functional Overview of SPC Digital Exchange
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• Each subscriber copper line is terminated on
the MDF.
• From MDF, an internal copper pair cable is run
within the exchange building to the appropriate
Subscriber Line Card (SLC) on the
concentrator switch-block.
• The SLC performs all the necessary subscriber
line termination functions as well as converting
the speech from the analogue signal on the
subscriber’s line to a 64 kbps digital stream.
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• The outputs from 30 of these line cards are
multiplexed together to form a standard 30channel PCM frame, with a line rate of 2
Mbps.
• Each multiplexer is connected at the 2 Mbps
level to the input of the concentrator switchblock.
• From the other side of this switch-block, 2
Mbps links are taken to the co-located
route switch-block for connection to the 2
Mbps digital trunk links to other exchanges.
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• The 2 Mbps output from the switching units
are multiplexed together with other links onto
either PDH or SDH higher-speed transmission
links at the serving core transmission station
(CTS), which is usually in the same exchange
building as the switch-blocks.
• The route switch-block is also connected with
2 Mbps links to the exchange’s SS7 signaling
equipment, for signaling to the control
systems of distant exchanges.
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• Also as shown in the figure, each subscriber’s
line card is monitored by a controller which is
used by the exchange-control system to manage
the setting up and clearing of calls, while the
signaling from subscribers’ telephones is
received by the MF equipment.
• The tone equipment supplies supervisory tones,
e.g. ‘network busy’, as well as recorded
announcements, such as ‘Sorry we are unable
to connect your call’.
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2.1 Subscriber Line Card
The Components of a Subscriber Line Card
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•
The set of functions required by a
copper-line termination and provided by
a subscriber’s line card are:
1. Battery: application of the -48 V DC
power supply to the subscriber’s line.
2. Overload: protection of the delicate
semiconductor equipment of the digital
switch-blocks from any induced voltages
on the external copper line
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3. Ringing current: provision of an
interrupted electrical signal, with an
appropriate cadence, of about 200 mA is
required to ring a subscribers’ set of
telephone instruments.
4. Supervision: that is, the detection of the
subscriber going off-hook to signify call
initiation, and detection of off-hook
conditions in the presence of ringing current
to signify call answer.
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5. Codec: coding and decoding, the ADC in
the Go direction and the DAC in the Return
direction.
6. Hybrid: 2-to-4 wire conversion from the
local loop to the Go and Return format of the
switches and core transmission network.
• Typical configurations for a concentrator
switch-block are TS and TST and for a route
switch-block is TST.
• The concentration block serves the Intraexchange calls and the rout block serves the
Inter-exchange calls.
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Local Exchange Structure with Remote Concentrator Units
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2.2 Inter-working of the SS7 system
and the exchange systems
Digital Exchanges and the SS7 System
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• The diagram shows trunk exchanges A
and B connected by a traffic route
comprising two 2 Mbps digital streams,
each with 30 speech channels.
• The SS7 link terminates in either
exchange on a sender/receiver (S/R)
system linked to the exchange-control
systems and the digital route switchblocks.
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• Exchange A’s control system initiates a
message, say an initial address message
(IAM), by sending an instruction to the
SS7 signaling-control subsystem through
the control link, passing all relevant
information, including dialed number,
circuit or channel identity and the point
code of the destination SS7 node (i.e. the
SS7 S/R at exchange B).
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• The formatted message is then passed to the
signaling-termination subsystem, where it joins a
queue with other messages waiting to be sent.
• This subsystem then creates a signaling unit to
carry the message by the addition of the SU
sequence numbers, length indicator, flags, etc.,
and the generated check sum from the errorcontrol subsystem.
• As soon as a vacant slot appears on the
signaling link, the SU is transmitted via the time
slot 16 (TS16) in one of the 30-channel 2 Mbps
systems, as shown in figure.
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• The transmission rate of SS7 signaling
over the network is therefore 64 kbps.
• At exchange B the TS16 contents from the
2 Mbps link are permanently switched
through to the SS7 signaling-termination
subsystem.
• On receipt of the SU the error-control
subsystem checks for errors and corrects
or seeks a re-transmission if necessary.
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• The SU is then decomposed and the
message part transferred via a queue to
the signaling-control subsystem for
decoding.
• The relevant information is then passed to
exchange B’s control system.
• A corresponding procedure occurs for the
SS7 messages in the reverse (backward)
direction.
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5 Digital Network
• The North American and European PSTN
are 100% digital.
• The International Interconnecting Network
is nearly 100% digital.
• The Transport Networks, whether National
or International, are SDH/SONET.
• International Networks are no more
hierarchal after the emergence of Satellite
System.
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• International high-usage (HU) and Direct
Routes became practical.
• Unified Signaling System called SS7.
• Pervasiveness of fiber-optic cable on
many, if not most, terrestrial routes and all
undersea trunk routes.
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