Transcript TN-Lecture8

Chapter 9
Digital Switching and Networks
09/04/2013
Bahman R. Alyaei
1
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.
09/04/2013
Bahman R. Alyaei
2
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
3
Continue…
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.
09/04/2013
Bahman R. Alyaei
4
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.
09/04/2013
Bahman R. Alyaei
5
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.
09/04/2013
Bahman R. Alyaei
6
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.
09/04/2013
Bahman R. Alyaei
7
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
8
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.
09/04/2013
Bahman R. Alyaei
9
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
10
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.
09/04/2013
Bahman R. Alyaei
11
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
12
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.
09/04/2013
Bahman R. Alyaei
13
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
14
Continue…
A space-division switch showing connectivity from user C to user G
09/04/2013
Bahman R. Alyaei
15
Continue…
• 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).
09/04/2013
Bahman R. Alyaei
16
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
17
Continue…
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)
09/04/2013
Bahman R. Alyaei
18
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.
09/04/2013
Bahman R. Alyaei
19
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
20
Continue…
Lucent 5ESS TSSSST Switch
09/04/2013
Bahman R. Alyaei
21
Continue…
Northern Telecom DMS-100 Line Card Drawer showing line cards
09/04/2013
Bahman R. Alyaei
22
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.
09/04/2013
Bahman R. Alyaei
23
Continue…
• 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”.
09/04/2013
Bahman R. Alyaei
24
Continue…
•
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.
09/04/2013
Bahman R. Alyaei
25
Continue…
Time-slot interchange: time switch (T). Sequential write, random read.
09/04/2013
Bahman R. Alyaei
26
Continue…
Time-switch, time-slot interchange (T). Random write, sequential read.
09/04/2013
Bahman R. Alyaei
27
Continue…
• 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).
09/04/2013
Bahman R. Alyaei
28
Continue…
• 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,
09/04/2013
Bahman R. Alyaei
29
Continue…
• 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 .
09/04/2013
Bahman R. Alyaei
30
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
31
Continue…
Space switch connects time slots in a spatial configuration.
09/04/2013
Bahman R. Alyaei
32
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.
09/04/2013
Bahman R. Alyaei
33
Time-division space switch cross-point array showing enabling gates.
09/04/2013
Bahman R. Alyaei
34
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
35
Continue…
•
•
•
•
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.
09/04/2013
Bahman R. Alyaei
36
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
37
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
38
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
39
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
40
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
41
3.4 Time-Space-Time Switch
A time–space–time (TST) switch. TSI, time-slot interchanger.
09/04/2013
Bahman R. Alyaei
42
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
43
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
44
3.5 Space-Time-Space Switch
A space–time–space (STS) switch.
09/04/2013
Bahman R. Alyaei
45
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
46
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
47
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.
09/04/2013
Bahman R. Alyaei
48
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.
09/04/2013
Bahman R. Alyaei
49
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
50
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
51
The make-up of the 16-bit internal time slot Lucent 5ESS.
Bit mapping in the DMS-100
09/04/2013
Bahman R. Alyaei
52
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.
09/04/2013
Bahman R. Alyaei
53
Continue…
•
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”.
09/04/2013
Bahman R. Alyaei
54
Continue…
• 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
and 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.
09/04/2013
Bahman R. Alyaei
55
Continue…
• 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.
09/04/2013
Bahman R. Alyaei
56
Continue…
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.
09/04/2013
Bahman R. Alyaei
57
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 lowspeed data rates of copper pairs used to
provide access to a group of residences.
09/04/2013
Bahman R. Alyaei
58
Continue…
• But, what is the difference between DXC
and PSTN Digital Switch?
• A PSTN Digital Switch, whether serving the
local area, tandem, or toll, sets up a shortterm virtual circuit where a connection may
last just seconds, minutes, or several hours.
• A DXC has more permanency where the
duration of a connection may be minutes,
hours, days, weeks, or years.
09/04/2013
Bahman R. Alyaei
59
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.
09/04/2013
Bahman R. Alyaei
60
Continue…
•
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.
09/04/2013
Bahman R. Alyaei
61
Continue…
Diagram of different network topologies
09/04/2013
Bahman R. Alyaei
62
Continue…
Centralized
Decentralized
Distributed
Centralized, Decentralized, and Distributed Networks
09/04/2013
Bahman R. Alyaei
63
Continue…
• 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.
• However, where traffic originates in a metropolitan network and
terminates in some other net-work, 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.
09/04/2013
Bahman R. Alyaei
64
Continue…
• Backhaul: is to 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 high-speed lines to a private
network or the Internet.
09/04/2013
Bahman R. Alyaei
65
Continue…
Fiber tower backhaul Network Architecture
09/04/2013
Bahman R. Alyaei
66