1. Digital Switching and Networks
Chapter 9
Digital Switching and Networks
09/04/2013
Bahman R. Alyaei

2. 1 Introduction
Philosophically, Data Communication and Digital Telephony are very different specially from Signaling aspects.
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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3. 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

4. Continue… 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

5. 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

6. 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:
Circuit Switching.
Packet Switching.
ATM Switching.
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7. 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
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8. 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

9. 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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10. 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.
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11. 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
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12. 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.
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13. 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:
Space-Division Switch.
Time-Division Switch.
Combination of Space-Division Switch and Time-Division Switch construct the Digital Switch.
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14. 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.
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15. A space-division switch showing connectivity from user C to user G
Continue…
A space-division switch showing connectivity from user C to user G
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16. 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).
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17. 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.
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18. 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)
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19. 3.1 Approaches To Digital Switching
A classical Digital Switch is made up of two functional elements:
A Time Switch called “T”.
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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20. 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.
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21. Lucent 5ESS TSSSST Switch
Continue…
Lucent 5ESS TSSSST Switch
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22. Northern Telecom DMS-100 Line Card Drawer showing line cards
Continue…
Northern Telecom DMS-100 Line Card Drawer showing line cards
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23. 3.2 Time Switch
Time-Division Switch or simply, Time-Switch is a Time-Slot Interchanger (TSI).
We know that E1 consists of 32 time-slots in 125 µs, with time slot duration of 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 time-slot.
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24. 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”.
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25. Continue… 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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26. Time-slot interchange: time switch (T). Sequential write, random read.
Continue…
Time-slot interchange: time switch (T). Sequential write, random read.
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27. Time-switch, time-slot interchange (T). Random write, sequential read.
Continue…
Time-switch, time-slot interchange (T). Random write, sequential read.
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28. 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).
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29. 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,
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30. 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 .
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31. 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.
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32. Space switch connects time slots in a spatial configuration.
Continue…
Space switch connects time slots in a spatial configuration.
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33. 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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34. Time-division space switch cross-point array showing enabling gates.
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35. 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.
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36. 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 time-slot passes from the input horizontal to the desired output vertical.
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37. 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.
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38. 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 time-slots are identical on the horizontal and on the vertical.
It switches in the space domain, not in the time domain.
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39. 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.
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40. 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
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41. 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.
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42. 3.4 Time-Space-Time Switch
A time–space–time (TST) switch. TSI, time-slot interchanger.
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43. 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.
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44. Continue…
The multiplier is either the value for M or value for N , whichever is smaller.
We also saw earlier that space-stage time-slots 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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45. 3.5 Space-Time-Space Switch
A space–time–space (STS) switch.
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46. 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.
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47. 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.
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48. 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
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49. 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
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50. Continue…
The common internal digital format of a switch might or might not use 8-bit time-slots, 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 bit time-slots into 128 time-slots.
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51. 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.
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52. 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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53. 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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54. Local Exchange Structure with Remote Concentrator Units
Continue…
Local Exchange Structure with Remote Concentrator Units
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55. Continue… 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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56. 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 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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57. Continue… The advantages of Remote Switching:
It can serve as a community dial office (CDO) where a full-blown switch would not be justified.
It can dramatically extend the operational area of a switch.
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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58. Continue…
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
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59. 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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60. 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 short-term virtual circuit where a connection may last just seconds, minutes, or several hours.
The bandwidth of the switched signals is in the range 64 kbps to 140 Mbps.
It is controlled by embedded control information in the transmitted signals
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61. Continue…
A DXC:
Has more permanency where the duration of a connection may be minutes, hours, days, weeks, or years.
The bandwidth of the switched signals is in the range 155 Mbps to 10 Gbps or higher.
It is controlled by commands from an operating system or an operator.
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62. 4.2.1 DXC Strategies There are two strategies for DXC: 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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63. Continue…
The disadvantage is that, if the central node goes down the entire network is down, i.e. single point of failure.
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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64. Centralized, Decentralized, and Distributed Networks
Continue…
Centralized
Decentralized
Distributed
Centralized, Decentralized, and Distributed Networks
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65. 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.
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66. Continue…
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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67. Continue…
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 high-speed lines to a private network or the Internet.
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68. Fiber tower backhaul Network Architecture
Continue…
Fiber tower backhaul Network Architecture
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69. 4.3 A New Direction—Programmable Switching
A typical programmable-switching system consists of three major elements:
Switching System.
Host Computer.
Real-time Application Programming Interface (API) messaging.
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70. Continue…
A typical programmable switch in a client-server configuration with an API
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71. 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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72. 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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73. Continue… These open strategies involve:
Application Programming Interfaces (APIs),
Industry-standard interconnection devices,
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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74. Continue…
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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75. 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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76. Continue…
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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77. Continue…
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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78. Continue…
The new API switches will be uni-service 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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79. 1 Digital Local Exchange
Local-access network and digital local exchange site
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80. Continue…
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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81. Continue…
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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82. Continue…
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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83. Continue…
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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84. 2 Digital Exchange Structure
Functional Overview of SPC Digital Exchange
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85. Continue… 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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86. Continue…
The outputs from 30 of these line cards are multiplexed together to form a standard 30-channel PCM frame, with a line rate of 2 Mbps.
Each multiplexer is connected at the 2 Mbps level to the input of the concentrator switch-block.
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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87. Continue…
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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88. Continue…
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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89. The Components of a Subscriber Line Card
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90. Continue…
The set of functions required by a copper-line termination and provided by a subscriber’s line card are:
Battery: application of the -48 V DC power supply to the subscriber’s line.
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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91. Continue…
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.
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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92. Continue…
Codec: coding and decoding, the ADC in the Go direction and the DAC in the Return direction.
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 Intra-exchange calls and the rout block serves the Inter-exchange calls.
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93. Local Exchange Structure with Remote Concentrator Units
Continue…
Local Exchange Structure with Remote Concentrator Units
09/04/2013
Bahman R. Alyaei

94. 2.2 Inter-working of the SS7 system and the exchange systems
Digital Exchanges and the SS7 System
09/04/2013
Bahman R. Alyaei

95. Continue…
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 switch-blocks.
09/04/2013
Bahman R. Alyaei

96. Continue…
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).
09/04/2013
Bahman R. Alyaei

97. Continue…
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 error-control 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.
09/04/2013
Bahman R. Alyaei

98. Continue…
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.
09/04/2013
Bahman R. Alyaei

99. Continue…
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.
09/04/2013
Bahman R. Alyaei

100. 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.
09/04/2013
Bahman R. Alyaei

101. Continue…
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.
09/04/2013
Bahman R. Alyaei