09/04/2013Bahman R. Alyaei1 Chapter 9 Digital Switching and Networks.

09/04/2013 Bahman R. Alyaei 1

Chapter 9Chapter 9

Digital Switching and NetworksDigital Switching and Networks


1 Introduction1 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.


Continue…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.



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.


1.1 New Direction1.1 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.


2 Introduction To Switching2 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.


2.1 Circuit Switching2.1 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.



• 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.


2.2 Packet Switching2.2 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.



• 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.


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.



• 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.


3 Digital Switching3 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.



• 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.



A space-division switch showing connectivity from user C to user G



• 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).



• 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.



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)


3.1 Approaches To Digital 3.1 Approaches To Digital Switching 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.



• 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.



Lucent 5ESS TSSSST Switch



Northern Telecom DMS-100 Line Card Drawer showing line cards


3.2 Time Switch3.2 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 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 time-slot.


Continue…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”.



• The three basic functional blocks of a time switch are:

1. Memory for speech.

2. Memory for control.

3. Time-slot counter or processor.• There are two choices in handling the time

switch:

1. Sequential write, random read

2. Random write, sequential read.



Time-slot interchange: time switch (T). Sequential write, random read.



Time-switch, time-slot interchange (T). Random write, sequential read.



• 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).



• 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,



• 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 .



• 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.


Continue…

Space switch connects time slots in a spatial configuration.


3.3 Space Switch3.3 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.


Time-division space switch cross-point array showing enabling gates.



• 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.



• If M = N, the switch is Non-blocking.• If M > N, the switch Concentrates;• If N N > > MM, 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.


Continue…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.



• 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.



• 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.



• 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.



• 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.


3.4 Time-Space-Time Switch3.4 Time-Space-Time Switch

A time–space–time (TST) switch. TSI, time-slot interchanger.



• 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.



• 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.


3.5 Space-Time-Space Switch3.5 Space-Time-Space Switch

A space–time–space (STS) switch.



• 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.



• 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.


3.6 TST Compared to STS3.6 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.


4 Digital Switching Concepts4 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.



• 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 120 8-bit time-slots into 128

time-slots.



• 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.


Bit mapping in the DMS-100

The make-up of the 16-bit internal time slot Lucent 5ESS.


4.1 Remote Switching4.1 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.



• The functions carried out in the remote module as minimum:

1. Interface with a subscriber.

2. Battery supply, often −48 volts DC.

3. Signaling: supervisory and address signaling.

4. Alerting the subscriber, some form of “ring-down”.



• 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.



• 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.



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.


4.2 Digital Cross-Connect4.2 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.



• 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.

• A DXC has more permanency where the duration of a connection may be minutes, hours, days, weeks, or years.


4.2.1 DXC Strategies4.2.1 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.



• 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.



Diagram of different network topologies


Continue…

Centralized DistributedDecentralized

Centralized, Decentralized, and Distributed Networks


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.



• 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.



Fiber tower backhaul Network Architecture

09/04/2013Bahman R. Alyaei1 Chapter 9 Digital Switching and Networks.

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