switching (c) manzur ashraf1 switching manzur ashraf

Switching (c) Manzur Ashraf 1

Switching

Manzur Ashraf


Definition

• The ITU-T defines switching as:

"the establishing, on demand, of an individual connection from a desired inlet to a desired outlet within a set of inlets and outlets for as long as is required for the transfer of information".


Historic preview

• One hundred and twenty years ago, switching meant 'an operator interconnecting two subscribers with each other'

• Present-day switching equipment must be capable of handling more services than before, including high-quality audio, video of different quality standards, LAN-to-LAN communication, the transfer of large data files, and new interactive services based on the cable-TV network. But there is more to it than the switching of information related to the service user. Information used by the network - signalling information, for example - must also be switched.


Contd.

• From the beginning we had only circuit switching, which is very suitable for isochronous services, such as telephony. Since then, subscribers have demanded better utilisation of transmission capacity and larger bandwidth, and other techniques have emerged. As a result of the requirements imposed by data communication, circuit switching was supplemented in the 1970s with the packet-mode switching technique. Today we also have frame relay and two types of cell switching: asynchronous transfer mode ( ATM) and distributed queue dual bus (DQDB). The origin of frame relay and the techniques for cell switching can be traced to packet switching.


Whole picture( Different switching techniques in

public networks )


Multiplexing

>>In addition, concepts such as packet and cell switching have been given a broader meaning than just switching, namely multiplexing, which means that traffic from different sources are brought together into a shared traffic flow with a higher bit rate.

>>Developments in the area of switching are driven by a few basic factors or requirements:

1. accessibility, or the ability to establish desired connections;

2. transparency; and 3. network economy.


Developments in the area of switching

1. One should be able to establish a "successful" and chargeable connection from A to B through the network. In this ambition, the switching equipment itself is only one of several important factors. If the switching capacity is underdimensioned, congestion will result in the network, implying that network planning is somewhat inadequate or that operation does not run smoothly The switching equipment must also have a very high level of availability - a factor that depends on how well the system architecture and the equipment software are structured

2. Good transparency means that delays through the switching equipment are minimised, that the flow of information is not distorted in any way, and that the switched bandwidth can match service requirements.


Contd..

3. The requirement for good network economy applies especially to the capacity of the switching equipment to handle multiplexed traffic that consists of packets. The packet-mode technique is a consequence of the requirement for more efficient use of lines. This technique can also be used to integrate services. For example, ATM technology enables both integrated transmission and integrated switching.


Bandwidth requirements for different telecommunications

services


History

• Manual systems

In the infancy of telephony, telephone exchanges were built up with manually operated switching equipment. The first manual exchange was installed in New Haven, USA, in 1878.

• 500-line selector (1923);

• crossbar systems (1937).


Exchange

• A technique for saving expensive connections was introduced into the long-distance networks: frequency division multiplexing (FDM). This technique was developed around 1910, but was not implemented until 1950 when about 1,000 channels were transferred on the same cable (the coaxial cable).

• Digital multiplexing (based on PCM), which was introduced around 1970, also made transmission networks less expensive while at the same time improving transmission quality. Costs were further reduced when digital group switches (the actual switching equipment in the telephone exchange) were combined with digital transmission systems, eliminating the need for many relatively expensive analog-digital (A/D) converters


Contd..

• The first computer-controlled exchange was put into service in 1960 in the US; the first digital exchange in Europe was opened for traffic in 1968 (Tumba, Sweden).


Special nodes for data communication

• The strong growth in data traffic and in the number of users of data communication has resulted in the development of separate data networks and data switches. In many cases, these can meet users' increasingly stringent quality requirements and the need for higher transmission rates in a better and less expensive way. Packet mode and frame relay, for example, provide efficient network utilisation, enable packets to be retransmitted when errors occur on a link (applies to packet mode only), and allow for sorting, routing, and buffering.


Nodes for N-ISDN

• Developments for providing service-integrated networks (for voice, video, and data services) require both public and private N-ISDN nodes (N stands for narrowband). In principle, a complete ISDN node can be seen as a combination of today's telephone exchanges and packet data switches (circuit switching and packet mode), with an important sorting function for subscriber traffic.


Nodes for B-ISDN

• The technology called ATM, which applies the cell-switching technique and which forms the basis of B-ISDN (B stands for broadband), is not yet completely standardised.


Optical switches

• Why? It is primarily the switching equipment that limits the bandwidth of a connection. Today, we can make use of very high bit rates, up to tens of billions of bits per second (tens of Gbit/s) in optical transmission systems. However, in switching equipment, we must change over to electrical signals and considerably lower bit rates.

• How? The next step is to use optical switching with electronic switch control. And in time, we will most assuredly have fully optical switching systems. Indeed, in view of the intensive research and development that is being carried out in this area, it should not be long before the first optical space switches are commercially available.


Total Picture


Nodes for circuit switching


The design of a telephone exchange


Centralized control


Centralized non-hierarchical control

• RP Regional processors


Distributed control with independent switching parts


Overall view of processor structures


Switching part

• Group switch:

The main task of the switching part is to interconnect an incoming time slot and an outgoing time slot. The unit responsible for this function - a group switch - is a system for handling time slots


The group switch interconnects incoming and outgoing time slots

• Suppose that A makes a local call to B. The control system has assigned time slot 3 to the call on its way into the group switch, and time slot 1 on its way out of the group switch (to B). The time slots, which have been reserved in the signalling phase, will be used for the entire duration of the call


Voice time slots are switched in both directions in the group switch

• The group switch ensures that the contents of incoming time slot 3 are sent in outgoing time slot 1, which means that B can hear what A says. For the opposite case to apply (that is, A hears B) we must also have a connection of time slots in that direction


• This means that it must be possible to switch time slots in the time domain (changing their numbers) as well as

in the space domain (to the right outgoing link).

• There are two types of building block in the digital group switch: time switch and space switch


Time & Space switch


Time switch

• The time switch works as follows. Each incoming time slot is stored in sequence in a speech store (SS). The control store (CS) determines in which order the time slots are to be read from the SS. This means that a voice sample can be "moved" from, say, incoming time slot 3 to outgoing time slot 1. The information in the control store is obtained in the signalling phase, before the actual conversation is started.


Space Switch

• the space switch consists of a matrix with n x n switching points (designed as electronic gates). Each column of the matrix is related to one additional control store (crosspoint control) with the same number of positions as the number of internal time slots in the switch. The control store specifies which crosspoints are to be set up in the matrix. During each internal time slot, one matrix crosspoint is activated per column. In the shift to the next internal time slot, the control store is incremented by one step, and a new crosspoint pattern is formed in the matrix.


Small telephone exchanges

• In small exchanges (with less than 500 connection channels) it is often sufficient to have a group switch that consists of a time switch only.

• Likewise, in the subscriber switch , only a time switch is used to connect a subscriber to a unit for receiving dialled digits, or to the group switch itself.


Large telephone exchanges

• When the number of channels to the group switch exceeds the capacity of a single time switch, additional switching resources are needed. If, for instance, the A-subscriber for the moment is connected to one time switch and the B-subscriber is connected to another one, then we must have a system that can interlink the two switches. In a connection of this kind, the space switch is handy. This complex structure is called "time-space-time" (TST).


The principle of TST switching


More complex scenario…

• SPM = space switch module • TSM = time switch module


Capacity

• As an example of the capacity offered by digital group switches, let us consider a system whose group switch has the TST structure. Each time switch (TSM = time switch module) can receive 16 PCM links of 32 channels each, giving 16 X 32 = 512 multiple positions (MUP).

• Thirty-two such modules can be connected to a space switch (SPM = space switch module) to produce a capacity of 512 X 32 = 16,384 multiple positions.

• Since a telephone call always seizes at least two inputs and outputs; that is, two multiple positions, a group switch of this size can theoretically switch a maximum of 8,192 simultaneous calls


((128X16)PCM)X32=65536 multiple positions


Subscriber connections

Radio in the local loop (RLL)


Nodes for packet switching


Switching Principle

In packet-mode networks, information is transferred in packets of varying lengths. Thus, the task of the nodes is to distribute each packet to the right receiver. Each node reads the address field of each packet and routes the packet to the proper outgoing link. The node can be thought of as a mail terminal where personnel forward packets according to their attached address labels


Switching in the X.25 network

• The address in an X.25 packet indicates a logical channel on which the user sends data to the receiver. When the data call is started, the first packet - the start packet - is sent through the network to the receiver. In each node that it passes, the packet notifies the processor how to route all subsequent packets belonging to the same connection. Thus, the rest of the packets need only contain a "label" - logical channel number (LCN) - that specifies to which logical channel the packet belongs.


Contd..

• In the node, the packets are first stored in input buffers. This is done in order to give the processors time enough to process the packets (read the address, for example) when bursts of data arrive at the node.

• Located in a central position in the node is the processor part (or the packet sorter) which fetches packets from input buffers and, after analysing their addresses (reading the LCN), places the packets in buffers for outgoing traffic.


Packet-node structure


Contd..

• The addresses are analysed by means of tables that contain routing information. The data in the tables has been generated by the start packets of the different connections. The routing function belongs to layer 3 (the network layer) in the OSI model. All packets pass through the processor part of the node, where they are processed by many different software routines. Switching takes 15-20 ms.

• Output buffers are needed to store a copy of each packet in case of a request for retransmission.


Switching in the frame-relay network

• the frame-relay technique was developed in an attempt to combine the idea of shared bandwidth - as in the packet-mode X.25 network - with higher rates and less delay, which we have in circuit-switched networks.

• The higher rate is mainly the result of simplified protocol handling, notably for error handling.


Data rate

• In terms of transmission rates, it is estimated that frame relay can be developed to deliver at least 40 Mbit/s, compared with today's ordinary bit rate of 2 Mbit/s. The corresponding figures for X.25 are 2 Mbit/s and 64 kbit/s. This increase, which is particularly dramatic for frame relay, is largely due to developments in component technology.


Summary: Different connections in dedicated networks


Nodes for N-ISDN


• In ISDN, you may not only integrate transmission between the nodes, but you can also permit the transfer modes - for example, circuit mode and packet mode - to use the same subscriber line and to be switched in the same node.


N-ISDN

• If the ISDN subscriber wants to make an ordinary call, only the circuit-mode switch in the ISDN node will be used. The call is then routed either in ISDN or the PSTN, depending on which bearer network the called subscriber is connected to.

• On the other hand, if the subscriber wants to send packet data, the circuit-mode switch must route the traffic to a packet handler. The most important part of the packet handler is a packet switch whose task is to ensure correct set-up to a subscriber connected to the same node; or to a node in the packet switched public data network (PSPDN); or to another packet handler in ISDN. In this case as well, the choice depends on which network the receiving computer is connected to.


broadband ISDN

• In contrast, in the case of broadband ISDN, full integration from A to B is possible at the cell level


Why B-ISDN?

• Future telecommunications networks must be able to offer today's range of services as well as services with new features - for example, high bit rate (in excess of 2 Mbit/s) and a high degree of burstiness - in an integrated services network. For this reason, B-ISDN is presently being developed and standardised.


And why ATM?

• ATM was chosen as the technique to support B-ISDN, mainly because it is flexible and well suited for broadband switching. ATM combines the advantages of circuit and packet switching, and permits broadband switching and transport of all types of service in a common, digital format.

• ATM technology is well suited for use in backbone networks in LANs; for the interconnection of LANs in private networks; for use in virtual private networks (VPNs); and for large digital cross-connects in B-ISDN's transport network. Emerging applications are video-on-demand for residentials and a backbone network function in the Internet.


The basic function in an ATM switch

• In an ATM switch, ATM cells are transported from an incoming logical channel to one or more outgoing logical channels. A logical channel is indicated by a combination of two identities:

1. the number of the physical link; and 2. the identity of the channel - the virtual path

identifier (VPI) and the virtual channel identifier (VCI) on the physical link.

Switching of cells through an ATM node requires a tie between the identities of incoming and outgoing logical channels.


• Two transport functions that are needed in the ATM switch are described below; they are also compared with the corresponding functions in a circuit-mode switch.

First function can be compared to the change of time slot numbers in a circuit-mode switch This is the function that transfers a voice sample from an incoming time slot to an outgoing time slot. In an ATM network, the identities of the different logical channels correspond to the time slots. The identity is composed of two values in two different fields in the header of the cell: VPI and VCI. They have the same task as the time slot in a circuit-switched system; that is, to identify each individual connection on each physical link between two nodes.


ATM switch

• the channel identity (VPI and VCI) is specified by the digits in the cell header.

Second function can be compared to the function for space switching in a circuit-mode switch. Here payload A being shifted from the upper physical input to the lower output, and vice versa for payload B.


• a virtual path (VP) contains many virtual channels (VCs), which are the individual channels. The physical transmission medium, such as an optical fibre, carries several VPs.


Contd..

• The VPI level can be compared to the digital cross-connect function in so far as certain network elements (ATM cross-connects or VP switches) perform switching based on the VPI address alone. This means that - on a specific incoming link - all connections with different VCI values, but with the same VPI values, will be kept together during the switching process

• An example is the communication between two company sites, where the network operator has assigned the customer one or more VPs. The customer can create his own virtual private network by using all the VCs contained in a VP.


Mesh-shaped network with direct connections between every node


The network hierarchy according to the ITU


Example of telephone network hierarchy


Non-circuit-switched networks - Example

switching (c) manzur ashraf1 switching manzur ashraf

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