TelephoneNumbering. Each telephone set (or subscriber card) has a unique identification number

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1 TelephoneNumbering 1 International telephone connection from any telephone to any other telephone made possible by unique identification of each subscriber socket in the world Each telephone set (or subscriber card) has a unique identification number Numbering is hierarchical: internationally standardized country code at the highest level (eg., Italy +39) Numbering hierarchy: International Prefix Country Code Trunk Code (or Area Code) Subscriber Number Structure of telephone number 32

number Numbering is hierarchical: internationally standardized country code at the highest level (eg.

2 TelephoneExchange 2 The speech channel is connected from the time when the circuit was established to the time when the call is cleared. This principle is called the circuit switching concept Electronic storedprogramcontrol(spc) exchange and signaling principles used between exchanges 33

This principle is called the circuit switching concept Electronic

3 LocalExchange Tasks of the digital local exchange: Detect off-hook condition, analyze dialed number, determine if route is available Connect the subscriber to a trunk exchange for longer distance calls Connect the subscriber to another in the same local area Determine if the called subscriber is free and connect ringing signal toher Provide metering and collect charging data for its own subscribers Convert 2W local access to 4W circuit of the network Convert analog speech into a digital signal (PCM) 3 Local-access network and digital local exchange site 34

called subscriber is free and connect ringing signal toher Provide metering and collect charging data for its own subscribers Convert 2W

4 Signaling: CAS 4 Control unit of the local exchange receives the subscriber signaling, such as dialed digits, from the subscriber line and makes consequent actions according to its program: two methods, Channel Associated Signaling (CAS)orCommon Channel Signaling (CCS). (speechchanneland the related signaling channel are seized) In CAS speech channel reserved between exchanges for a call and, at the same time, another channel is reserved for signaling purposes only. Channel Associated Signaling(CAS) between exchanges 35

5 Signaling: CCS 5 Data frames information are exchanged between computers only when required. Only one data channel between two exchanges is required to serve all established calls When a set of digits is received by Exchange A, it analyzes the dialed digits to determine to which direction it should route the call. Common Channel Signaling(CCS) between exchanges 36

a set of digits is received by Exchange A, it analyzes the dialed digits to determine to

6 TrunkNetwork The national switching hierarchy includes multiple levels of switches above local exchanges Local exchanges are connected to trunk exchanges, which are linked to provide a network of connections from any customer to any other subscriber in the country Usually optical line systems, with very higycapacities (10 Gb/s or more), interconnect trunk exchanges Transport network has alternative routes: if one of these transmission systems fails, switches are able to re-route new calls. 6 The IEEE defines a trunk as a transmission path between exchanges or central offices. Two-layer network and links between trunk and local exchanges 37

(10 Gb/s or more), interconnect trunk exchanges Transport network has alternative routes: if one of these transmission systems fails, switches are able to

7 International Network 7 Each country has at least one international switching center to which trunk exchanges are connected High-capacity optical systems interconnect international exchanges Submarinecables(coaxialcableor optical cable systems), microwave radio systems, and satellites connect continental networks In addition to speech, submarine systems carry intercontinental Internet traffic The international network 38

Submarinecables(coaxialcableor optical cable systems), microwave radio systems, and satellites connect

8 SwitchingHierarchy 8 Early years of the telephone: switching office or exchange located at a central point in a service area (thus the name central office, CO). Later it became necessary to interconnect the individual service areas with trunks between central offices. Currently national networks have several switching levels. Actual implementation of hierarchy and number and names of switching levels differ from Country to Country. The structure of the telephone number supports simple basic principle of routing up and down in the switching hierarchy. Modern exchanges can do more than the simple strictly hierarchical routing: if there is a sufficient volume of traffic, calls may pass by a hierarchy level or may be connected directly to another low level switch, as illustrated in Figure. An example of switching hierarchy 39

Actual implementation of hierarchy and number and names of switching levels differ from Country to Country.

9 RoutingExamples Global rules for the highest-level numbering, country codes, and overall numbering (maximum length and so on) given by ITU-T. National TLC authority coordinates the national numbering plan. Telephone call routing Example 2 Example 1 Example 1: Finnish subscriber makes a call to Stockholm, Sweden, and dials the international prefix 00, country code 46 for Sweden, area code (0)8 (leaves out zero) for Stockholm, and subscriber number xxxxx. The international prefix is all that thelower-level exchanges in Finland need to know to route this call up toward the international exchange. The international exchange analyzes the country code and selects an outgoing route to Sweden. Example 2 : Subscriber dials for a long-distance call to Helsinki. First digit 0 tells exchanges this is a longdistance call to be routed to the regional exchange. Due to next digit, 9, regional center routes call to Helsinki, helped by other regional centers. Helsinki regional center analyzes next two numbers, 13, selects the route down to the primary center where subscribers are located. (Operator has defined in his numbering plan that the subscriber numbers 2xxxx and 1xxxx are placed on the left-hand branch.) Primary center checks following numbers, 131, notices that this is not my subscriber but destination subscriber is located below me. So, routes the call to corresponding lower-level exchange, e.g., the local exchange. Local exchange selects the subscriber loop of the telephone number and sends to subscriber a ringing signal. 40 9

Telephone call routing Example 2 Example 1 Example 1: Finnish subscriber makes a call to Stockholm, Sweden, and dials the international prefix 00, country code 46 for Sweden, area code (0)8 (leaves

10 GenericStructureofa TelecommunicationsNetwork 1 Structure of a modern telecommunications network: three main components Transport Network: it is the long-distance national backbone (or core ) network. Metro Network: it is the local high-speed metropolitan or regional ring to transport aggregated (i.e., multiplexed) signals. Access network: it is the local loop between the customer and the central office. 10

Metro Network: it is the local high-speed metropolitan or regional ring to transport aggregated (i.e., multiplexed) signals.

11 GenericStructureofa TelecommunicationsNetwork 2 11 Backbone networks used for the transport of signals over long and very long distances Metro networks collect the traffic generated in the more densely populated areas and prepare it to be delivered to the network backbone Access network has particular characteristics: traffic usually modest but discontinuous, characterized by high variability, ie. high ratio between peak and average value ('bursty'). Furthermore, the cost of each link is not shared, but rests on a single customer or, more rarely, on a small number of customers. Optical technologies are well established in transport networks and metro networks, as they are the only capable of transporting large amounts of traffic. Presently, in the access network copperis mostly used (twisted-pairs). In future, optical technologies in the access network too. 42

variability, ie. high ratio between peak and average value ('bursty'). Furthermore, the cost of each link is not shared, but rests on a single customer or, more rarely, on a small number of customers.

12 Some examples in telecommunications: Network Topologies Single star topology: each user is connected to the local exchange with a dedicated physical carrier Tree topology (active): each user is served through a tree configuration in which a first branch is shared (multiple access) and then, a second branch is dedicated (single access) as in the case of star access (es. Access) 12 Tree topology (two-level): as above, but with remote terminal, passive or active, in which the division of the signals takes place at two levels, the second of which in general in proximity to the enduser Mesh topology(es. Backbone) Bus topology(es. LAN) Ring topology(es. Metro) L = N(N-1) /2 43

access) as in the case of star access (es.

Fundamentals of Telecommunications

Fundamentals of Telecommunications Professor of CIS Columbus, OH 43210 [email protected] http://www.cis.ohio-state.edu/~jain/ 1 Overview Time Division Multiplexing T1, T3, DS1, E1 T1 Framing Echo Cancellation