Telecommunication Networks Categories Up to now we explained the operation of the public switched telecommunications network (PSTN) and used the conventional telephone network as an example. 1 Public network include many other networks that are optimized to provide services with different characteristics. We can divide telecommunications networks into categories in many different ways: two broad categories, public networks and private (or dedicated) networks. Telephone Network (PSTN) ISDN Mobile Telephone Networks Private Networks: Professional mobile radio (PMR) Virtual Private Networks Public Networks: Telex Network Paging Networks Public Data Networks Internet Radio and Television Networks 44
PSTN 2 Public switched telephone network (PSTN) in the most ancient TLC network Involvesanaloguevoice transmissionoverthe frequency band 300 3400 Hz: this was the only service it was originally designed to carry on it, called POTS (Plain Old Telephone Service) Whendigitaldata havetobetransmittedoverthe PSTN theymustbetreatedas analogue in nature The PSTN consists of local networks interconnected by a long-distance network 45
ISDN 3 ISDN: all information is transmitted in digital form from end-to-end Requires replacement of analog subscriber interface units with digital ones Ordinary two-wire subscriber loop of the telephone network (PSTN) is upgraded to the basic rate access of ISDN by a network termination (NT) at the subscriber premises and by a basic rate interface (BRI) unit and ISDN software in the local exchange ISDN basic rate interface 46
ISDN Access Types 4 ISDN provides two types of access, called basic access and primary access, respectively: Basic access: is the typical access for residential users that offers three digital channels on a single twisted pair, two 64 kbit/s channels can be used as independent phone lines, referred to as B-channels, and one 16 kbit/s channel (optional), called D-channel, used for signaling and for the transmission of packet data. Therefore, the basic access, characterized by a total speed of 144 kbit/s in each direction of transmission, is also known as 2B + D access. Primary Access (PRI): scheduled for business users, offers 31 channels of which thirty 64 kbit/s, with the addition of one D-channel at 64 kbit/s used only for signaling between the user and the network. The primary access, also called 30 B+D access, is realized with a conventional 2 Mbit/s line. 47
IntelligentNetwork (IN) Conventional telephone network able to establish a connection only to a socket identified by the number of a B subscriber: dialing a certain number makes every time a connection to a certain socket. IN is an ordinary digital telephone network with some additional capabilities like flexible routing of calls and voice notifications In an IN the physical number and service number have no fixed relation and may change with time. The basic structure of an IN, illustrated in Figure below, is based on centralized intelligence. 5 48
IN: DistributedIntelligence 6 Implementation of supplementary services in local exchanges Examples of supplementary services: Call forwarding permits to direct incoming calls to another telephone Call waiting means that, during a call in progress, a subscriber is notified of an incoming call Automatic callback can be used when the number you are trying to callisbusy Abbreviated dialing permits a subscriber to specify short numbers that correspond to complete telephone numbers Screening of incoming and outgoing calls allows a subscriber to specify which telephone numbers he does not want to receive calls from, or makecallsto. 49
Internet 7 Internet is a worldwide packet-switched network Itisa computer network ofnetworks having public access It realizes the global interconnection between networks of different nature and extent, made possible by a common OSI Layer 3 network protocol called IP (Internet Protocol) ARPANET, the Internet ancestor, was developed in the late 1960s by the U.S. Department of Defense In the 1990s the user-friendly graphical user interface World Wide Web (WWW) was introduced The Internet as of 2003 Source: http://www.opte.org/maps/tests/ 50
Circuit-switching vs. packet-switching Circuit-switching In circuit-switching, the path is decided before the data transmission starts. The system decides which route to follow, and transmission goes according to the path. For the whole length of the communication session between the two parties, the route is dedicated and exclusive. Therefore, it is released only when the session terminates. Packets Let us first clarify what a packet is. IP breaks data into chunks and wraps them into structures, the packets. Each contains, along with the data load, information on the IP address of source and destination nodes, sequence numbers and some control information. Once packets reach destination are reassembled to make up the original data flow. Obviously, to transmit data in packets, it has to be in digital form. Packet-switching In packet-switching, packets are sent towards the destination irrespective of each other. Each packet has to find its own route to the destination. There is no predetermined path; the decision as to which node to hop in the next step is taken only when a node is reached. Each packet finds its way using the information it carries, such as the source and destination IP addresses. 8 51
Public Telecommunication Networks Worldwide public TLC networks, even if based on different standards and able to carry differentservicesare generallyinterconnected: to day, more and more they are based on the common fabric represented by the global Internet. This worldwide TLC network contains the Public Switched Telephone Network (PSTN) and the ISDN which together provide basic analog/digital telephony, the Public Land Mobile Network (PLMN), which provides access for cellular subscribers, as well as many other networks. Internet users are connected to the global Internet via the hosts of their Internet Service Provider (ISP). Showntwomainmethodsfor accessing the Internet: a) telephone or ISDN network used for dial-up connections b) ADSL providing permanent higher data rate Internet service. Digital PBX/PABX connected to a local exchange with a 1,544/2,048-kb/s digital line with capacity of 23/30 simultaneous calls. This connection is called the primary rate interface in the case of ISDN. Overview of the public TLC network 52 9