Telecom networks is mostly used today for wide area communication. Stringing a wire between every pair of telephones that might want to communicate was not a good long term strategy. A better idea was to connect all the telephones to a central switching office. There an operator could connect one telephone to another via a switch board.
Routing a telephone call:
A call is routed up through higher level switching offices until it reaches a switching office that can reach the destination telephone by connecting with lower level switching offices, which examine the digits of the telephone number you dialled to make these decisions.
Connection Oriented Services – I :
A dedicated connection between the end points are maintained throughout the session. Usually, this means that the quality of service can be reasonably guaranteed to the extent of the bandwidth for the channel that is established. Message bits arrive in the same order in which they are sent. Traditional telephone circuit is a circuit switched connection oriented system.
Transmission Media in Telephone Systems:
In traditional analogue telephone systems, the telephone is connected to the local exchange via category 3 UTP cables. This connection is called the local loop. It is typically between 1km and 10km length. Higher up in the hierarchy, higher bandwidth cables are used to carry multiple telephone calls. This is far cheaper than using separate cables for separate calls. Specifically, digital lines on fiber is used. Analogue systems used a technique called frequency division multiplexing (FDM) to do this.
The local loop:
The subscriber hand sets are powered by a battery bank in the exchange. With echo suppressors, the transmission is half duplex. With echo cancellers, it is possible to have full duplex communication. Since local loop is still analogue, we need modems for sending digital data. Since one bit is used for control purposes, we usually get 56kbps speed. Depending on the line condition, modems may automatically negotiate a lower speed.
Signaling refers to the information exchanges between terminal devices, exchanges and routers for setting up circuits, termination, billing, advanced network services etc. In common Channel signaling which is in band, some of the bits in the frame is used for this purpose, in which is SS7 is considered standard. Whether In band or Out of band, logically the switch controllers maybe considered to be an overlay network in the control plane. Out of band signaling (CCIS – Common Channel Interoffice Signaling) is more flexible as it allows arbitrarily complex message transfer, as they do not interfere with the regular channel.
Control Plane Protocol Stack in SS7:
- Application Service Element (ASE) – Application level functionality such as interpreting signaling messages. Transaction Capabilities Application Part(TCAP) allows system to invoke procedure calls on remote machines.
- Message Transfer Part 1 (MTP-1) – it is physical bit transfer usually on a digital line like E1. One of the main application of SS7 is Telephone User Part (TUP) which is responsible for setting up voice calls. The TUP interprets dialed digits, routes, reserves resources, maintains accounts etc.
Digital Technology in Telephone Networks:
Over the past 30 years, much of the traditional analogue telephone network has been replaced by digital technology. A device called codec (coder/decoder) is used to convert analogue voice signals into digital information that can be handled by the digital technology. The codec is also used to convert the digital signals back into analogue voice signals that can be handled by the older analogue technology.
Only the local loop is still analogue and this loop can be replaced by Integrated Services Digital Network (ISDN) connections. It was envisaged as an end to end digital service. Home users would be connected by same Cat3 cable. the digital bit streams are time multiplexed. The system uses Out of band signalling and uses the D channel for that. NT1 is a network terminating device which connects to the ISDN exchange on one side and a local passive bus on the other from which up to eight terminal devices can hang. ISDN turned out to be very expensive and obsolete even before the standard was finalized. The vision of broadband ISDN was sought to be realized by ATM.
Digital Subscriber Loop:
The next attempt for converting the analog local loop went in two directions:
- Large users could be serviced by taking fiber to their premises which could terminate on an integrated digital outlet.
- The market for small users was driven by the demand for more bandwidth from the subscriber for Internet connection over the same Cat3 cable.