1. The Origins of Circuit-Switching: from a fully meshed topology to a star topology. Circuit switching facilitates the addition of new user. The switch still handles a number of circuits, but the user equipment –the telephone set – remains simple.
2. At present, several hierarchical levels of switches are implemented all together to make up the PSTN. The user equipment is called a terminal and the set of switches is called the network.
3. The key to circuit-switching: switches are oblivious to the contents of the message they are transporting.
Strengths of circuit-switching: fast and remains the same for the duration.
Weaknesses of circuit-switching: Once a path is established between two terminals, the resources associated with it cannot be used for another connection until the path is torn down.
4. delay: establishment delay and switch delay.
5. The purpose of signaling is to exchange control information between systems of any kind.
6. The presence of signaling in telephony implies two kinds of traffic: signaling traffic and voice traffic.
7. The evolution path of telephone system:
l Local and central battery systems: simple signaling, operator, switchboard, manually, battery
l DC and AC Analog Systems: allow signaling backwards, caller-busy tones-callee
l FDM and In-band Signaling: where each circuit used for telephony consisted of a copper wire, so a switch handling 500 circuits needed 500 cables. FDM made it possible to use a simple wire for several simultaneous calls. Then the signals within a cable between two switched were needed to control many circuits instead of one. The solution would prove to be in-band signaling system, meaning that signaling is carried on the same frequency as speech.
l Analog Transmission: every signal concentrates most of its energy within a certain range of frequencies – so-called spectrum of the signal. It is necessary to transmit voice ranged from 300 Hz to 3,400 Hz.
l Digital Transmission: simple user equipment and complex networks. Its weakness lies in delay and complexity.
l TDM: easier to implement and cheaper to run than FDM. Connecting exchanges allocate time slots between them to certain channels. TDM too requires the use of buffers in the exchanges.
l Digital Signaling Systems: analog signaling is expensive and inefficient exchange-to –exchange. Digital signaling is usually described by means of protocol state machines.
l Access and Trunk Signaling: access signaling – signaling that passes between terminals and the network; network signaling – trunk signaling.
8. Access Signaling: off/on hook signals, dialed numbers and information tones. Digital Subscriber Line No.1 (DSS-1) for GSM and BTS.
9. Trunk Signaling: Channel Associated Signaling(CAS) and Common Channel Signaling(CSS)
l CAS: the signaling and the voice associated with a certain call are transferred along the same channel. Use time slot number 16 in each PCM link.
l CCS: voice and signaling traverse different paths through the network. These intermediate nodes are called Signaling Transfer Points. SS7 is the most widespread CCS system at present.
Trackback: http://tb.donews.net/TrackBack.aspx?PostId=1186145