Routing in telephone networks
- 1. Renjith V Ravi Assistant Professor(ECE) M.E.A Engg. College Perinthalmanna, Kerala,India Routing in Telephone Networks
- 2. Circuit Switching Circuit switching is used for call routing in the telephone network. Circuit switching refers to the mechanism of communication in which a dedicated path with allocated bandwidth is set up in an on- demand basis before the actual communication can take place. On-demand means that the path is set up quickly when the request is made. The dedicated path is released immediately when the communication is over. The most well-known application of circuit switching is telephone network calls.
- 3. Circuit Switching The call bandwidth for a wire-line telephone circuit is 4 kilohertz in the analog mode or 64 Kbps in the digital mode. That is, a voice connection in the wired telephone network takes up a voice circuit established through circuit switching, requiring 64 Kbps of bandwidth. When a circuit is considered on a link, it is also referred to as a trunk. Thus, the terms circuit and trunk will be used interchangeably. The term trunkgroup refers to a group of circuits or trunks on a link between two directly connected switches; a trunkgroup is also referred as an inter-machine trunk (IMT) while considered in the context of connecting two
- 4. Local Exchange Carrier (LEC) A telephone company which operates within a local area and provides telecommunication services within that area. A local exchange carrier (LEC) is the term used in the U.S.
- 5. Simplified view of the telephone network Subscribers are connected to a local exchange carrier (LEC). Central offices owned by LECs connect to a toll (wide-area) switch. Toll switches are connected to each other using single-hop paths.
- 6. Local Exchange Carrier (LEC) Three-level hierarchy 1. Subscriber telephone instruments or modems, 2. Central offices, 3. Long-distance or toll switching offices • The set of toll switches as the network core • The core is structured as a fully connected mesh or clique, with every toll switch connected to every other toll switch by a logical one-hop path.
- 7. Routing Policies This dense connectivity simplifies routing. The routing algorithm is as follows: i. If a call's source and destination are within a central office, directly connect them. ii. If the call is between central offices within a local exchange carrier, use a one-hop path between central offices. iii. Otherwise send the call to (one of) the core(s). The only major decision is at a toll switch, which chooses either a one-hop or a two-hop path to a destination switching system.
- 8. Toll switch A telephone central office switch (telco switch) that generates toll call transaction. End office switch A telephone central office switch that connects directly to the customer. Also called a "Class 5 switch" or "TDM switch," an end office switch is a computer specialized for TDM-based, circuit-switched telephone calls. It handles services such as call forwarding and call waiting. Tandem switch A telephone central office switch that links telecom end offices together and does not connect to the customer directly. Also called a "Class 4 switch" or "TDM switch," a tandem switch is a computer that is specialized for
- 9. Features Of Telephone Network Routing Stable load can predict pairwise load throughout the day can choose optimal routes in advance Extremely reliable switches downtime is less than a few minutes per year can assume that a chosen route is available can’t do this in the Internet Single organization controls entire core can collect global statistics and implement global changes Very highly connected network Connections require resources (but all need the same)
- 10. Dynamic Nonhierarchical Routing (DNHR) Simplest core routing protocol accept call if one-hop path is available, else drop DNHR divides day into around 10-periods in each period, each toll switch is assigned a primary one-hop path and a list of alternatives(two hop) can overflow to alternative if needed(spilling or Overflow) drop only if all alternate paths are busy The process in which a call rejected on a primary path is retried on an alternative path is called crankback Problems does not work well if actual traffic differs from prediction
- 11. Metastability Burst of activity can cause network to enter metastable state high blocking probability even with a low load Removed by trunk reservation prevents spilled traffic from taking over direct path
- 12. Trunk status map routing (TSMR) In DNHR, a central computer gives each toll switch a set of alternative paths based on past measurements, which are updated once a week. If a sudden surge of calls arrives on a trunk, the only adaptability in the network is to start trying the previously prescribed alternate paths. Although this was suitable for earlier toll switches that had very limited computational power, modern switch controllers can do much better.
- 13. Trunk Status Map Routing (TSMR) In this scheme, each switch controller measures the load on each of its outgoing links and tells this to a central computer. The central computer periodically computes optimal alternative paths for each toll switch (based, for example, on the current load) and loads these into all the toll switches. Thus, the central computer updates the choice of alternative paths more often than with DNHR.
- 14. Real-time Network Routing(RTNR) No centralized control Each toll switch maintains a list of lightly loaded links Intersection of source and destination lists gives set of lightly loaded paths Example At A, list is B,C,E => links AB, AC, AE lightly loaded At D, list is C, E, G => links DC, DE, DG lightly loaded A asks D for its list Intersection = D => AC and CD lightly loaded => ACD lightly loaded => it is a good alternative path Very effective in practice: only about a couple of calls blocked in core out of about 250 million calls attempted every day
- 15. REFERENCE 1. S. Keshav,” An Engineering Approach To Computer Networking: Atm Networks, The Internet, And The Telephone Network”, Addison-Wesley professional computing series. Pearson Education