TELECOMM SWITCHING

PACKET SWITCHING:
 In packet-based networks, the message gets broken into small data packets. These packets are sent out from the computer and they travel around the network seeking out the most efficient route to travel as circuits become available. This does not necessarily mean that they seek out the shortest route.
 Each packet may go a different route from the others.
 Each packet is sent with a ‘header address’. This tells it where its final destination is, so it knows where to go.
 The header address also describes the sequence for reassembly at the destination computer so that the packets are put back into the correct order.
 One packet also contains details of how many packets should be arriving so that the recipient computer knows if one packet has failed to turn up.
 If a packet fails to arrive, the recipient computer sends a message back to the computer which originally sent the data, asking for the missing packet to be resent.
 Two types of packet switching: “datagram” and “virtual circuit”.
• Datagram
 In datagram packet switching, each packet is treated as a completely separate entity.
 It may be sent on a different route through the network from the previous one and/or the next one, and arrive out of sequence. Connectionless.
 Each packet must carry a header containing source and destination addresses, message identifier and sequence number. This is an overhead.
 The message can then be re-assembled at the receiver.
• Virtual Circuit
 In virtual circuit operation, the same route is followed by all packets belonging to the same message. Connection oriented. Now they cannot arrive out of sequence, so no sequence number is necessary. The allocation of a virtual circuit to a message also removes the need for source and destination addresses on every packet.
 However, virtual circuit needs to be set up by sending a special packet from source to destination and back to mark the route before traffic can flow. A different kind of overhead.
 For short messages datagram is used, for long ones virtual circuit is used.
• Advantages:
» Security
» Bandwidth used to full potential
» Devices of different speeds can communicate
» Not affected by line failure (rediverts signal)
» Availability – do not have to wait for a direct connection to become available
» During a crisis or disaster, when the public telephone network might stop working, e-mails and texts can still be sent via packet switching
• Disadvantages
» Under heavy use there can be a delay
» Data packets can get lost or become corrupted
» Protocols are needed for a reliable transfer
» Not so good for some types data streams e.g real-time video streams can lose frames due to the way packets arrive out of sequence.
Examples of circuit switched networks
• LAN and Internet
• New Mobile Phone Technology (GPRS)
• Asynchronous Transfer Mode(ATM) (uses Virtual Circuit Switching)
• Multiprotocol Label Switching (MPLS) and Resource Reservation Protocol (RSVP)
• X.25 Protocol

CIRCUIT SWITCHING:
 There are three phases in circuit switching:
• Establish
• Transfer
• Disconnect
 The telephone message is sent in one go, it is not broken up. The message arrives in the same order that it was originally sent.
 In modern circuit-switched networks, electronic signals pass through several switches before a connection is established.
 During a call, no other network traffic can use those switches.
 The resources remain dedicated to the circuit during the entire data transfer and the entire message follows the same path.
 Circuit switching can be analogue or digital.
 With the expanded use of the Internet for voice and video, analysts predict a gradual shift away from circuit-switched networks.
 A circuit-switched network is excellent for data that needs a constant link from end-to-end. For example real-time video.
 A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two locations in exchange for a monthly rent (hence the term lease). It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN (Circuito Diretto Numerico) in Italy. Unlike traditional PSTN lines it does not have a telephone number, each side of the line being permanently connected to the other. Leased lines can be used for telephone, data or Internet services. Some are ringdown services, and some connect two PBXes.
 A permanent telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the speed of the circuit. Because the connection doesn't carry anybody else's communications, the carrier can assure a given level of quality.
 An internet leased line is a premium internet connectivity product, delivered over fibre normally, which is dedicated and provides uncontended, symmetrical speeds. It is also known as an ethernet leased line, DIA line, data circuit or private circuit.
 For example, a T-1 channel can be leased, and provides a maximum transmission speed of 1.544 Mbps. The user can divide the connection into different lines for multiplexing data and voice communication, or use the channel for one high speed data circuit. Increasingly, leased lines are being used by companies, and even individuals, for Internet access because they afford faster data transfer rates and are cost-effective for heavy users of the Internet.
 In computer networks and telecommunications, a dedicated line is a communications cable or other facility dedicated to a specific application, in contrast with a shared resource such as the telephone network or the Internet.
 In practice, such services may not be provided by a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guaranteed for more public systems. Such properties add a considerable premium to the price charged.
 As more general-purpose systems have improved, dedicated lines have been steadily replaced by intranets and the public Internet, but they are still useful for time-critical, high-bandwidth applications such as video transmission.
• Advantages:
» Circuit is dedicated to the call – no interference, no sharing
» Guaranteed the full bandwidth for the duration of the call
» Guaranteed Quality of Service

• Disadvantages:
» Inefficient – Coz the equipment may be unused for a lot of the call, if no data is being sent, the dedicated line still remains open
» Takes a relatively long time to set up the circuit
» During a crisis or disaster, the network may become unstable or unavailable.
» It was primarily developed for voice traffic rather than data traffic.
Examples of circuit switched networks
• Public Switched Telephone Network (PSTN)
• ISDN B-channel
• Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD) service in cellular systems such as GSM
• Datakit
• X.21 (Used in the German DATEX-L and Scandinavian DATEX circuit switched data network)

PACKET SWITCHING vs CIRCUIT SWITCHING:
 It is easier to double the capacity of a packet switched network than a circuit network – a circuit network is heavily dependent on the number of channel available.
 It is cheaper to expand a packet switching system.
 Circuit-switched technologies, which take four times as long to double their performance/cost, force ISPs to buy that many more boxes to keep up. This is why everyone is looking for ways to get Internet traffic off the telephone network. The alternative of building up the telephone network to satisfy the demand growth is economically out of the question.
 The battle between circuit and packet technologies has been around a long time, and it is starting to be like the old story of the tortoise and the hare. In this case, the hare is circuit switching—fast, reliable and smart. The hare starts out fast and keeps a steady pace, while the tortoise starts slow but manages to double his speed every 100 meters.
 If the race is longer than 2 km, the power of compounding favours the tortoise.