Author: Neftaly Malatjie

• This section describes the difference between circuit switching (CS) and packet switching (PS).

  PS is used for time insensitive applications such as internet/email/SMS/MMS/VOIP etc. In CS even if user is not talking the channel cannot be used by any other users, this will waste the resource capacity at those intervals.

   
  The example of circuit switched network is PSTN and example of packet switched network is GPRS/EDGE. Following table summarizes difference between circuit switching and packet switching of type datagram and virtual circuit.

  Circuit Switching	Packet Switching(Datagram type)	Packet Switching(Virtual Circuit type)
  Dedicated path	No Dedicated path	No Dedicated path
  Path is established for entire conversation	Route is established for each packet	Route is established for entire conversation
  Call setup delay	packet transmission delay	call setup delay as well as packet transmission delay
  Overload may block call setup	Overload increases packet delay	Overload may block call setup and increases packet delay
  Fixed bandwidth	Dynamic bandwidth	Dynamic bandwidth
  No overhead bits after call setup	overhead bits in each packet	overhead bits in each packet

   

• Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.
• Telephone switching equipment, for example, establishes a path that connects the caller’s telephone to the receiver’s telephone by making a physical connection.
• With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated.
• Routing decisions must be made when the circuit is first established, but there are no decisions made after that time
• Circuit switching in a network operates almost the same way as the telephone system works.
• A complete end-to-end path must exist before communication can take place.
• The computer initiating the data transfer must ask for a connection to the destination.
• Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer.

Advantages:

• The communication channel(once established) is dedicated.

Disadvantages:

• Possible long wait to establish a connection, (10 seconds, more on long- distance or international calls.) during which no data can be transmitted.
• More expensive than any other switching techniques, because a dedicated path is required for each connection.
• Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it.

Packet Switching  

• Packet switchingcan be seen as a solution that tries to combine the advantages of message and circuit switching and to minimize the disadvantages of both. 
• There are two methods of packet switching:Datagram and virtual circuit.
• In both packet switching methods, a message is broken into small parts, calledpackets.
• Each packet is tagged with appropriatesource and destination addresses.
• Since packets have a strictly defined maximum length, they can bestored in main memory instead of disk; therefore access delay and cost are minimized.
• Also the transmission speeds, between nodes, are optimized.
• With current technology, packets are generally accepted onto the network on afirst-come, first-served basis. If the network becomes overloaded, packets are delayed or discarded (“dropped”).

The size of the packet can vary from 180 bits, the size for the Datakit virtual circuit switch designed by Bell Labs for communications and business applications; to 1,024 or 2,048 bits for the 1PSS switch, also designed by Bell Labs for public data networking; to 53 bytes for ATM switching, such as Lucent Technologies’ packet switches

• Inpacket switching, the analog signal from your phone is converted into a digital data stream. That series of digital bits is then divided into relatively tiny clusters of bits, called packets. Each packet has at its beginning the digital address — a long number — to which it is being sent. The system blasts out all those tiny packets, as fast as it can, and they travel across the nation’s digital backbone systems to their destination: the telephone, or rather the telephone system, of the person you’re calling.
• They do not necessarily travel together; they do not travel sequentially. They don’t even all travel via the same route.
• But eventually they arrive at the right point— that digital address added to the front of each string of digital data — and at their destination are reassembled into the correct order, then converted to analog form, so your friend can understand what you’re saying.
• Datagram packet switching is similar to message switchingin that each packet is a self-contained unit with complete addressing information 
• This fact allows packets to take a variety of possible paths through the network.
• So the packets, each with the same destination address, do not follow the same route,and they may arrive out of sequence at the exit point node (or the destination).
• Reordering is done at the destination point based on the sequence number of the packets.
• It is possible for apacket to be destroyed if one of the nodes on its way is crashed momentarily. Thus all its queued packets may be lost.
• In the virtual circuit approach, a preplanned route is established before any data packets are sent.
• A logical connection is established when a sender send a “call request packet” to the receiver and the receiver send back an acknowledge packet “call accepted packet” to the sender if the receiver agrees on conversational parameters.
  • The conversational parameters can be maximum packet sizes, path to be taken, and other variables necessary to establish and maintain the conversation.
  • Virtual circuits imply acknowledgements, flow control, and error control, so virtual circuits are reliable. That is, they have the capability to inform upper-protocol layers if a transmission problem occurs
  • In virtual circuit, the route between stations does not mean that this is a dedicated path, as in circuit switching.
• A packet is still buffered at each node and queued for output over a line.

Switching Techniques – In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels.

There are four typical switching techniques available for digital traffic.

    * Circuit Switching

    * Packet Switching

The architecture on which you choose to base your network is the single most important decision you make when setting up a LAN. The architecture defines the speed of the network, the medium access control mechanism it uses (for example, collision detection, token passing, and so on), the types of cables you can use, the network interface adapters you must buy, and the adapter drivers you install.

The Institute of Electrical and Electronic Engineers (IEEE) has defined and documented a set of standards for the physical characteristics of both collision-detection and token-passing networks. These standards are known as IEEE 802.3 (Ethernet) and IEEE 802.5 (Token-Ring), respectively. IEEE 802.11 (Wi-Fi) defines wireless versions of Ethernet.

The most common choice today for new networks is Ethernet (both wired and wireless). In rare cases, you may encounter a Token-Ring or ARCnet network. Network data-link architectures you might encounter are summarized in the following table. The abbreviations used for the cable types are explained in the following sections.

On completion of this section you will be able to Explain LAN architect