tailieunhanh - Practical TCP/IP and Ethernet Networking- P8

Practical TCP/IP and Ethernet Networking- P8: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | Networking fundamentals 17 Circuit switching In a circuit switched process a continuous connection is made across the network between the two different points. This is a temporary connection which remains in place as long as both parties wish to communicate that is until the connection is terminated. All the network resources are available for the exclusive use of these two parties whether they are sending data or not. When the connection is terminated the network resources are released for other users. A telephone call is an example of a circuit switched connection. The advantage of circuit switching is that the users have an exclusive channel available for the transfer of their data at any time while the connection is made. The obvious disadvantage is the cost of maintaining the connection when there is little or no data being transferred. Such connections can be very inefficient for the bursts of data that are typical of many computer applications. Packet switching Packet switching systems improve the efficiency of the transfer of bursts of data by sharing the one communications channel with other similar users. This is analogous to the efficiencies of the mail system as discussed in the following paragraph. When you send a letter by mail you post the stamped addressed envelope containing the letter in your local mailbox. At regular intervals the mail company collects all the letters from your locality and takes them to a central sorting facility where the letters are sorted in accordance with the addresses of their destinations. All the letters for each destination are sent off in common mailbags to those locations and are subsequently delivered in accordance with their addresses. Here we have economies of scale where many letters are carried at one time and are delivered by the one visit to your street locality. Efficiency is more important than speed and some delay is normal -within acceptable limits. Packet switched messages are broken into a .