Communication Fundamentals in Computer Networks

Transcription

1 Lecture 5 Communication Fundamentals in Computer Networks M. Adnan Quaium Assistant Professor Department of Electrical and Electronic Engineering Ahsanullah University of Science and Technology Room 4A07 adnan.eee@aust.edu URL- 1

2 Topics Circuit-Switched Networks From Circuit Switching to Packet Switching The Principle of Packet Switching Advantages of Packet Switching Packet header Disadvantages of Packet Switching 2

3 Circuit-Switched Networks In circuit-switched networks (also called switching networks), the communication partners have a fixed connection available. It is set up over a series of switching locations. 3

4 Circuit-Switched Networks An important example of a circuit-switched network is an analog telephone network. The connection to the desired party is established via the automated exchanges of the telephone network by dialing a number. After the participant on the opposite end has picked up the receiver, the connection remains active until the receiver is hung up. In a telephone network, speech is transmitted between the sender and the receiver with a more or less constant bandwidth. The maximum amount of data that can be transmitted per unit of time over a specific transmission medium is referred to as bandwidth. 4

5 Circuit-Switched Networks A circuit-switched network offers the communication partners a fixed data transmission rate at all times. Delay on a network transmission path is always uniform and remains minimal. The bandwidth of a connection is always limited by the weakest section of the route through the network. If one of the exchanges involved fails, the connection is lost and communication ends. In the worst case scenario it can happen that after the failure of just one exchange, entire subnets of the whole network become separated and inaccessible. 5

6 From Circuit Switching to Packet Switching The data traffic generated by networked-functioning computers is not steady in comparision to the normal telephone call, but occurs in varying clusters of activity, called as bursts. Bursts occur as intervals of maximum activity on which follow intervals of inactivity. How can a fail-safe data flow be ensured in computer communications, also when exchanges do not work with absolute reliability or can even fail completely? 6

7 The Principle of Packet Switching Packet switching is based on the principle of the disassembling the message to be sent into individual data packets. After disassembly (fragmentation), the packets can be transported over the communication network individually and independent of one another. The route taken by the individual packets is not fixed from the outset. The sender only determines the path to the next exchange. On the receiver s side the data packets are subsequently reassembled back into the original message. 7

8 The Principle of Packet Switching 8

9 The Principle of Packet Switching Special routing algorithms serve to find the best path through the network. On their path through the network, packets can able to avoid any congestion and interference occurring at the exchanges. Although their way might be longer, they can reach their target faster than packets traveling on a shorter but blocked path. In addition, the entire network can also be utilized. A connection is never occupied for a long time and as a result all potential senders have equal and fair access to the network. No user can block a connection for a long time as happens in circuit-switched networks. 9

10 The Principle of Packet Switching In packet switching it is necessary that it be possible for data packets to be buffered for as long as is necessary at the network exchange. Every packet switch has its own intermediate storage (input buffer and output buffer) for its incoming and outgoing data lines. If an incoming packet is to be forwarded via an output that is momentarily occupied with sending another packet, the packet is buffered in the output buffer until the output is free again. This type of exchange is thus called store-and-forward. 10

11 The Principle of Packet Switching 11

12 The Principle of Packet Switching The latency that occurs is variable and depends on the network load. If a data packet cannot be accepted or forwarded because the input or output buffer is full packet loss occurs. Either the incoming packet or a packet from the output buffer is discarded. The order in which the packets are sent from the output buffer corresponds to their arrival. Because packets can arrive from different inputs arbitrarily, i.e., in random order, this bundling of different inputs to one output queue at the packet switch is referred to as statistical multiplexing or as asynchronous multiplexing. The procedure contrasts to regular time division multiplexing (TDM), in which alternately each participant receives an equally large time slot. 12

13 Message Switching Message switching is a special kind of memory switching. Messages are not disassembled as in packet switching but sent in their entirety, i.e., in one single packet, through the network. Just as in circuit switching, no explicit connection is established in this type of message transfer. The entire message has to be buffered at the respective switching stations; this also determines to which exchange the message will be sent next. In the early days of telegraphy, telegrams were transmitted this way. Message switching is entirely unsuitable for data networks common today. This procedure would mean exchanges being blocked for a lengthy time or under certain circumstances their buffer not having enough storage space for even a single message. 13

14 Packet Switching vs. Message Switching In this example we can see that data transmission in the packet-switched network is three times faster than in the message-switched network. 14

15 Advantages of Packet Switching High network utilization Since the individual data packets in packet switching are usually very small, a high degree of network utilization can be reached. The waiting times for all other communication participants remains minimal. Fair resource allocation The communication network is available to all participants equally. All connected devices can send data packets alternatively according to a predetermined multiplex procedure. Rapid error detection If small data packets are always sent, transmission errors can be quickly detected and if possible corrected immediately. It s only necessary to re-transmit the faulty packets and not the whole message. High reliability If one switching station fails, the whole message is not lost as in the case of circuit switching. 15

16 Packet header So that the complete message in a packet switched network can reach its destination and be reassembled correctly again on the receiver side, all data packets must contain a range of additional information. This additional information is usually transmitted in a segment prefixing the package and is known as a packet header. 16

17 Packet header A Packet Header consists mainly of the following 4 parts : Address information Packet headers contain address details about the sender and receiver. Packet number The individual data packets have to be numbered consecutively. Padding data It is possible that the amount of data sent is smaller than the required packet size. In this case, the data packet must be supplemented with padding data. Error detection mechanisms To recognize the transmission errors, extra error detection data must be added. This does not only make it possible to identify errors but in some cases to also correct them. 17

18 Comparison 18

19 Transmission time 19

20 Disadvantages of Packet Switching Overload (Congestion) Because there are no dedicated, exclusive connections fixed in packet switching, it can happen that an exchange is not able to handle a sudden rush of incoming data packets, i.e., the available buffer overflows and as a result data packets are lost. Complex communication protocol All communication partners must commit to a network based on a common network protocol. For example, the basic data packets and parameters for the communication process must be established such as bit rates and data fragmentation. No quality of service guarantee Another disadvantage of packet switching is that it is not possible to guarantee a constant bandwidth for transmission without taking extra steps. 20