15.4 ISSUES IN COMPUTER NETWORKING

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To design and develop computer networks is a challenging task because there are many issues to be considered. Here are some of the major issues:

Geographic area: The area to be covered by the network is of foremost importance. The area of coverage can be a single floor in a building, multiple floors in a building, a large campus with different buildings, an entire city with offices in different locations, a country with offices in different cities, or the world with offices in different countries. Based on the area of coverage, the communication media is chosen. A typical corporate network can use different media—fiber for LAN, copper wire or terrestrial radio for MAN (metropolitan area network), fiber or satellite for WAN (wide area network), and so on. In addition to the geographic coverage, what services are to be supported also determines the type of medium—voice and video communication require larger bandwidths.

Note

A wide area network can use a combination of different transmission media—such as satellite radio, coaxial cable and optical fiber. Generally, the speeds of wide area networks are much lower compared to the speeds of local area networks.

Services to be supported: Nowdays, computer networks need to support not just data, but voice, fax, and video services as well. This requirement needs to keep in mind the available bandwidths and cost considerations. The higher the bandwidth requirement, the higher the cost. For different types of services to be supported, different application-level protocols are required.

Note

Though in the initial days of computer networks, mostly data services were supported, nowdays voice and video services are also becoming predominant. Real-time communication of voice and video over computer networks requires additional protocols.

Security: To ensure that the network provides secure communication is of paramount importance. To ensure secrecy, necessary protocols need to be implemented. To support applications such as e-commerce, security is a must.

Different computing platforms: Computers have a wide variety of processors and operating systems. Because of these differences, filesystems will be different, data formats will be different, and filename conventions will be different (for instance, Unix versus Windows 9x). The protocols need to be designed in such a way that computers with different operating systems and data formats can communicate with one another.

Error control: The transmission media introduce errors during the transmission. Also, because of congestion in the networks, some packets may be lost. The protocols need to take care of the errors. Error detection is the first step: the receiver has to check whether the data that has been transmitted is received correctly using some additional bits called CRC (cyclic redundancy check). If the data is incorrect, the receiver has to ask for retransmission of the packet. Alternatively, error-correcting codes can be used to correct a few errors at the receiver without asking for retransmission.

Note

If errors are detected at the receiving end, the receiver has to ask for retransmission of packets. If the transmission medium is not very reliable, this results in a lot of retransmissions, causing waste of bandwidth. To reduce retransmissions, error-correction is a good approach and is generally used in satellite communication systems.

Flow control: As computers on a network will be of different capacities in terms of processing power, memory, and so forth, it may happen that one computer may not be able to receive packets with the same speed with which the other computer is sending. Protocols should be designed to control the flow of packets—the receiver may have to inform the sender "hold on, do not send any more packets until I ask you to." This mechanism is handled through flow control protocol. Flow control poses special problems in high-delay networks, such as satellite networks where the roundtrip delay is quite high.

Addressing: When two computers in a network have to share information, the sender has to specify to whom the packets are addressed. The receiver has to know from where the packets have come. So, addressing needs to be handled by the protocols.

Note

The global Internet solves the addressing problem by assigning a unique address to every machine connected to it. This address is known as the IP address.

Type of communication: Generally, two computers need to talk to each other. Cases also arise when a computer has to broadcast a packet to all the computers on a network. Also, there may be a packet being sent from one computer to a selected number of computers (for video conferencing between computers). This type of communication can be point-to-point, broadcasting, or multicasting. The protocols must have the capability to take care of these different types of communications.

Note

Multicasting is a requirement in applications such as video conferencing. For instance, if five persons situated at five different locations want to participate in a video conference, the data from each location needs to be sent to the other four locations.

Signaling packets: Before the actual data transfer takes place, it may be necessary to set up the call (as in the case of virtual circuit service) and disconnect the call. Protocols need to be designed for this to happen through transmission of special packets.

Congestion control: The packet switch will have a limited buffer for queuing the incoming packets. If the queue is full and the packet switch cannot take any more packets, then it results in congestion. Protocols need to take care of congestion to increase the throughput and reduce the delay. Two alternatives can be implemented: (a) send a control packet from the congested node to the other nodes informing them to reduce the number of packets, or (b) inform the other nodes to follow different routes. The strategies followed for congestion control are similar to the strategies adopted by traffic police at traffic islands.

Note

In computer networks, it is difficult to predict the traffic. Suddenly, there may be heavy traffic as a result of which some packet switches cannot handle incoming packets at all. In such cases, the packets might be discarded.

Internetworking: Based on the need, different networks need to be connected together, such as a LAN and a WAN. The protocols used in the two networks are likely to be different, and to inter-work, there must be some machines that do the protocol conversion. This protocol conversion is achieved by entities known as routers or gateways.

Segementation and reassembly: When two networks are interconnected, there is a possibility that the packet sizes supported by the two networks are different. There must be some designated machines that will take care of the differences in packet size. The large packets have to be broken down into smaller packets (segmented) and later put together (reassembled).

In developing computer networks using packet switching, a number of issues need to be considered. These include the geographical area to be covered, services to be supported, security, different computing platforms, mechanisms to control errors and the different speeds of the computers, addressing, support for real-time communication services such as voice/video, whether networking of networks is required, etc.

Real-time communication: For data communication (such as e-mail and file transfer), there may not be a strict limit on the delay with which a packet has to be received at the receiver. However, for real-time communication such as fax, voice, or video over packet networks, the delay is an important parameter. For instance, once the speech starts being played at the receiving end, all the packets should be received with constant delay; otherwise there will be breaks in the speech. Special protocols need to be designed to handle real-time communication.

Network management: To ensure that the computer networks operate as per the user requirements, to detect faults, to analyze traffic on the network, to rectify problems if any, etc., we need to design protocols whereby the networks can be managed well. Network management protocols take care of these issues.

These are just a few issues in computer communication, we will discuss how these issues are resolved for developing user-friendly computer networks, throughout this book.

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