Main types of services offered by computer networks to applications

Computer networks primarily offer two types of services to the applications. They are

i) a reliable connection oriented service byte stream service –

• In this type of service, the network offers a reliable byte streamed logical pipe between the sending and the receiving computers (similar to a registered post service offered by the postal department).
• By reliability, it means that every byte/bit sent via. the logical pipe from the sender is guaranteed to be received a) in-order as it was sent b) without a single bit/byte loss and c) without corruption of a single bit/byte even though the data travels over an underlying unreliable and erroneous media.
• By byte-oriented, it means that the basic unit of transfer is a byte.
• TCP (transmission control protocol) is the primary reliable protocol provided by computer networks for applications.

ii) an unreliable message oriented best effort service –

• In this type of service, the network offers an unreliable best effort message oriented logical pipe (similar to the ordinary post service) between the source and the receiving computers.
• By best effort, it means that the network would try its best to deliver data between the end points, but it cannot provide a guaranteed assurance that every bit/byte sent by a sender is received intact at the receiver.
• By unreliable, it means that if data packets are lost due to the erroneous nature of the underlying transmission channels (wired/wireless telecommunication links), then the network would not make any effort to detect such losses and take appropriate corrective action.
• By message-oriented, it means that the basic unit of transfer is a sequence of variable length bytes termed as a message
• UDP (User Datagram protocol) is the primary unreliable best effort service protocol provided by computer networks for applications.

There are other variations of these two primary types like a message oriented reliable service (e.g. Streamed Control Transport Protocol (SCTP)), but this discussion confines to the two primary types.

The table given below lists out the differences between the two types of service described above:

Comparison of two primary types of services offered by computer networks

Some of the points given in the above table are explained below:

• Flow control is the ability of the sender to dynamically control the sending rate/speed based on the receiver’s buffering/processing capability, so as to prevent loss of data at the receiver.
• Congestion control is the ability of the sender to control the sending rate/speed based on the underlying network’s bandwidth/processing capability (which may vary instantaneously), so as to prevent loss of data inside the network.
• Real time applications are those applications that cannot tolerate variable delays during transmission between the sending and the receiving computers. Examples are audio, video conferencing, voice transmissions etc.
• Non-Real time applications are those applications that are not affected by variable delays during transmission between the sending and the receiving computers. Examples are file transfer, email etc.

Similarities between Computer Networks and Postal Networks

While computer networks and postal networks have some differences, they have a lot of things in common, especially the basic principles of operation.

As we already know, computer networks use the principles of packet switching and next hop routing to achieve communication between remote computers.

Before comparing and contrasting computer and postal networks, a brief overview of the theory of operation of a postal network is given below. In postal networks,

• Communication happens in the form of short letters
• Letter is put inside an envelope
• Recipient and Sender address are written on envelope
• Envelope is then handed over to the postal department by posting it inside a post box
• Postal Department takes responsibility of letter delivery from sender to recipient
• Each letter is independently handled by postal department based on its address information
• Each post office has a routing table
• Routing table is consulted for routing an envelope to its destination
• Each post office passes on an envelope to one of its directly connected neighbors which it thinks is closer to the destination than itself
• Envelope is thus taken hop by hop by different post offices, closer and closer to the destination till it reaches destination
• Each hop is usually connected to its neighboring hop through either road, railway track, sea or air transport modes
• An envelope is carried between hops through one of these modes through appropriate vehicles and the mode may vary from hop to hop

Assume an envelope has to be taken from chennai (India) to Connecticut (USA)

A person writes a letter, seals it inside an envelope and posts it in the nearest post office, as illustrated in the diagram below:

A Letter creation process

The complete path typically taken by the envelope is illustrated in the diagram below:

Typical operation of a postal network

• Envelope is first taken from post box to head post office in chennai by road (van)
• It is then passed on to Mumbai by railway track (train) as Mumbai is India’s International gateway
• From Mumbai it is passed on to NewYork (USA) by air(plane)/sea(ship), as NewYork is a peer International gateway 
• It is then passed on from NewYork to Connecticut by road (van)
• From Connecticut head post office it is then delivered to the recipient by road (cycle/car)
• Envelope is thus delivered to recipient after being passed on between multiple hops through different transport modes
• No post office knows complete route taken by envelope !!
• Letters between same end points may take different routes
• Multiple letters of different end points are carried jointly through intermediate hops

After understanding the basic principles of computer and postal networks, let us compare them now.

There are lot of things common between computer and postal networks and they are given below:

• Communication happens through small envelopes/packets
• Each home/computer requires Unique postal/IP address
• Intermediate Networks needed
• End-End path needed for both (road/air/rail, wired/wireless links)
• No reservation of End-End path ( shared by multiple letters/packets)
• Signaling information sent along with data in each envelope/packet (Postal address/IP address)
• Next Hop Routing used in both
• Envelopes/Packets between same end points may take different intermediate paths
• Both are best-effort delivery mechanisms

While there are lot of things in common, there are still a few differences between a computer network and a postal network. They are given below:

• While postal networks use a physical envelope to carry information, computer networks use electronic packet data to carry information.
• While postal networks use road, rail, air with their respective vehicles like van, train, plane as information carriers, computer networks use wired/wireless telecommunication links with electromagnetic/optical signals to carry information.

Since the common things are much more than the differences, it can be concluded that computer networks and postal networks have similar basic principles of operation.