Connection-oriented vs. Connectionless Transfer

 Connection-oriented vs. Connectionless Transfer

When we call abroad using telephone lines, the entire telephone line including the expensive intercontinental trunk is occupied for the duration of the call. This is known as connection oriented solution. Almost half of our verbal communication is punctuated with pauses without which our sentences don't make sense. The problem with connection-oriented transfer is that it keeps the line busy even though there is no data transfer.

The connectionless mechanism does not establish the connection beforehand, so the entire line is not occupied throughout the duration of the call. The only line which is transferring our packet (the voice travels as small digitized chunks known as packets in connectionless mechanism) is occupied. Consider the following example depicted in Figure 1.3 to understand connection-oriented communication.

The caller calls from Ahmedabad. The call is connected to an exchange at Vadodara. From Vadodara, it gets connected to Mumbai from where an international trunk operates it through Port Elizabeth. The ultimate destination, Johannesburg, is reached via an exchange at Bloemfontein. It is assumed that the traffic flows through this route which is geographically correct, but telephone companies might follow some other route. For our purpose, the route is not going to make much of a difference, so we will not be worried about that. Now in case of telephone lines, the entire line from Ahmedabad to Johannesburg is occupied, conceptually.

Let us now consider the connectionless transfer. In this case, the packets being transferred work in a different way. When the caller calls from Ahmedabad, a specific portion of the line from Ahmedabad to Vadodara is occupied and when the packet reaches there, it is released. When the packet starts travelling from Vadodara to Mumbai, the line connecting only these two cities is used. When the packet reaches Mumbai, the line between Vadodara to Mumbai is released. In the same way, as soon as the packet reaches Port Elizabeth, the portion is released for other usage. Here onwards, only local lines are occupied.

Thus the advantage of connectionless scheme is that the lines can be used for other callers when we are not using that line and thus we have to pay less. There are two more advantages of this scheme. First, the packet, being in digital form, can be compressed, which can be as less as 8 kb (kb in networking parlance is kilobyte, i.e., 1000 bytes and not 1024 bytes) per second. Normal telephone lines use 64 kb per connection¹0. So the requirement is 8 times less. The other advantage is that the packet is multiplexed with other packets. Thus the sender packs as many packets as possible on the line. If we are not speaking and not generating packets, then the line is free to send other packets at that point of time. That means we are not paying when we halt to take a breath while talking! Now you can understand why calling through Internet is cheaper.

While transferring packets, the connectionless transfer offers a few additional advantages. The mechanism is more robust. Assume in the above case, our voice communication takes 10 packets to communicate with the other end. After two packets pass through the Vadodara-Mumbai line, the line crashes when only one packet travels on that line. It takes Vadodara a while to realize that Vadodara-Mumbai line has crashed. Vadodara now starts sending the data through another line, say, Vadodara-Surat-Mumbai. What is lost? The earlier two packets now traveling on the Mumbai-Port Elizabeth line are not even aware of this crash. The remaining packets are going to take a different route. Only one packet which was on the line between Vadodara to Mumbai is lost." Thus, even when the line crashes, our mechanism provides minimum loss and can continue from there to complete the job. On the contrary, while using connection-oriented transfer, if the line chocks up, the entire set of connections passing through is lost.

In summary, the connectionless transfer is cheaper and more robust. These two are the advantages of connectionless transfer. The connection-oriented mechanism has its own advantages though. When the line is reserved, we get guaranteed quality in voice without any delay and compression. Internet, by design itself, does route each packet independently. It makes each packet reach the destination with delay differences which creates distortion in voice. (If we have to sing songs, may be for telephonic Indian Idol, we should choose a normal telephone line and not an Internet connection!) The quality of voice through Internet is poor as compared to telephone lines due to the compression and delay differences between packets.

When we use the connection-oriented mechanism to transfer data, for example, while browsing the Net, downloading a file or remote login to a server using Telnet, the connection between the sender and the receiver is established before the actual communication starts. Thus every intermediate router comes to know about that connection beforehand. All of them expect it to occupy some resources like memory and processor time. Hence there won't be problems like-when the packet comes, there is no space to hold it or the CPU is not available for a long period to process it.

One more advantage that is always associated with a connection-oriented system is the order. A connection-oriented system is always expected to maintain the order of packets. The packet which is sent first is received first. On the other hand, in a connectionless system, the packets pumped into the network are free to take different routes and it is possible with non-negligible probability that the packet which is sent first is received second (or third or may be last!).

One more advantage associated with connection-oriented transfer is its reliability. As the entire line is at our disposal, we are not going to be bothered till the connection is over. In connectionless transfer, a single greedy user sharing a line with us can make us feel the difference. He can start pushing a lot of traffic into the network and slow down the connection considerably. For example, it is quite common in Internet to have a flying speed at one point and almost a snail's pace at the other point. It is because the Internet traffic is connectionless and anybody can join and share the resources at any point of time, without any rationing. In a normal telephone line, on the contrary, when all lines are occupied and a new user is trying to call, he gets a busy tone. Thus the users currently using the services are not affected unlike Internet.