A protocol is more like a language that can be shared by many people. If all the people would like to use the same language, the protocol becomes a standard. The same is applicable with networks. Most networks have one feature in common that they transmit information by breaking the original information into a set of messages (called packets), transmitting these packets sequentially, and then reassembling these packets back into the original information.
Each packet is a string of bits. But these packets sometimes have to travel a long way, over a bunch of different networks, there are typically lots of computers sending and receiving information over the same network, and the information getting sent can often get altered during transmission (this is called a transmission error). In order to make sure that the original information gets transmitted correctly, to the right place, computer networks use a set of rules, called networking protocols.
Thus, a data communication protocol is a set of rules that must be followed for two electronic devices to communicate with each other. These rules cover:
- Syntax: Data format and coding
- Semantics: Control information and error handling
- Timing: Special matching and sequence
The tasks which a protocol does fall into two categories
- Network layer tasks
- Data-link layer tasks
The rules associated with each category are called network layer protocols and data-link layer protocols. Network-layer protocols
Network-layer protocols are also known as Layer-3 protocols. These rules specify how a network does the following three tasks:
- Packetizing: breaking up the information into packets, and reassembling the packets at the receiving end.
- Addressing: Determining which computer and network the packets are headed to.
- Routing: Determining the best way for the packets to get to where they are headed.
The most common example of Network-layer Protocol is TCP/IP.
Data-link layer protocols
The Data-link layer protocols are also known as Layer-2 protocols. These rules specify how a network does the following three tasks:
- Delineation: Figuring out when a message (packet) begins and ends.
- Error control: Figuring out when a transmission error has occurred, and correcting it.
- Channel access: Controlling when a particular client or server accesses the channel.
Ethernet is the most common example of Layer-2 protocol. The token ring protocol, developed by IBM is another example. The networks that we connect to from home using a telephone line typically use one of two data link protocols: SLIP or PPP.