Communication protocol and its need

A communication protocol is a set of agreed rules and conventions that govern how data are formatted, transmitted, received and interpreted by two or more communicating entities. Without a common protocol, devices can send bits to each other but cannot understand them, just as two people need a common language to talk.

Key elements of a protocol

1. Syntax: the format and structure of data, e.g. first 8 bits are the address, next 8 bits the control field, then data.
2. Semantics: the meaning of each field and the action to take, e.g. what a "NAK" means.
3. Timing: when data should be sent and how fast; sequencing and speed matching.

Example: HDLC frame

| Flag | Address | Control | Data | FCS | Flag |
  8b      8b        8b      var   16b    8b

Why a protocol is required (functions)

1. Encapsulation: adds headers (address, control, error-check bits) to user data.
2. Segmentation and reassembly: breaks long messages into blocks of suitable size and rebuilds them at the receiver.
3. Connection control: sets up, maintains and releases logical connections (e.g. TCP three-way handshake).
4. Ordered delivery: sequence numbers allow the receiver to put data back in the correct order.
5. Flow control: prevents a fast sender from overrunning a slow receiver (stop-and-wait, sliding window).
6. Error control: detects errors (CRC, checksum) and recovers by retransmission (ARQ) or correction.
7. Addressing: identifies source, destination, ports and connections.
8. Multiplexing: many connections share one link or one lower-layer connection.
9. Synchronisation: both ends agree on timing, state and frame boundaries.
10. Transmission services: priority, QoS, security (encryption, authentication).
11. Interoperability: equipment from different vendors and countries can work together.

Layered protocol architecture

Because communication is complex, protocols are arranged in layers; each layer uses the services of the layer below and serves the layer above. Peer layers talk using their protocol.

  Host A                          Host B
 Application  <-- HTTP -->    Application
 Transport    <-- TCP  -->    Transport
 Network      <-- IP   -->    Network
 Data link    <-- Eth  -->    Data link
 Physical     ============    Physical

Types of protocols

• Direct or indirect: data passed directly between two stations, or through switches/routers.
• Monolithic or structured (layered).
• Symmetric (peer-to-peer) or asymmetric (client/server, master/slave).
• Standard (ISO, ITU-T, IETF) or non-standard (vendor specific).

Example

When a browser loads a web page, HTTP defines the request and reply format; TCP divides data into segments, numbers them, and retransmits lost ones; IP routes each packet; Ethernet frames carry packets on the LAN with a CRC. If any of these rules were missing, data would be lost, mixed up or not understood.