Communication protocol and OSI architecture

Communication protocol

A communication protocol is a set of rules and conventions that govern the exchange of data between two communicating entities. Both ends must follow the same protocol to understand each other. A protocol defines three key elements:

• Syntax: format of data and control information (field order and sizes, e.g. a header of 20 bytes).
• Semantics: meaning of each field and the action to take (e.g. what an ACK or error code means).
• Timing: when and how fast data is sent, sequencing and speed matching.

Examples: HDLC (data link), IP (network), TCP (transport), HTTP and SMTP (application), Q.931 in ISDN.

OSI reference model

The Open Systems Interconnection (OSI) model was developed by ISO (ISO 7498, 1984) so that systems from different vendors can communicate. It divides the communication task into seven layers. Each layer performs a defined set of functions, uses the services of the layer below and gives services to the layer above. Layers on two machines talk to each other by peer protocols, while actual data moves down one stack, across the medium and up the other.

Layering principles: a layer for each different level of abstraction, well-defined functions, small interface flow across boundaries, and enough layers to separate functions but not too many.

 Host A                                  Host B
+----------------+                  +----------------+
| 7 Application  |<---------------->| 7 Application  |
| 6 Presentation |<---------------->| 6 Presentation |
| 5 Session      |<---------------->| 5 Session      |
| 4 Transport    |<---------------->| 4 Transport    |
| 3 Network      |<->[3]<---->[3]<->| 3 Network      |
| 2 Data link    |<->[2]<---->[2]<->| 2 Data link    |
| 1 Physical     |<->[1]<---->[1]<->| 1 Physical     |
+-------+--------+  router  router  +-------+--------+
        +========= physical medium =========+

Layers 1–3 are network-dependent (also run in intermediate nodes); layers 4–7 are end-to-end (only in hosts).

1. Physical layer

• Sends the raw bit stream over the medium.
• Defines mechanical (connectors, pins), electrical (voltage levels, bit duration), functional (meaning of pins) and procedural specifications.
• Line coding, bit synchronisation, data rate, simplex/half/full duplex, topology.
• Examples: RS-232/V.24, X.21, 10Base-T, ISDN I.430. Data unit: bit.

2. Data link layer

• Turns the raw line into a reliable link between adjacent nodes.
• Framing (start/end of frame, bit stuffing), physical (MAC) addressing.
• Error control with CRC and ARQ (retransmission), flow control (sliding window).
• In LANs it is split into LLC and MAC (medium access, e.g. CSMA/CD) sublayers.
• Examples: HDLC, LAPB, LAPD, PPP, IEEE 802.3. Data unit: frame.

3. Network layer

• Delivers packets from source host to destination host across many networks.
• Logical addressing (IP address), routing (choosing paths), switching, congestion control, fragmentation, internetworking.
• Connection-oriented (virtual circuit) or connectionless (datagram) service; accounting.
• Examples: IP, X.25 packet layer, ICMP. Data unit: packet.

4. Transport layer

• End-to-end (process-to-process) reliable delivery of the complete message.
• Segmentation and reassembly, port addressing, end-to-end error and flow control, connection management, multiplexing several sessions on one network connection.
• Hides network details from upper layers; gives the quality of service required.
• Examples: TCP, UDP, ISO TP0–TP4. Data unit: segment.

5. Session layer

• Establishes, manages and terminates sessions (dialogues) between applications.
• Dialogue control: who talks when (half/full duplex, token management).
• Synchronisation: inserts checkpoints so that after a failure transfer restarts from the last checkpoint, not from the start.
• Examples: RPC, NetBIOS, ISO 8327.

6. Presentation layer

• Concerned with syntax and semantics of the information exchanged.
• Translation between different codes (ASCII, EBCDIC) and data formats (ASN.1).
• Encryption/decryption for security and compression to reduce bits.
• Examples: JPEG, MPEG, SSL/TLS encryption, ASN.1.

7. Application layer

• Gives the user and application programs access to network services.
• Services: file transfer and access (FTAM, FTP), e-mail (X.400, SMTP), directory (X.500), virtual terminal, web (HTTP), network management.

Data flow and encapsulation

Each layer adds its own header (and the data link layer also a trailer) to the data from the layer above; the receiver removes them in reverse order.

          Data                       Layer 7
        AH|Data                      7  (headers
      PH|AH|Data                     6   added at
    SH|PH|AH|Data                    5   each layer)
  TH|SH|PH|AH|Data       = segment   4
NH|TH|SH|PH|AH|Data      = packet    3
DH|NH|....|Data|DT       = frame     2
0110100101110...         = bits      1

Summary table

Advantages of the layered OSI model

• Divides a complex task into small, manageable parts.
• Changes in one layer do not affect others (modularity).
• Equipment from different vendors can work together (open standards).
• Clear framework for teaching, design and troubleshooting.

The OSI model is mainly a reference model; practical networks mostly use the TCP/IP suite, whose layers map onto OSI (application ≈ 5–7, transport = 4, internet = 3, network access = 1–2).