Data Communications

Architecture Models

What is a Protocol?

• For two entities to communicate successfully, they must “speak the same language”.

• What is communicated, how it is communicated, and when it is communicated must conform to some mutually acceptable conventions.

• These conventions are referred to as a protocol.

Key Elements of a Protocol

• Syntax– Data formats– Signal levels

• Semantics– Control information for coordination– Error handling

• Timing– Speed matching– Sequencing

Protocol Architecture

• Task of communication broken up into modules

• For example file transfer could use three modules– File transfer application– Communication service module– Network access module

Simplified File Transfer Architecture

A Three Layer Model

• Application Layer

• Transport Layer

• Network Access Layer

Application Layer

• Support for different user applications

• e.g. e-mail, file transfer

Transport Layer

• Reliable data exchange

• Independent of network being used

• Independent of application

Network Access Layer

• Exchange of data between the computer and the network

• Sending computer provides address of destination

• May invoke levels of service

• Dependent on type of network used (LAN, packet switched etc.)

Addressing Requirements

• Two levels of addressing required

• Each computer needs unique network address

• Each application on a (multi-tasking) computer needs a unique address within the computer– The service access point or SAP

Protocols in Simplified Architecture

Protocol Data Units (PDU)

• At each layer, protocols are used to communicate• Control information is added to user data at each

layer• Transport layer may fragment user data• Each fragment has a transport header added

– Destination SAP– Sequence number– Error detection code

• This gives a transport protocol data unit

Network PDU

• Adds network header– network address for destination computer– Facilities requests

Operation of a Protocol Architecture

TCP/IP Protocol Architecture

• Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET)

• Used by the global Internet• No official model but a working one.

– Application layer– Host to host or transport layer– Internet layer– Network access layer– Physical layer

TCP/IP Protocol Suite

• Dominant commercial protocol architecture

• Specified and extensively used before OSI

• Developed by research funded US Department of Defense

• Used by the Internet

Physical Layer

• Physical interface between data transmission device (e.g. computer) and transmission medium or network

• Characteristics of transmission medium

• Signal levels

• Data rates

• etc.

Network Access Layer

• Exchange of data between end system and network

• Destination address provision

• Invoking services like priority

Internet Layer (IP)

• Systems may be attached to different networks

• Routing functions across multiple networks

• Implemented in end systems and routers

Transport Layer (TCP)

• Reliable delivery of data

• Ordering of delivery

Application Layer

• Support for user applications

• e.g. http, SMTP

TCP/IP Protocol Architecture Model

Some Protocols in TCP/IP Suite

OSI Model

• Open Systems Interconnection

• Developed by the International Organization for Standardization (ISO)

• Seven layers

• A theoretical system delivered too late!

• TCP/IP is the de facto standard

OSI - The Model

• A layer model• Each layer performs a subset of the required

communication functions• Each layer relies on the next lower layer to

perform more primitive functions• Each layer provides services to the next higher

layer• Changes in one layer should not require changes

in other layers

OSI as Framework for Standardization

OSI Layers

• Application

• Presentation

• Session

• Transport

• Network

• Data Link

• Physical

The OSI Environment

OSI v TCP/IP

Standards

• Required to allow for interoperability between equipment

• Advantages– Ensures a large market for equipment and software

– Allows products from different vendors to communicate

• Disadvantages– Freeze technology

– May be multiple standards for the same thing

Standards Organizations

• Internet Society

• ISO

• ITU-T (formally CCITT)

• IEEE

• ANSI

Functions of Standards

• Encapsulation• Segmentation and reassembly• Connection control• Ordered delivery• Flow control• Error control• Addressing• Multiplexing• Transmission services

Encapsulation

• Addition of control information to data– Address information– Error-detecting code– Protocol control

Segmentation (Fragmentation)• Data blocks are of bounded size• Application layer messages may be large• Network packets may be smaller• Splitting larger blocks into smaller ones is

segmentation (or fragmentation in TCP/IP)– ATM blocks (cells) are 53 octets long

– Ethernet blocks (frames) are up to 1526 octets long

• Checkpoints and restart/recovery

Why Fragment?

• Advantages– More efficient error control– More equitable access to network facilities– Shorter delays– Smaller buffers needed

• Disadvantages– Overheads– Increased interrupts at receiver– More processing time

Connection Control

• Connection Establishment• Data transfer• Connection termination• May be connection interruption and recovery• Sequence numbers used for

– Ordered delivery

– Flow control

– Error control

Connection Oriented Data Transfer

Ordered Delivery

• PDUs may traverse different paths through network

• PDUs may arrive out of order

• Sequentially number PDUs to allow for ordering

Flow Control

• Done by receiving entity

• Limit amount or rate of data

• Stop and wait

• Credit systems– Sliding window

• Needed at application as well as network layers

Error Control

• Guard against loss or damage• Error detection

– Sender inserts error detecting bits– Receiver checks these bits– If OK, acknowledge– If error, discard packet

• Retransmission– If no acknowledge in given time, re-transmit

• Performed at various levels

Addressing

• Addressing level

• Addressing scope

• Connection identifiers

• Addressing mode

• Let’s look at two of these in more detail

Addressing level

• Level in architecture at which entity is named• Unique address for each end system (computer)

and router• Network level address

– IP or internet address (TCP/IP)– Network service access point or NSAP (OSI)

• Process within the system– Port number (TCP/IP)– Service access point or SAP (OSI)

Address Concepts

Addressing Mode

• Usually an address refers to a single system– Unicast address– Sent to one machine or person

• May address all entities within a domain– Broadcast– Sent to all machines or users

• May address a subset of the entities in a domain– Multicast– Sent to some machines or a group of users

Multiplexing

• Supporting multiple connections on one machine

• Mapping of multiple connections at one level to a single connection at another– Carrying a number of connections on one fiber

optic cable– Aggregating or bonding ISDN lines to gain

bandwidth

Transmission Services

• Priority– e.g. control messages

• Quality of service– Minimum acceptable throughput– Maximum acceptable delay

• Security– Access restrictions

Review Questions

• What are the layers of the TCP/IP model?

• What are the layers of the OSI model?

• What is meant by encapsulation?

• Trace an FTP command as it moves down through the layers, across the medium, and up the layers on the receiving side.