Data Communication Networks Computer Architecture Department of Computer Science
Communication Level
• User‐to‐User Communication
• Computer‐to‐Computer Communication
• Computer‐to‐Network Communication
Data Communication Network
CommunicationSystem
Application Application
Computer BComputer A
User‐to‐User Communication
Computer‐to‐ComputerCommunication
Computer‐to‐NetworkCommunication
CommunicationSystem
ISO Reference Model
• A communication systems is complex piece of hardware and software.
• ISO has adopted a layer approach for a reference model
• Network Environment– Data communication protocols
and standards.
• OSI‐Environment– adds application oriented
protocols and standards.
• Real System Environment– Proprietary Software and
Services
Data Communication Network
Network‐dependentfunctions
Application Application
Computer‐to‐NetworkCommunication
Real System Environment
Computer A
User‐to‐User Communication
Computer B
Application‐orientedfunctions
ISO‐ Environment
Network Environment
Application‐orientedfunctions
Network‐dependentfunctions
ISO Model Structure
• Network‐dependent and application oriented components are – Implemented as number of layers– Layer boundaries defined based on
experience – Each layer performs well‐defined
functions– Layer operates according to a defined
protocols by exchanging messages.– Each layer communication with its
peer, similar layer in a remote system.
– Each layer is aware of the layer structure and may depend on other layers.
The ISO Layer
• Up to seven protocol layers• Three layer (1‐3) are network
dependent.– Data communication protocols
• Three layers (5‐7) are application dependent– End application user process – Services offered by the operating
system• The intermediate transport layer
– Masks the application oriented layers from network dependent layers.
The application Layers• The application layer
• The presentation Layer
• The session layer
• The transport layer
The application layer
• Provides an interface to a range of network distributed information services.– File transfer and access management
– Web‐browsing
– Electronic mail Services
• The application access a set of primitives– Supported by operating system (system calls).
– The operating system use a communication subsystem (software and hardware).
The application layer Services• Identification of the intended communication partner
• Determining the current availability of an intended partner
• Establishing authority of communicate
• Agreement on privacy mechanism
• Authentication of intended communication partner
• Selection of the dialog discipline, including the initiation and release procedure
• Agreement of responsibility of error recovery
• Identification of constrains on data syntax– Character Set, Data Structure
The Presentation Layer
• Manage the representation of data during the transfer between two communicating applications.
• Negotiates and selects the appropriate transfer syntax during a transaction.
• It performs all necessary conversions.
• This layer is also concerns with data security.
The session Layer
• Interaction management
• Synchronization
• Exception Reporting
Allows two application layer protocol entities to organize and synchronize their dialog and manage their data exchange
The transport Layer
• Provide services to compensate for the varying quality of services provided by the network layers.
• Full error control
• Flow control
Acts as an interface between the higher application‐oriented layers and the network‐dependent protocol layers.
Network‐Dependent Layers• The network layer
– Responsible for establishing and clearing a network‐wide connection between two transport layer protocol entities.
• Network routing, flow control across computer‐to‐network interface, etc.
• The link layer – Responsible for providing reliable information transfer facility.
• Error detection in transmission errors and retransmission of messages.
– Services • Connectionless, which treats each information frame as self‐contained and transferred using a best‐try approach.
• Connection oriented, which aims to provide error‐free information transfer facility.
Network‐Dependent Layers• The physical layer
– Responsible for the physical and electrical interfaces between the user equipment and network termination equipment.
– It provides the link layer with a means of transmitting a serial bit‐stream between two equipments.
Error Detection Methods
• Forward error control– Each transmitted character or frame contains additional redundant information which aims
• To detect when errors are present• Determine where in the received bit stream the errors are
• Backward (feedback) error control– Each transmitted character or frame include sufficient information to enable the receiver to when errors are present.
– Can not determine the error laocation.
Error Detection and Recovery
• Parity – The transmitter adds additional bit to each character prior to transmission.
• Block Sum Check – A set of parity bits computed from a complete block of data.
• Cyclic Redundancy Check – A single set of check digits is generated for each frame transmitted.
Parity
• The parity bit of a character is computers as follow– The number of 1 bits in the code are added together modulo 2.
– The parity bit is chosen so that the number of 1 bits is
• Even Parity • Odd Parity
• Parity can detect an error in on bit, but not two
Hamming Distance
• Codeword is defines as the combined messages unit that include– The useful data bits– The error control and detection
bets
• The Hamming distance is defined as the minimum number of bit positions in which two valid codewordsdiffer.
Example:If we assume even parity0000000 00000001 10000010 10000011 0
Block Sum Check (BSC)
• The resulting set of parity bit for each column is the Block Sum Check character.
Cyclic Redundancy Check
• A single set of check digits is generated for each frame transmitted based on – The content of the frame
– Appended to the tail of the frame
• The check digits is selected based on – The type of anticipated transmission errors
– 16 and 32 are the most common
• The check bits are computed based on the frame check sequence or the cyclic redundancy check.