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Chapter 2. Protocols & Architecture A Protocol Architecture is the layered structure of hardware & software that supports the exchange of data between systems

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Protocols architecture

Chapter 2

Protocols & Architecture

  • A Protocol Architecture is the layered structure of hardware & software that supports the exchange of data between systems

  • At each layer, one or more common protocols are implemented in communication systems. Each protocol provides a set of rules for the exchanges of data between systems

  • Key Functions of a protocol: encapsulation, segmentation & reassembly, connection control, ordered delivery, flow control, error control, addressing, and multiplexing

Functions of protocols
Functions of Protocols

  • Segmentation and reassembly

  • Encapsulation

  • Connection control

  • Ordered delivery

  • Flow control

  • Error control

  • Addressing

  • Multiplexing

  • Transmission services

Segmentation fragmentation
Segmentation (Fragmentation)

  • Data blocks are of bounded size at each layer

  • 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

      Why Fragment?

  • Advantages: 1) .More efficient error control; 2) More equitable access to network facilities; 3) Shorter delays; 4). Smaller buffers needed

  • Disadvantages: 1). Overheads; 2). Increased interrupts at receiver; 3). More processing time


  • Addition of control information to data

    • Address information

    • Error-detecting code

    • Protocol control: data + control information -> PDU

Connection control
Connection Control

  • Connection Establishment

  • Data transfer

  • Connection termination

Ordered delivery flow control
Ordered Delivery & Flow Control

  • 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
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 level

  • Addressing scope

    • Globally unique : Global address identifies unique system

    • Locally Unique: Multiple simultaneous applications

  • Addressing mode

    • Unicast address: Sent to one machine or person

    • Broadcast: Sent to all machines or users

    • Multicast: Sent to some machines or a group of users

Addressing level
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)





















  • Supporting multiple connections on one machine

  • Mapping of multiple connections at one level to a single connection at another: e.g.

    • Carrying a number of connections on one fiber optic cable

    • Aggregating or bonding low speed lines to gain bandwidth

Transmission services
Transmission Services

Some common service example:

  • Different Priorities for Different messages

  • Quality of Service Guarantee

    • Minimum acceptable throughput

    • Maximum acceptable delay

  • Security: Access Restrictions

Osi 7 layers model
OSI - 7 Layers 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 layers layer 1 4
OSI Layers – Layer 1 - 4

Provide fundamental network functionalities/services

  • Physical: Covers the physical interface between devices & the rules by which bits are passed from one to another

  • Data Link

    • Provide a reliable link by Error detection and control

    • Higher layers may assume error free transmission

  • Network: routing/switching; establish/maintain/terminate connections

  • Transport: Exchange of data between end-to-end systems

    • Error free; In sequence; No losses; No duplicates; QoS

Osi layers layer 5 7
OSI Layers – Layer 5 - 7

Provide finer network service (more software orientated)

  • Session:Control of dialogues between applications in end system

    • Dialogue discipline: full-duplex or half-duplex

    • Recovery: provide checkpoint mechanism.

  • Presentation: Defines data formats and coding. e.g.,

    • Data compression

    • Encryption

  • Application: provide means for application program to access OSI environment

    • management functions; email; FTP

Tcp ip protocol architecture
TCP/IP Protocol Architecture

  • Application Layer

    • Communication between processes or applications

  • End to end or transport layer (TCP/UDP/…)

    • End to end transfer of data

    • May include reliability mechanism (TCP)

    • Hides detail of underlying network

  • Internet Layer (IP): Routing of data

  • Network Access Layer

    • Logical interface between end system and network

  • Physical Layer

    • Transmission medium

    • Signal rate and encoding

Further reading reference
Further Reading Reference

  • Stallings chapter 2

  • Comer,D. Internetworking with TCP/IP volume I

  • Comer,D. and Stevens,D. Internetworking with TCP/IP volume II and volume III, Prentice Hall

  • Halsall, F> Data Communications, Computer Networks and Open Systems, Addison Wesley

  • RFCs