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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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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
encapsulation
Encapsulation
  • 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
Addressing
  • 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)
multiplexing

(a)

(b)

A

A

A

A

Trunk

group

B

B

B

MUX

MUX

B

C

C

C

C

Multiplexing
  • 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
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