Reference model
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seminar surveyer
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04-10-2010, 03:12 PM


-Although the protocols associated with the OSI model are rarely used any more ,the model itself is quite general and still valid

-The TCP/IP model has the opposite property :the model itself is not of much use but the protocol are widely used .

-For this reason we look both them detail

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.ppt   chapter 4.ppt (Size: 375 KB / Downloads: 46)
seminar flower
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06-10-2012, 11:20 AM

Reference Models

.pptx   Reference Models.pptx (Size: 890.23 KB / Downloads: 8)

Functions of Protocols

Segmentation (Fragmentation)
Connection control
Ordered delivery
Flow control
Error control
Transmission services

Segmentation (Fragmentation)

Data blocks are 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 easy to access network facilities; 3) Shorter delays; 4). Smaller buffers needed
Disadvantages: 1). Overheads; 2). Increased interrupts at receiver;
3). More processing time

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

Guard against loss or damage data.
Error detection
Sender inserts error detecting bits
Receiver checks these bits
If OK, acknowledge
If error, discard packet
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

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)

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