OSI MODEL

Introduction An open system is a model that allows

any two different systems to communicate regardless of their underlying architecture.

OSI is not a protocol, it’s a model for understanding and designing a network architecture which should be robust, flexible and interoperable

It’s a layered framework for the design of network that allows communication across all types of computer systems

The Layers

Seven layers can be classified into three sub groups.

1. Network support layers- layers 1,2,3(physical, data link and network)

2. User support layers- layers 5,6,7(session, presentation, and application)

3. Transport layer

Functions of Physical LayerDeals with the mechanical and electrical

specification of the interface and transmission medium.

Also defines the procedures and functions that physical devices have to perform for transmission

Its mainly concerned with:◦Physical characteristic of interfaces and media◦Representation of bits◦Data rate◦Line configuration◦Physical topology and transmission media

Functions of data link layerResponsible for node to node delivery and its main

responsibilities are:1. Framing: converts the stream of bits into small

units called as frames2. Physical addressing: it adds a header to the

frame to define the physical address of the sender and/or the receiver of the frame

3. Flow control: implements a flow control mechanism to prevent loading of receiver

4. Error control: it detects and retransmits damaged or lost frames. This is usually done with addition of trailer

5. Access control: it defines the control over link by a device

In Figure, a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection

Functions of Network LayerResponsible for source to

destination delivery of a packet across multiple links

Major role of network layer is:1. Logical addressing2. Routing

If we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.

Functions of transport layerResponsible for source to

destination delivery of entire message (end to end)

Major functions include:1. Segmentation and reassembly2. Flow control3. Error control4. Connection control

Functions of Session layerEstablishes, maintains and

synchronizes the interaction between communicating systems.

Its role includes1. Dialog control: allows the

communication between two process to take place either in half duplex or full duplex

2. Synchronization: allows to add checkpoints in to stream of data

Functions of Presentation LayerMainly concerned with syntax and

semantics of the information exchanged between two systems

Specific functions include:1. Translation: provides for

interoperability between two different encoding methods

2. Encryption: to assure privacy of the message

3. Compression

Function of Application Layer Provides user interface and support for

services such email, remote file access, shared database management and other types of distributed information services

Summary of OSI Layers

TCP/IP PROTOCOL SUITE

The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP/IP protocol suite do not match exactly with those in the OSI model. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model with the layers named similarly to the ones in the OSI model.

Figure 2.8 TCP/IP and OSI model

Addressing Four levels of addresses are used in an

internet employing the TCP/IP protocols: physical address, logical address, port address, and application-specific address. Each address is related to a one layer in the TCP/IP architecture.

TCP/IP Protocol Suite 23

Figure 2.15 Addresses in the TCP/IP protocol suite

In Figure a node with physical address 10 sends a frame to a node with physical address 87. As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. The data link layer at the sender receives data from an upper layer. It encapsulates the data in a frame. Each station with a physical address other than 87 drops the frame because the destination address in the frame does not match its own physical address.

Example

TCP/IP Protocol Suite 25

Example: physical addresses

Data87 101

packetdiscarded

2

packetdiscarded

3

packetaccepted Data87 10

4

As we will see in Chapter 3, most local area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

07:01:02:01:2C:4BA 6-byte (12 hexadecimal digits) physical address

Figure shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks. So each router has three pairs of addresses, one for each connection. The computer with logical address A and physical address 10 needs to send a packet to the computer with logical address P and physical address 95.

Example 2.5

TCP/IP Protocol Suite 28

Figure 2.17 Example 2.5: logical addresses

DataA P20 10 DataA P20 10

Physicaladdresseschanged

DataA P33 99

DataA P33 99

Physicaladdresseschanged

DataA P95 66 DataA P95 66

TCP/IP Protocol Suite 29

The physical addresses will change from hop to hop, but the logical addresses

remain the same.

Note

Figure shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer.

Example 2.6

A Sender Receiver P

Internet

Figure 2.18 Example 2.6: port numbers

a DatajA PH2

a DatajA P

a Dataj

Data

a DatajA PH2

a DatajA P

a Dataj

Data

As we will see in future chapters, a port address is a 16-bit address represented by one decimal number as shown.

Example 2.7

753A 16-bit port address represented as one single number