An Open Systems Interconnection (OSI) reference model provides a common basis that aids in the development of systems interconnection standards. The model covers all aspects of network communication as envisaged in the International Standards Organization (ISO). ISO is an organization and not a model, whereas the OSI reference model is not a protocol or a set of rules but is an overall framework that forms the basis to define protocols. OSI has a layered architecture that facilitates design of network systems allowing communication between all types of computer systems. It consists of seven layers and is shown in Figure 3.3.
Application data
Transport
header Transport
header
Transport PDU Transport PDU
Network PDU Network PDU
Network
header Network
header
A packet A packet
FIGure 3.2 Transport and network PDUs. (From W. Stallings. Data & Computer Communications, 6th Edition. Pearson Inc., New Delhi, India, p. 17, 2000.)
The seven layers can be visualized to be divided into three subgroups.
Layers 1, 2, and 3 are known as network support layers, while layers 5, 6, and 7 are called user support layers. The layer lying in between, i.e., layer 4, links the two subgroups. The network support layers help in moving data from one device to the other by taking care of physical connections, physical addressing, electrical specifications, transport tim-ings, etc. The user support layers help in interoperability among software systems that are different from each other. The bottommost layer, i.e., layer 1, is mostly hardware based; other lower layers are a combination of hardware and software; while the upper layers comprise mostly of software.
Each layer of the seven-layer OSI model performs the following tasks, shown in Figure 3.4. While developing the model, similar types of net-working functions were clubbed together and put into a specific layer. This way, the different layers were assigned different functions and architec-ture developed that is comprehensive and at the same time flexible. Since the functionalities of each layer are separate and well defined, standards can be developed independently and simultaneously of each other—thus speeding up the whole process of standardization. Again, since the layers are independent of each other, any change in standard in one layer would not affect the existing software in another layer.
Layer 6 Layer 7
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Application
Presentation
Session
Transport
Network
Data link
Physical
User support layer
Links upper and lower layers
Network support layer
FIGure 3.3 Seven-layer OSI model. (From B. A. Forouzan. Data Communication and Networking, 4th Edition, Special Indian Edition. Tata McGraw Hill Companies Inc., New Delhi, India, p. 30, 2006.)
Network Models 49
Source data is encapsulated in packet form, which starts at the upper layer and moves down the successive layers, adding control information in the form of header at each layer and also trailer at the data link layer. When the packet (with headers added at each layer and trailer) reaches layer 1, i.e., the physical layer, it is sent across a physical communication link that passes through the intervening nodes before finally reaching the destina-tion stadestina-tion. This is shown in Figure 3.5. A layer on a device communicates with the same layer in another device. Such communications are based on a set of rules and conventions already agreed on. Such communications are termed peer-to-peer communications and shown in Figure 3.5. Figure 3.6 shows data exchange between two computers with headers and trailers placed at appropriate places in each layer.
3.3.1 PhysiCal layer
Figure 3.7 shows the mechanism of transporting data from the physical layer onto the physical medium. The physical layer receives data from the data link layer. It deals with the physical and electrical specifications of the inter-face and the medium, as also the functions and procedures that the physical devices and interfaces have to perform for transmission to take place.
Characteristics associated with the physical layer are as follows:
• It transforms the bits into signal, i.e., how the 0’s and 1’s are to be encoded for transmission over the physical medium.
Layer 7
FIGure 3.4 Tasks performed by seven-layer OSI model.
• It defines the data rate or transmission rate of the bits.
• It is the duty of the physical layer to synchronize the transmitter and receiver clocks.
• It defines the physical topology—i.e., how the devices are con-nected, viz., mesh or star or ring or hybrid.
7
Data link Data link Data link Data link
Physical Physical Physical Physical
FIGure 3.5 Data transfer between devices and peer-to-peer processes.
Network Models 51
• Line configuration, i.e., either point-to-point or multipoint con-figuration of the devices is the responsibility of the physical layer.
• The physical layer is concerned with the mode of transmission, i.e., simplex, half duplex, or full duplex.
• The physical layer defines the characteristics of the interface between the devices and the transmission medium.
• The physical layer provides the necessary specifications for differ-ent types of hardware such as cabling, connectors and transreceiv-ers, network interface cards (NICs), and hubs.
3.3.2 Data link layer
The data link layer is shown in Figure 3.8. Characteristics associated with the data link layer are as follows:
• It divides the whole message received from the network layer into smaller manageable data units—termed frame.
• It is the responsibility of the data link layer to move the frames from one node to another node (hop-to-hop).
From data link layer To data link layer
Transmission medium
Physical layer Physical
layer 101000010 101000010
L2 data L2 data
FIGure 3.7 Operation of physical layer. (Available at www.eazynotes.com/
notes/computer-networks/slides/osi-model.pdf.)
From network layer To network layer
L3 data L3 data
Frame Frame Data
linklayer Datalink
layer T2 H2 T2 H2
To physical layer From physical layer 10101000000010 10101000000010
FIGure 3.8 Operation of data link layer. (Available at www.eazynotes.com/
notes/computer-networks/slides/osi-model.pdf.)
• It imposes a flow control mechanism when data produced by the sender is at a rate higher than the rate at which data can be absorbed by the receiver.
• The layer has an error control mechanism by virtue of which it can detect and retransmit damaged or lost frames. It also can rec-ognize duplicate frames. It is achieved by adding a trailer at the end of each frame.
• The data link layer is subdivided into an upper sublayer called log-ical link control (LLC) and a lower sublayer called media access control (MAC). LLC is responsible for flow and error control.
LLC ensures that protocols such as Internet Protocol (IP) can function regardless of the type of physical technology used. Multi-point access is resolved by MAC, i.e., MAC acts as a mediator.
Technologies that are used to achieve the above are Carrier Sense Multiple Access with Collision Detection for Ethernet and token for token ring systems.
• The data link layer adds a header to a frame that needs to be dis-tributed to different systems. Then the header includes the sender and receiver addresses. This is known as physical addressing.
3.3.3 network layer
The mechanism of data flow through the network layer is shown in Figure 3.9. The responsibilities of the network layer include the following:
• The network layer takes the responsibility for the source to desti-nation delivery of a packet across multiple networks.
From transport layer To transport layer
Packet Network Packet
layer Network
layer
H3 H3
L3 data L3 data
L4 data L4 data
To data link layer From data link layer
FIGure 3.9 Operation of network layer. (Available at www.eazynotes.com/
notes/computer-networks/slides/osi-model.pdf.)
Network Models 53
• If a packet, residing in a network, is to be sent to another network, the network layer then adds the logical (network) address of the sender and receiver to each packet.
• Such addresses are assigned to local devices by a network admin-istrator. This is assigned dynamically by a special server called Dynamic Host Configuration Protocol (DHCP).
• Several networks are connected by routers and switches to form a large network. The network layer identifies the best path to route the packets to the final destination.
Network layer protocols are IP and Novell’s Internetwork Packet Exchange, although the latter is almost deprecated.
3.3.4 transPort layer
The transport layer is shown in Figure 3.10. Responsibilities carried out by the transport layer are as follows:
• It ensures process-to-process delivery of an entire message.
• While the network layer treats each packet independently, the transport layer treats the entire message en masse and sees to it that all the packets are in order.
• The transport layer can be either connectionless or connection ori-ented. In connection-oriented transmissions, the receiving device sends an acknowledgment back to the source after a packet is received. This is not so for a connectionless transmission; thus, while the former is a slower transmission method, the latter is rela-tively faster.
To session layer From session layer
L5 data L5 data
Transport layer Transport
layer
H4 H4 H4 H4 H4 H4
L4 data L4 data L4 data
L4 data L4 data L4 data To network layer From network layer
FIGure 3.10 Operation of transport layer. (Available at www.eazynotes.com/
notes/computer-networks/slides/osi-model.pdf.)
• A computer may run several processes at the same time. A trans-port layer header assigns a trans-port address to each such process.
• The transport layer divides a message into segments, with each segment containing a sequence number. The sequence numbers enable the assembly of the message at the receiver. It also identi-fies and replaces packets lost in transmission.
Flow control in the transport layer is end to end rather than a single link.
Error control in the transport layer is process-to-process rather than across a single link.
Transport layer protocols include Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The former is connection oriented while the latter is connectionless.
3.3.5 session layer
Figure 3.11 shows the operation of the session layer. It acts as the dialog controller for the network. Its job includes establishing, maintaining, syn-chronizing, and lastly terminating the interaction among the devices that communicate with each other. If a session is broken, it attempts to retrieve the session.
Responsibilities carried out by the session layer include the following:
• Dialog control involves determining which of the two devices are to communicate data between themselves. Data sharing may be simplex, half duplex, or full duplex.
To presentation layer From presentation layer
L6 data L6 data
L5 data L5 data
Session
layer Session
layer
H5 H5
syn syn
syn syn syn syn
To transport layer From transport layer
FIGure 3.11 Operation of session layer. (Available at www.eazynotes.com/
notes/computer-networks/slides/osi-model.pdf.)
Network Models 55
• The session layer adds checkpoints, also called synchronization points, to a stream of data. For a huge amount of data, it adds checkpoints in between at predetermined intervals to ensure that data up to each consecutive checkpoint are received and acknowl-edged properly. The process of adding checkpoints and markers to a stream of data is called dialog separation.
3.3.6 Presentation layer
Syntax and semantics of a data message are taken care of by the presen-tation layer. Figure 3.12 shows the operation of a presenpresen-tation layer. Its responsibilities include the following:
• It is the responsibility of the presentation layer to ensure that data encoded differently by different computers are interoperable.
• Sensitive information, to be exchanged between the sender and the receiver, must be kept away from possible eavesdroppers. Data is encrypted in a manner that hides the information from such malafide data poachers. Decryption is done to transform the mes-sage back to its original form at the receiver.
• Data compression is a method that reduces the bit numbers con-tained in a data stream, without losing vital information. It is par-ticularly applied in multimedia systems.
Presentation layer formats include text (ASCII, EBCDIC, RTF), images (JPG, TIF, GIF), audio (MP3, WAV), and movies (MPEG, AVI, MOV).
To application layer From application layer
Presentation
layer Presentation
layer Encoded, encrypted, and
compressed data Decoded, decrypted, and
decompressed data
H6 H6
To session layer From session layer
L6 data L6 data
L7 data L7 data
FIGure 3.12 Operation of presentation layer. (Available at www.eazynotes.
com/notes/computer-networks/slides/osi-model.pdf.)
3.3.7 aPPlication layer
Figure 3.13 shows the operation of the application layer whose main char-acteristics are given below:
• It provides user interface and supports various services such as e-mail, file transfer, and access to the World Wide Web.
• It supports e-mail service to any part of the world.
• It allows users to locate data from a remote location, retrieve the same, and use the same at the user’s place.
• A user can log into a remote computer and use its resources.