To reduce their design complexity, most networks are organized as a series of layers , i.e., network protocols are designed in terms of layered architecture ( layering ).
The major advantage of layering is that it clearly delineates the responsibilities of various protocols, by dividing responsibilities hierarchically among layers, with each layer offering services needed by the layer above.
The key to protocol families is that communication occurs logically at the same layer of the protocol in both sender and receiver, but it is implemented via services of the lower level (peer-to-peer) .
The common purpose of layers 4-7 is to provide interoperability: all the system elements can exchange data regardless of the vendor of the equipment (open system).
Connectivity is provided in the layers 1-3 of the model, which provides a working connection between the sender and receiver, i.e., the ability to move data anywhere in the network, regardless of the transmission technology or medium.
This layer's responsibility is to interface the user application with the rest of the layers in the model. The Application layer is also responsible for providing an API ( Application Programming Interface ) to the user applications so the programmers who write code for the user interface don't have to worry about the implementation details of the interface. This means that the Application layer takes the responsibility of the networking details away from the user application so the user application does not have to know anything about the underlying implementation of the network. Some examples of user applications are file transfer services, printing services, e-mail services, network management consoles, client-server processes, and so on.
This layer's responsibility is to provide encoding standards for the network. The Presentation layer is also responsible for negotiating between the Application layer and the rest of the protocol stack. It provides a standard encoding streamer for the Application layer so that communication between the Application layer and the rest of the protocol stack is standardized across different operating environments. In other words, the Presentation layer provides translation and conversion functions to successfully transfer data to the underlying protocol stack. As an example, if the Application layer of a PC sends information in ASCII format, the Presentation layer is responsible for formatting the information in the standard network type. This standard network type, which is generic for the underlying protocol stack across different operating environments, would be transferred without further conversion. At the receiving end, it is again the Presentation layer's responsibility to convert the generic network format to a format that the receiving application can understand. Data encryption and decryption can also take place at the Presentation layer.
This layer's responsibility is to provide a communication channel between hosts. It provides a definition for managing the individual network channels, also called sessions , between two hosts. The Session layer is responsible for establishing a session between the hosts, as well as maintaining and ending the session. Some examples of the Session layer protocol are RPC ( Remote Procedure Call ), AppleTalk, and NFS ( Network File System ).
This layer's responsibility is to control the transmission of data on the network. In other words, it provides flow control mechanisms to ensure data integrity between the nodes. The flow control mechanism acknowledges the receipt of every segment from the sending host and the proper sequencing of the segments. If the sender does not receive an acknowledgement from the recipient, the flow control mechanism at the sender's end is responsible for resending the segment. On the whole, the Transport layer's responsibility is to segment the data received from the Session layer and forward it to the Network layer. In addition, the Transport layer receives segmented data from the Network layer to reassemble the segments to forward them to the Session layer. The Transport layer is also responsible for establishing a logical connection between the destination node and the source node. Some examples of the Transport layer are TCP and UDP ( User Datagram Protocol ).
This layer's responsibility is to ensure the addressing of the hosts. It also ensures the routing of information between hosts across networks. In other words, the Network layer handles all of the transmission and traffic management among hosts. It also provides address resolution for the segments forwarded by the Data Link layer.
This layer's responsibility is to define how data is accessed from a physical medium. It provides a mechanism to format the information presented from a physical medium so the information can be passed to the Network layer. The information presented from a physical medium can be in the form of bits. The Data Link layer collates this information and formats it into frames . A frame is a unit of information that contains the destination address, the source address, an error checksum, and the data itself. The Data Link layer is also responsible for converting the information obtained from the Network layer into bits to forward it to the Physical layer. In addition, the Data Link layer is responsible for ensuring that the messages traversing the network reach the appropriate physical devices. This is possible because the Data Link layer manages the unique identity of the physical device on the network. It uses the concept of hardware addressing (MAC address) to identify a physical device. Some examples of the Data Link layer protocol are ARP ( Address Resolution Protocol ) and RARP ( Reverse Address Resolution Protocol ).
This layer's responsibility is to manage the hardware details of sending and receiving binary data over a physical channel. The physical channel is typically made up of wires such as twisted-pair and fiber optic cables. It can also be made up of wireless media such as infrared or radio waves. In general, the Physical layer provides a specification for interfacing with a physical channel based on the electrical and mechanical functions of the medium. The connectors at the Physical layer have different topologies defined for different network designs. Topologies are the structures in which you set up your network. Some examples are the star topology, the ring topology, and the bus topology. One example of the Physical layer is the Ethernet standard, which is the network protocol that defines how different devices on the network communicate with each other over the Physical layer.
Application layer. This layer's responsibility is to provide a common interface for any user application to communicate with the underlying layers. In other words, the Application layer is responsible for providing an interface between the user application and the network.
Transport layer. This layer's responsibility is to control the flow of data between two communicating hosts. The Transport layer is responsible for breaking down data into packets and sending and receiving them from the Network layer.
Network layer. This layer's responsibility is to route packets across the network. It is also responsible for some message control and group management.
Link layer. This layer's responsibility is to handle the hardware-related details of the system. In other words, the Link layer is responsible for interfacing the operating system to the network interface card within the computer.
Data Link Physical Network Layer Transport Layer Application Layer
In LAN, network control is distributed among the devices on the network, it resides in the NIC firmware in each machine.
LAN communication may be connectionless or connection-oriented .
Connectionless messages ( datagrams ) are sent with the expectation that they will be received correctly. There is no acknowledgment of correct receipt. A higher layer must ask for retransmission if the message is received incorrectly.
Connection-oriented communications include the acknowledgment of message as correct before they are passed on to the recipient.
LANs have five major communications characteristics:
Medium : the means by which data is sent.
Transmission technique : In baseband technique, the LAN signal is carried directly on the medium; in broadband system, the LAN signal is modulated on to an analog carrier signal, which allows several LANs to share the same medium.
Network topology : the layout of the cabling.
Access control method : contention (Ethernet) and token passing ( token ring ) for shared medium.
Data Rate : the raw ability to transfer information in mega bits per second.
Wide area networks (WANs) carry message at a lower speed between computers that are separated by large distance.
Many WANs and LANs can be combined to produce a single internetwork – a communication system that interconnects large collections of geographically dispersed computers.
The computers interconnected by a WAN are called host computers . The communication medium is a set of communication circuits linking a set of dedicated computers called packet switches or packet switching exchanges (PSEs).
The OSI layer architecture, TCP/IP layer architecture, or other layer architectures can be used in building WANs.
In WANs (packet networks), a message is divided into packets before transmission and the packets are reassembled at the receiving computer (transport layer). A packet consists of a header and a data field. The header contains a transport address composed of the network address of a host and a port number.
The PSEs operate the network by forwarding packets from one PSE to another along a route from sender to the recipient. PSEs are responsible for defining the route (network layer).
Every packet of data is stored temporarily by each PSE along its route before it is forwarded to another PSE ( store-and-forward communication). The routing operations introduce a delay at each point in the route, and the total transmission time for a message depends on the route it follows.
Two types of data transport service can be provided: connection-oriented -- a `virtual connection is set up between a sending and a receiving process and is used for the transmission of a stream of data; connectionless – individual messages (datagrams) are transmitted to specified destinations.
Intranet LAN LAN SW1 LAN LAN SW2 SW3 A D DATA T2 10 87 10 12 15 19 87
LAN LAN WAN S1 F1 F2 F3 LAN LAN F1 F2 F2 F1 S2 S3 A D D A S1 S3 Internet
Receives service from physical layer and provides service to the network layer.
Receives service from network layer and provides service to physical layer.
Responsible for carrying data from one hop to the next hop.
DATA T2 10 87 10 12 15 19 87
Duties packetinzing Addressing Error control Flow control Access control Duties of DLL Frame or cell MAC or VC CRC Prevent conflict or collision DLL is for point to point, or node to node on a common link LAN and WAN operate in DLL 12 15 19 87 bridge
Physical, data link and Network layers are independent. Uses message to communi.
Connection oriented, reliable channel.
Infinite supply of data.
No processing delay.
DL waits for a packet form NL. When and if DL recv a pack from NL, it encapsulates it into a frame adding some control bits (header), and then handed over to physical layer. Transmitting HW appends checksum bits and then transmits frame on the cable.
DL in rcvr waits for a frame from physical layer. DL may wait in an infinite loop or for an interrupt from physical layer. Recv HW recv a frame and computes the checksum. If checksum is ok frame is recvd undamaged and passed to DL. DL checks if the destination matches with its own id. If everything is ok, DL drops the frame header and passed the packet to NL.
Sending device keeps a copy of the last frame transmitted until it recvs an ack for that frame.
Both data frames and ack frames are numbered alternatively o and 1. A data 0 frame is ack by an ACK 1 frame, indicating that it has recved data frame 0 and expecting data frame 1.
If the recvr detects an error in the recevd frame, it simply discard the frame and send no ack. If the recvr recives a frame out of order, it knows that a frame is lost. It discards the out-of –order recvd frame.
Sender has control variable S, that holds the number iof the recently sent frame (0 or 1). The rcvr has a control variable, R, that holds the number of the next frame expected (0 or 1).
The sender starts a timer when it sends a frame. If an ack is not received within timeout period, the sender assumes that the frame is lost and resends it.
Rcvr sends only positive ack for frames received safely. Ack number always defines the next frame expected.
Operation: Normal S=0 S=1 R=0 R=1 Ack 1 Ack 0 time
Three kinds of frames: Information, Supervisory, Unnumbered.
Seq – frame sequence no. 3 bit sliding window. Next – piggybacked ackn. P/F – poll/final, P is used when computer is inviting the terminal to send data. All the frames sent by the terminal set the bit to P , except the final one, which is set to F .
Information Supervisiony Unnumbered Control fields for three kinds of frames
In some of the protocols the P/F bit is used to force the other machine to send a supervisory frame immediately rather than waiting for the reverse traffic onto which to piggyback the window information.
Type – for different kinds of supervisory frames. Type 0 is RECEIVE READY, 1 is REJECT. Next field indicates frames to be retransmitted, Type 2 – receive not ready, Type 3 - selective reject.
Unnumbered Frames – used for connectionless services. Differs greatly on implementation.