group11_DNAA:protocol stack and addressing

Application layer

Application layer provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network.

 

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model.

 

Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.

 

Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

 

group11_dnaa:stop and wait ARQ

Algorithm for stop and wait ARQ

Sender-site algorithm for stop and wait ARQ

Sn=0;                                                                                 //Frame  0 should be sent first

cansend=true;                                                               //allow first request to go

while(true)                                                                     //repeat forever

{

waitforevent();                                                           //sleep until an event occurs

if(event(request to send) AND cansend)

{

GetData();

MakeFrame(Sn);                                                        //The seqno is Sn

StoreFrame(Sn);                                                       //keep copy

startTimer();

Sn=Sn+1;

cansend=false;

}

Waitforevent();                                                            //sleep

if(event(Arrivalnotification))                               //an ACK has arrived

{

receiveFrame(ackno);                                            //receive the ACK frame

if(not corrupted AND ackno==Sn)                   //valid ACK

{

stopTimer();

PurgeFrame(Sn-1);                                               //copy is not needed

cansend=true;

}

}

if(event(timeout))                                             //The timer has expired

{

StartTimer();

ResendFrame(Sn-1);                                         //resend a copy check

}

}

Receiver-site algorithm for stop-wait-ARQ protocol

Rn=0;                                                                       //frame 0 is expected to arrive first

while(true)

{

waitForEvent();                                                //sleep until an event occurs

if(Event(ArrivalNotification))                   //data frame arrives

{

ReceiveFrame();

if(corrupted(frame));

sleep();

if(seqno==Rn)                                                 //valid data frame

{

ExtractData();

DeleiverData();                                              //deleiver data

Rn=Rn+1;

}

SendFrame(Rn);                                          //send an ACK

}

}

group11_dnaa:protocol stack and addressing

NETWORK LAYER

The Network Layer is Layer 3 of the seven-layer OSI model of computer networking. The Network Layer resides above the Data link layer and below the Transport layer.

It provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, ie establishing paths for data transfer through the network. The Network Layer is also responsible for packet sequencing, congestion control, error handling, creating logical paths, known as virtual circuits, for transmitting data from node to node.

The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. Addressing and internetworking are the main functions of this layer. The addressing scheme is hierarchical.

This Layer is responsible for end-to-end (source to destination) packet delivery including routing through intermediate hosts. It provides the functional and procedural means of transferring variable length data sequences from a source to a destination host via one or more networks while maintaining the quality of the service and error control functions.

Functions of the Network Layer include:

• Logical Addressing: Every device that communicates over a network has associated with it a logical address, sometimes called a layer three address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. addressing is done at the data link layer as well, but those addresses refer to local physical devices. In contrast, logical addresses are independent of particular hardware and must be unique across an entire internetwork.
• Routing: Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go.
• Datagram Encapsulation: The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.
• Fragmentation and Reassembly: The network layer must send messages down to the data link layer for transmission. Some data link layer technologies have limits on the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up, send each piece to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination machine.
• Error Handling and Diagnostics: Special protocols are used at the network layer to allow devices that are logically connected, or that are trying to route traffic, to exchange information about the status of hosts on the network or the devices themselves.

The best-known example of a Layer 3 protocol is the Internet protocol (IP). It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting the packet into sufficiently small packets that the medium can accept.

The Network Layer responds to service requests from the Transport layer and issues service requests to the Data link layer.

group11_dnaa:protocol stack and addressing

DATA LINK LAYER:

The data link layer is the second layer in the OSI seven-layer reference model.

Functions:

• It responds to service requests from the network layer above it and issues service requests to the physical below it.
• The data link layer is responsible for encoding bits into packets prior to transmission and then decoding the packets back into bits at the destination.
• The data link layer is also responsible for logical link control, media access control, hardware addressing, error detection and handling and defining physical layer standards.
• It provides reliable data transfer by transmitting packets with the necessary synchronization, error control and flow control.

The data link layer is divided into two sub-layers:

Media access control layer (MAC) : Controls how computers on the network gain access to the data and obtain permission to transmit it.

Logical control layer (LLC): Controls packet synchronization, flow control and error checking.

The data link layer is where most LAN(local area network) and wireless LAN technologies are defined. Among the most popular technologies and protocols generally associated with this layer are Ethernet, Token Ring, FDDI (fiber distributed data interface), ATM (asynchronous transfer mode), SLIP (serial line Internet protocol), PPP (point-to-point protocol), HDLC (high level data link control) and ADCCP (advanced data communication control procedures).

Data link layer processing is faster than network layer processing because less analysis of the packet is required.

group11_dnaa:protocol stack

A protocol stack is a complete set of network protocol layers that work together to provide networking capabilities. It is called as stack because it is designed as a hierarchy of layers, each supporting one above it and using those below it. A protocol is a mutually agreed upon format for doing something. With regard to computers, it most commonly refers a set of rules that enables computers to connect and transmit data to one another. A protocol can be implemented by hardware, software or a combination of the two.
Regardless of the number of layers, the lowest protocols always deal with low level, physical interaction of the hardware. Each higher layer adds additional features, an user applications typically interact only with the uppermost layers. The layers can be broadly classified as media, transport and application.
The media-to-transport interface defines how transport protocol software makes use of particular media and hardware types. Theapplication-to-transport interface defines how application programs make use of the transport layers.

Example of protocol stack and corresponding layers:

Protocol—– Layer
1. HTTP—– Application
2. TCP——- Transport
3. IP ———Internet
4. Ethernet— Link

group11_dnaa:protocol stack and addressing

International Standards Organization (ISO) developed OSI model as a first step towards international standardization of the protocols used in the various layers. The model is known as the ISO OSI Reference Model because it is related with connecting open system i.e. systems that are open for communication with other systems.

The OSI model includes seven layers:

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.

Different layers of ISO OSI model are as follows:

1) Physical Layer describes the physical & electrical properties of the communications media.
e.g., voltage levels, data rates, max distances.

2) Data Link layer describes the logical organization of data bits transmitted on a particular
medium.
e.g., frame sequencing, error notification

3) Network Layer describes how a series of exchanges over various data links can deliver data across a network.

e.g., addressing and routing

4) Transport Layer describes the quality and nature of data delivery
e.g., how retransmissions are used to ensure delivery

5) Session layer describes the organization of large data sequences & manages
communication session.
e.g., coordinates requests/responses

6) Presentation Layer describes the form of data being transferred & ensures that it will be
readable by receiver.
e.g., floating point formats, data compression, encryption

7) Application Layer describes how applications will communicate
e.g., HTTP, FTP, Telnet, SMTP

Physical layer:

The physical layer defines the hardware implementation, mechanical, electrical and procedural interfaces and the physical communication channel which lies below the physical layer. At this layer information is placed on the physical medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it. It is related with delivering raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received as a 1 bit by the other side but not as a 0 bit. The physical layer performs services requested by the Data Link Layer.

The major functions and services performed by the Physical Layer:

1. Definition of Hardware Specifications: The details of operation of cables, connectors wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer.
2. Physical topology: establishment and termination of a connection to a communications media. It defines how devices are connected to devices to media. Devices can be connected by using mesh topology, star topology, ring topology or bus topology.
3. Participation in the process whereby the communication resources are effectively shared among multiple users.
4. Representation of bits: Conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. To be transmitted bits must be encoded into signals electrical or optical.
5. Encoding and Signaling: The physical layer data consists of a stream of bits with no interpretation thus it is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
6. Date transmission rate: The transmission rate the no of bits sent each second is also defined by the physical layer.
7. Transmission mode: The physical layer also defines the direction of transmission between two devices:simplex,half dupolex,full duplex.

group11_dnaa:how does internet work?

Internet is a world wide system of computer networks in which users at any one computer can get information from any other computer. Each computer connected to internet must have a unique address called as an IP address. The message transfer between the computers is usually over wires. The message must be translated from alphanumeric text into electronic signals, transmitted over internet, then translated back to alphanumeric text. This is accomplished by using a protocol stack which is usually built into computer’s OS. The protocol stack has four layers- application protocol layer, transmission control protocol layer, internet protocol layer, hardware layer.
The message would start at the top of the protocol stack on the computer and work its way downward. The message would be sent as chunks of data called packets. Each packet will have a port number. The IP layer would help to know the address of the destination computer. TCP layer helps to know which program on the destination computer needs to receive the message because it will be listening on a specific port. The hardware layer takes care of turning the packets containing the alphanumeric text of the message into electronic signals and thus transmitting it.The ISP router examines the destination address in each packet and determines where to send it. The packets start at the bottom of the destination computer’s TCP/IP stack and work upwards. When the data reaches the top of the stack, the packets are reassembled into their original form.
One of the internet’s major services is the World Wide Web. Web documents can be linked together because they are created in a format known as hypertext. Hypertext systems provide an easy way to manage large collections of data. To support hypertext documents the web uses a special protocol called Hypertext Transfer Protocol orHTTP. A hypertext document is an encoded file which uses the hypertext mark up language or HTML. This language allows a user to embed hypertext links or hyperlinks in the document. HTTP and hypertext links are the foundation of the World Wide Web.