1.3 Protocol Architectures

A layered protocol architecture is usually used to break the communication task into subtasks, each of which is implemented in a separate layer in a stack.

Each protocol belongs to one of the layers in the architecture.

A layer provides its services by performing predefined internal actions and using the services provided by the layer below it.

A protocol layer can be implemented in software, hardware or in a combination of software and hardware components.

Among the advantages of the protocol layered architecture is providing a modular framework that eases the process of updating the communication system components.

1.3 Protocol Architectures

1.3.1 The Internet Protocol Architecture

The most widely used layered protocol architecture is the Internet protocol stack which consists of five layers:

The Application layer: Which includes a number of protocols supporting Internet applications, such as the HyperText Transfer Protocol (HTTP) that supports the World Wide Web (WWW) and the Simple Mail Transfer Protocol (SMTP) which supports the Internet's email service.

The Transport Layer: This layer transports application layer messages between the client and the server. Data to be communicated is divided into smaller sequenced segments. Two alternative Internet transport protocols can be used: the Transmission Control Protocol (TCP) or the User Datagram Protocol. The TCP protocol supports a connection-oriented service where a guaranteed delivery of application messages is provided through timing and error handling techniques. The UDP protocol supports a connectionless service with no guarantees for message delivery.

The Network Layer: Receives transport segments and a destination address from the transport layer, encapsulates such segments into datagrams (packets). The Internet network layer includes the Internet Protocol (IP) which determines the route that the datagram should follow to reach its destination.

1.3 Protocol Architectures

The Link Layer: Responsible for delivering a datagram to the next node along the route. At each node, the network layer passes the datagram down the link layer which forwards it to the next node directly linked with this node. The protocol used in this layer is link dependent based on the network over which the datagrams are sent.

The Physical Layer: Provides the physical interface between the device and the transmission medium. It transfers the bits representing the data to be communicated into electronic signals. The protocol used in this layer depends on the transmission medium.

1.3 Protocol Architectures

Example: Figure 1.3 illustrates how the Internet protocol architecture accomplishes the communication of a web page request made by a client to a server over the Internet.

Each layer of the Internet protocol stack is implemented at both the source and destination.

The request for the web page is generated as an HTTP message in the application layer of the source (the client).

The HTTP message is then passed to the source's transport layer where it is split into segments, transformed and encapsulated in TCP messages.

On receiving a TCP message, the network layer of the source adds an IP header to it containing the IP address of the destination.

In this example, the source machine resides in an Ethernet network. Thus, the link layer transforms the IP packet received from the upper layer into an Ethernet message to forward it to the next machine in the network.

The actual transmission of the message is done through the physical layer of the source.

1.3 Protocol Architectures

At the destination side, the message is received by the physical layer and passed upwards the software modules implementing the Internet protocol stack layers.

At each layer, the message is transformed and unpacked before being sent to the upper layer.

Finally, the HTTP message request for the web page is realized by the application layer of the destination server.