The Session Layer The Presentation Layer

www.pragsoft.com Chapter 1: Introduction 9 The network layer hides the above details from the higher layers. To the transport layer, it appears as a uniform data transfer service, regardless of the location of the communicating devices and how they are connected.

1.3.4. The Transport Layer

The aim of the transport layer is to isolate the upper three layers from the network, so that any changes to the network equipment technology will be confined to the lower three layers i.e., at the node level. Transport layer protocols are concerned with the following issues: • Establishment and termination of host-to-host connections. • Efficient and cost-effective delivery of data across the network from one host to another. • Multiplexing of data, if necessary, to improve use of network bandwidth, and demultiplexing at the other end. • Splitting of data across multiple network connections, if necessary, to improve throughput, and recombining at the other end. • Flow control between hosts. • Addressing of messages to their corresponding connections. The address information appears as a part of the message header. • Type of service to be provided to the session layer e.g., error-free versus error- prone connections, whether messages should be delivered in the order received or not. The transport layer hides the above details from the higher layers. To the session layer, it appears as a customized data transfer service between two hosts, isolating the underlying network technology from it.

1.3.5. The Session Layer

The session layer provides a structured means for data exchange between user processes on communicating hosts. Session layer protocols are concerned with the following issues: • Negotiating the establishment of a connection a session between user processes on communicating hosts, and its subsequent termination. This includes the setting of various communication parameters for the session e.g., synchronization and control. • Correct ordering of messages when this function is not performed by the transport layer. 10 Communication Networks Copyright © 2005 PragSoft • Recovery from interrupted transport connections, if necessary. • Grouping of messages into a larger message, if necessary, so that the larger message becomes available at the destination only when its constituent messages have all been delivered successfully. The session layer hides the above details from the higher layers. To the presentation layer, it appears as an organized communication service between user processes.

1.3.6. The Presentation Layer

The presentation layer provides a mutually-agreeable binary representation of the application data communicated between two user processes. Since there are many ways of encoding application data e.g., integers, text into binary data, agreement on a common representation is necessary. Presentation layer protocols are concerned with issues such as the following: • Abstract representation of application data. • Binary representation of application data. • Conversion between the binary representation of application data and a common format for transmission between peer applications. • Data compression to better utilize network bandwidth. • Data encryption as a security measure. The presentation layer hides the above details from the higher layers. To the application layer, it appears as a universal communication service between user processes, regardless of their system-specific idiosyncrasies, allowing them to converse in a common syntax.

1.3.7. The Application Layer