Now that you have completed this unit it is a good idea to reflect on what you have learned, and a good way of reflecting on your learning is to write a summary of the material. A brief summary is provided below, but this is no substitute for your own summary. You may also find it useful to note any points you are unsure of and wish to follow up by further reading. Posting a message on the OpenLearn forums board may elicit suggestions and support from fellow students.
There are two basic types of switching – circuit switching and packet switching. A physical communication channel is established for the exclusive use of a call in a circuit-switched network. In contrast, physical communication channels are shared in a packet-switched network. Packet switching is described as store-and-forward because a switch will store packets and then forward them when resources become available.
Networks can provide connection-oriented and connectionless services. In a connection-oriented service, a connection must be established before user data is transferred, whereas this is not necessary for a connectionless service. The connections need not be physical channels, and generally exist to organise the flow of packets rather than to reserve resources.
The OSI reference model is an approach to the separation of the communication functions of systems into seven layers: application, presentation, session, transport, network, data link and physical. The concept of layering is common with most protocols, although the seven layers advocated by OSI are not always followed. The vertical flow of data is represented by primitives being issued and received across layer boundaries. The four basic primitive types are: request, indication, response, and confirm. The flow of data between peer layers in different systems is called horizontal communication and is governed by protocols, which enable a layer to provide services to the layer above. One or more protocols is available to each layer. A protocol defines the structure and the meaning of protocol data units and therefore how the systems communicate. A protocol data unit in a higher layer is encapsulated in a protocol data unit in a lower layer.
Each layer, except the lowest, works at a level of abstraction based on the services provided by lower layers. For instance, higher layers can ignore the problems that arise with specific physical media, such as optical fibre and twisted pair, because the physical layer deals with problems associated with the types of physical medium.
The Internet is based on the TCP/IP protocol suite, which generally has a four-layer model: application, transport, internetwork, and host-to-network. The hypertext transfer protocol (HTTP) is an example of an application layer protocol, but there are several others. The principal transport layer protocol is the transmission control protocol (TCP), which provides a connection-oriented service. A three-way handshake provides a reliable means of establishing a connection. Sequence numbers allow any missing data to be identified. The main internetwork layer protocol is the internet protocol (IP), which provides a connectionless service. IPv4 addresses identify all hosts on an internetwork and have 32 bits. The number of bits allocated to host and network addresses is determined either by the uppermost bits if the class form of addressing is used, or by masks if the classless form of addressing is used. The forwarding of IP datagrams is accomplished by forwarding tables, which are set up by the routing process of an internetwork. Entries in a forwarding table denote either a direct path or an indirect path for a network address. In the former case the router is able to forward the datagram over a direct connection. In the latter case the final destination is on a network to which the router does not have a direct connection.
The asynchronous transfer mode (ATM) protocol is based on 53-byte cells. The ATM layer provides a connection-oriented service and several service categories are available: constant bit rate, real-time variable bit rate, non-realtime variable bit rate, unspecified bit rate, available bit rate and guaranteed frame rate. A virtual connection is identified by the combination of a virtual path identifier and a virtual channel identifier.
One approach to providing IP over an ATM network is label switching. Edge routers of an ATM network convert between forwarding equivalence classes (i.e. significant parts of IP network addresses) and labels. The labels are transferred in the virtual path identifier and/or virtual channel identifier fields in ATM headers.
Where possible, give examples and brief descriptions from TCP/IP and ATM protocols of the following OSI general functions identified in Section 2:
segmentation and re-assembly
addressing and routing
multiplexing and demultiplexing.
This unit has not explicitly addressed all the above issues, so you may not be able to give examples of every function.
My examples are in Table 12, but you may have given others.
Table 14: Answer to SAQ 20
|Layer function||TCP/IP example||ATM example|
|Connection control||The establishment and clearing of TCP connections||The establishment and clearing of switched and permanent virtual paths|
|Data flow||The window size, sequence and acknowledgement fields in TCP protocol data units may be used to control the flow of data, but this is not discussed||No examples are given, but control of the flow of data is an important part of ATM|
|Segmentation and re-assembly||Segmentation and re-assembly may involve both the TCP and IP protocols. The stream of bytes received by the transport layer is segmented into TCP segments, and these may be further fragmented at the internetwork layer. The appropriate layer at the receiving system re-assembles the original data units||The ATM adaptation layer segments the user data into 48-byte blocks of data. The ATM adaptation layer in the receiving system re-assembles the original data units|
|Sequencing||Sequence numbers are available in the TCP segments||This is not discussed, but sequencing may take place at the ATM adaptation layer|
|Acknowledgement||Acknowledgement numbers are available in TCP segments||This is not discussed, but acknowledgement may take place at the ATM adaptation layer|
|Error control||There are checksum fields in TCP and IP protocol data units, which imply that there may be some error detection available, but this is not discussed||There is a header error control field in ATM cells, so some form of error detection is available, but this is discussed only in the context of cell synchronisation|
|Synchronisation||Synchronisation of TCP segment sequence numbers takes place when connections are established||ATM cell synchronisation takes place at the ATM physical layer|
|Addressing and routing||Both the TCP and IP protocols use addresses. Network addresses are used at the internetwork layer for routing datagrams. Port addresses are used at the transport layer for identifying TCP connections||Network addresses and routing are not discussed in the context of ATM, but virtual connections are identified by virtual path and virtual channel identifiers|
|Multiplexing and demultiplexing||Multiplexing and demultiplexing of TCP connections are permitted through port numbers||Virtual path and virtual channel identifiers implement multiplexing and demultiplexing by allowing several virtual connections to be supported by a single physical transmission medium|