Content area
Full Text
The need to extend business applications, which are usually data-oriented and which run mostly on local area networks (LANs, over a larger geographical area via the public infrastructure and without any loss of quality has led Bellcore to define a switched multimegabit data service (SMDS). Up until now, the wide-area interconnection of LAN islands (over larger distances) has traditionally been implemented by means of connection-oriented networks, such as X.25, frame relay, or leased lines. The opportunity presents itself, however, to make use of the advantages of the idea of connectionless transmission/service used within LANs for networking, which was taken into consideration when defining SMDS. The greatest advantage of connectionless switching is the omission of the connection set-up, which is especially advantageous for bursty traffic. Yet another advantage is the simplicity of the switching functions which leads to high performance with regard to throughput and transfer delay. SMDS is based on the distributed queue dual bus (DQDB) technology according to IEEE 802.6.
Asynchronous transfer mode (ATM) was also developed at this time. ATM was endorsed by the ITU-T as the basic technology for the future broadband ISDN (B-ISDN). Taking the SMDS specifications and ATM technology into account, ETSI standardised a connectionless switching service, the connectionless broadband data service (CBDS) and described its use via connection-oriented ATM networks. As the differences between SMDS and CBDS are minimal, the term SMDS/CBDS will be used with regard to connectionless transmission service in the following.
SERVICE ASPECTS
The basic service provided by the SMDS/CBDS networks consists in the switching of data packets between the users (Figure 1). (Figure 1 omitted) As a connectionless service is being dealt with, the information required for the switching of the data packets is based on source and destination addresses. Since SMDS/CBDS is set up as a public service and intended for global coverage, the format for the addresses was chosen according to the ITU-T Recommendation E.164, so that compatibility with the ISDN numbering plan could be maintained.
In addition to the actual packet routing, the SMDS/CBDS network offers a number of access features which reflect the characteristics of the connectionless access, namely:
* verification of access rights through the use of 'source address validation';
* 'address screening' enabling the creation of closed user...