and some will be offering a commercial service by the end of 1991. These virtual leased lines will offer bandwidth in the way bursty applications require it, and charge for usage in a similar way to X.25 MANs are based on ring or bus topologies and are intended to services. However all the benefits of Frame Relay will apply. provide city-wide networking, to interconnect LANs and to carry Existing physical interfaces (e.g. V.35) are to be used initially, with digital voice and video. MANs are also seen as a means of accessing standard ISDN interfaces following on later. Private Frame Relay Broadband ISDN (ATM) networks in the longer term, hence the networks are also in use. alignment between MAN and ATM Cell formats, (i.e. 53 byte cells). Although MANs, in the form of SMDS, are very visibly supported The standard IEEE 802.6 Distributed Queue Dual Bus MAN in the U.S. by Bellcore and the Regional Bell Operating Companies comprises two contra-directional data buses up to 150Km long (RBOCs) in terms of market development and technology life cycle, running up to 140Mb/s, as shown in Figure 55.9. MANs are at a much earlier stage than Frame Relay. While the SLAVE FRAME UNI-DIRECTIONAL GENERATOR WRITE MASTER FRAME GENERATOR BUS E Figure 55.9 DQDB architecture 55/10 Fast Packet Switching RBOCs are pushing SMDS for city-wide networking, standards References have not been defined for interconnection of these networks to form wide area networking. In addition, SMDS requires hardware ANSI (1990) TI.606, Frame Relaying Bearer Service - Architec- changes to existing CPE equipment. It is most likely that SMDS will tural Framework and service description. ANSI Inc. not be viable as a public service until sometime late in 1992 or into CCITT (1988a) Recommendation 1.122, Framework for providing 1993. As if in confirmation of this the RBOCS are also actively additional packet mode bearer services. Blue book, ITU, engaged in Frame Relay trials. Geneva. International standards bodies have concluded that Asynchronous CCITT (1988b) Recom