One of the most recent developments in the communication area is the packetized voice transport over data networks and the Internet using protocols such as voice over IP VOI or voice telephony over ATM VTOA.
The packetized voice allows suppression of silence periods of natural speech, hence reducing the bandwidth requirement as well as allowing sharing of the same physical channel by multiple voice connections, i.e.,
statistical multiplexing.
In traditional wireless networks, digitized voice samples in Pulse Code Modulation PCM format are carried over 64 Kbps DS0 channels. However, by use of ATM technology in wireless networks as shown in Figure 4-1, the
packetized voice can be transported more efficiently from base stations to a central office location. At the central office, the voice can be converted to DS0 format to be carried over traditional PSTN networks or the packetized
voice can be carried over Public Data Networks PDN such as Frame Relay, ATM and the Internet networks. Carrying voice over PDNs offers a more cost-effective alternative. Additionally, more voice calls can be carried
out on the same physical link between base stations and central office. Advantages of using ATM between base stations and central office is twofold:
• Higher concentration on the physical links between base stations and central office, • Interoperability with PDNs to allow cost-effective voice transport alternatives.
The interoperability feature of ATM technology with PSTN and other data networks will be further discussed in Section 4.2 and Section 4.3, respectively.
Figure 4-1 Use of ATM in wireless networks
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II. Public Telephone Service Networks
The traditional PSTNs served well in the past to carry voice-only traffic. However, as the traffic pattern shifts from voice to data due to explosive growth experienced in Internet access, the typical telephone access is no longer a viable solution.
Although it make sense from the technology perspective to switch to a packetized transport mechanism for voice, data and video over a single universal network such as ATM, it is not economically possible to abandon PSTN networks due to the large installed base. The migration from PSTN to ATM
networks will take an evolutionary path and will occur over time. This has been a general trend in the telecommunication field from the beginning. Migration from an existing technology to a new one always took place gradually.
The most probable scenario for the transition from PSTN to ATM or any other packet transport technology such as IP is to add the capability to separate high speed and bursty data or video traffic from the PSTN networks at the user interface using access technologies such as ADSL as shown in
Figure 4-2. Once the data or video traffic is separated it can be routed to an ATM network. In this case, while the PSTN network handles the traditional voice traffic, the ATM network handles the packetized traffic data and video.
The key factors for continuing to use the PSTN network for the traditional voice traffic instead of immediately moving to ATM network are:
• Cost • Availability
The cost factor is the most obvious reason to maintain the PSTN network for sometime in the future. In most cases, the operators of the PSTN network have already recovered their investments on PSTN equipment. Hence, the only cost associated with the traditional voice service over the PSTN network
is maintenance and administration related expenses. On the other hand, to provide the same level of service and geographical coverage by implementing an ATM network would require a very large initial investment.
Figure 4-2 Separation of high-speed videodata traffic from PSTN network.
Particularly in the U.S., operators of the PSTN network offer a wide range of value-added voice services such as custom calling features call waiting, call forwarding, etc., Centrex and EKTS features for business customers. Implementation of such features for ATM networks will take enormous
amount of time and effort. Hence the availability of such calling features for voice service is an important factor in maintaining the current PSTN network for sometime into the future. However, there is no doubt that we will reach a crossover point when ATM networks can provide these
sophisticated voice services cost effectively.
The manufacturers of the traditional PSTN equipment already are considering various approaches to prolong the life cycle of their equipment. These approaches involve integration of ATM capability into their equipment or providing adjunct ATM capability for handling high bandwidth data and video
traffic.
III. Data Networks
At present, the data networks public or private are segmented into various sub-networks such as Frame-Relay, SMDS, SNA, X.25, etc. This segmentation can be attributed to the development of several data transport technologies to address very specific needs. In most cases, these sub-
networks are not compatible. Hence, internetworking between these networks is not possible or very costly.
Due to globalization of business and increasing dependence on information and computing, a need to interconnect these sub-networks has arisen
