16.3 Telephony and Its Rapid Evolution
| Figure 16.2 provides a schematic overview of the existing PSTN. This architecture provides a cost-effective method of delivering voice connectivity throughout the world. When combined with digital wireless (cellular) architectures, nearly universal access is available, certainly in the developed world and in large parts of the economically developing and even underdeveloped areas. However, there are a number of changes that are occurring that are causing rapid evolution to this architecture. These changes are among the important drivers for development and deployment of VoDSL:
16.3.1 Digital Loop Carriers (DLCs)Traditionally POTS and ISDN services are provided over loops that terminate in the CO. "Line cards," equipment that is an integral part of the CO switch, provides the interface between the loop serving the customer and the remainder of the PSTN. Those loops that support DSL services are supported from DSLAMs, which are stand-alone devices that terminate the DSL services. In the case of ADSL, the POTS (or ISDN) and the ADSL signals are separated in the CO by a splitter, the high-frequency signal being sent to the DSLAM, and the low frequency signal going to the telephone switch. For over forty years , remotes have been used to separate the location of the line card from the location of the telephone switch. In a remote, or DLC, the telephone services are multiplexed over a digital service from the remote to the switch. The line cards reside in the remote, while the signaling and switching functions remain on the switch. In the first generation of DLCs (sometimes known as subscriber loop carrier or SLC, pronounced "slick"), one or more channalized T1s are used to connect the remote to the switch. The use of remotes has several advantages. The loop lengths are shorter, which saves copper and improves the quality of the voice signal for the user . The use of a carrier system such as T1 meant that long runs of copper from the CO to the users could be replaced by several long pairs to support the T1 carriers rather then one loop per user. Typically this first generation of remotes supports between 96 and 256 users per remote. The interface between the remote and the switch is defined in Telcordia TR-008 [9]. DLCs were originally used to serve relatively small aggregations of users that were as far from an existing CO as might occur in certain rural areas or a developing suburb. Enhancements to this architecture include the use of a SONET/SDH fiber- optic connection between the remote and the CO and the use of enhanced protocols between the remote and the CO. These next generation digital loop carriers (NGDLCs) can support up to 2,000 customer lines per remote. The interfaces between the NGDLC and the CO switch are defined in Telcordia GR-303 [4] for North American “based systems and ITU-T V5.2 [11] for systems in Europe and the rest of the world. By the use of fiber, a very reliable high bandwidth interface between the remote and the CO is provided. The GR-303 and V5.2 interfaces support additional functionality and improved efficiencies in multiplexing. The use of SONET/SDH, which carries the traffic between remote and CO, means that highly efficient self-healing ring architectures can be used to connect multiple NGDLC to one CO. Figure 16.4 shows such an architecture. Figure 16.4. Digital loop carrier architectures. Because of the development of DLC systems, new COs are rarely built in North America. Instead, new construction (such as new housing developments in suburbs) and rehabilitation of the loop plant (as when population density increases in an existing area) typically are served by adding a new remote to be connected to the existing CO. Because of this increasing deployment of DLC, approximately 30 percent of all customers of U.S. phone companies are currently served by remotes; this figure is steadily increasing. The effect of DLC deployment on DSL deployment is discussed extensively in Chapter 9; however, the use of DLCs has several effects on the architectures and justifications for VoDSL.
16.3.2 Support for Voice-over Packet and Cell-based NetworksThe PSTN has been carrying voice for over 100 years, using a dedicated connection between the callers for each call. Originally these were physical connections, literally wire connections set up through the network between the two points. Although the PSTN no longer sets up copper connections between two callers , it still uses resources that are dedicated solely to each call. The analog POTS signal carried on the copper pairs is converted to a digital signal on the switch in the CO (or in the DLC). A series of dedicated 64 Kbps channels carries the signal to the terminating switch where the signal is converted back into an analog signal and transmitted to the receiver's telephone. Although these digital connections are multiplexed together on the transport systems that connect the switches, each call is allocated resources that are dedicated to the call for its duration. This transport architecture has proven to be reliable and cost effective for many years. However, it is also inflexible and difficult to change; each call must use the allocated resources even if it is not the most efficient use of the bandwidth. The quality of voice that is received is limited by the analog bandwidth of the loops (4 khz) and digital encoding techniques and 64 kbps digital paths used in the network. A series of complex telephone switches must be coordinated by a dedicated and specialized signaling protocols. Because of this the services that can be offered to telephone users are often limited, and these services are difficult and expensive for carriers to change. Much of the operational expense of operating the PSTN is spent to ensure that the connections are stable and able to be constructed as needed. The development of both the Internet protocols (IP) and asynchronous transfer mode (ATM) as data transport architectures allows for a fundamentally different structure for transporting voice. The IP suite of protocols were originally developed to provide an extremely robust data networking environment. Communications are connectionless at the network layer. Rather than a dedicated connection for the data, each packet of data is routed separately and may take a unique path through the network. This routed architecture makes for a network that is very reliable and very extendable, as each router needs to know a limited amount of information about the entire network topology. Routers do not need to keep information about the state of connections set up through the network. Instead, such information is, if required at all, kept in devices connected to the periphery of the IP network. Voice-over ATM conveys voice in an ATM virtual circuit (VC) and data in different ATM virtual circuits, at Layer 2 of the protocol stack. Unlike VoIP, VoATM uses the connections provided in the ATM network to ensure the proper quality to support voice adequately. However, unlike the TDM (time division multiplex ) based PSTN, VoATM uses the 53-byte cells of ATM to make very efficient use of the bandwidth for transporting voice. VoATM architectures have been defined specifically for the DSL environment ”DSL Forum TR 39 Annex A [TR 39] and ATM Forum af-vmoa-0145.0000 [1]. |
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