PDF of this tutorialThe deployment of a leading-edge voice-over-broadband service, such as VoDSL, significantly raises the level of network complexity that must be accommodated by the signaling system.
Instead of relatively simple telephones, the CPE in a VoDSL system must consist of more sophisticated integrated access devices (IADs) to handle the local multiplexing of voice and data traffic as well as interfacing to individual voice handsets.
With the deployment of IADs to thousands, and ultimately millions, of individual customer locations, service providers can leverage the extensibility of message-based signaling to cost-effectively conduct remote maintenance and software upgrades of CPE, without incurring the expense of on-site service calls.
Figure 3. Regional Packet Network
In addition to supporting a higher number of CPE, the delivery of VoDSL services also entails coordination of more sophisticated intervening equipment throughout the network. Instead of simply a copper wire loop between the customer’s telephone and the central office switch, a VoDSL service consists of a sophisticated packet-switching network with many key elements, such as DSLAMs, carrier switches, and VoDSL gateways, which typically are geographically dispersed. This geographic flexibility is quite important to CPECs because it allows them to minimize the use of expensive leased floor space in an ILEC’s central office; however, it also places greater demands on the underlying signaling system. Message-based signaling provides the deterministic end-to-end, two-way communications needed for ongoing dynamic network management, performance monitoring, fault isolation, and load balancing.
Building on Leading-Edge Messaging Methods
An ideal digital messaging system is built on the message-based signaling standard Q.931 and transport protocol Q.921, which offer a proven foundation for effective and extensible implementation of signaling in VoDSL environments. Q.931 is the native messaging mechanism already being used in V5, ISDN, VoIP, and GR–303 call control systems. It defines a full range of two-way messages for controlling both advanced functions and providing backward compatibility with legacy systems. DTMF in-band touchtone functions can be carried simply within the VoDSL system’s voice traffic, while the generation of loop voltage current can be handled by the IAD at the customer end.
Message-based signaling also provides a simple mechanism for interfacing to existing AB or ABCD bit-coded PSTN trunk lines. Because any VoDSL implementation needs to extract the bit-codes from the PSTN’s TDM bit stream and reformulate them for propagation over the packet network, it is a straightforward operation to convert them to message-based equivalents during the same process. In this way, the VoDSL system avoids the challenge of juggling hybrid signaling methods, while gaining all of the robustness and end-to-end deterministic control that can only be achieved through message-based signaling.
Achieving User Expectations for Voice
Maintaining acceptable voice quality of service (QoS) for multiple connections over a shared DSL link requires a higher level of signaling support than has been traditionally required for simply establishing and supervising individual isochronous TDM connections. Because the available DSL bandwidth has to be dynamically allocated across all of the voice and data traffic on the link, a message-based signaling system includes sufficient intelligence to stay abreast of bandwidth availability as well as the demands being placed upon the link by other connections.
For instance, if system conditions temporarily drive the DSL link’s available bandwidth down below specifications, the signaling system needs to have the capability for dynamically adjusting the number of simultaneously allowable voice connections in order to maintain required QoS levels. If this is not done—and an additional voice call is admitted to the link when bandwidth is not available—the results are disastrous. Not only will the new call be compromised, but calls already established will also be destroyed.
Similarly, the bidirectional acknowledgment capability inherent to digital messaging allows a network to robustly handle so-called glare issues in which calls initiated from both ends of the line attempt to seize the trunk simultaneously. While one-way, bit-based signaling requires cumbersome system work-arounds to reduce the risk of collisions, the built-in two-way intelligence of digital message-based signaling can completely resolve such situations through auto-negotiation within the signaling channel.
In addition to avoiding such problems, the use of digital message-based signaling can also increase system flexibility by reliably enabling the provision of more individual lines than could actually simultaneously use the available bandwidth. For instance, a user’s system might support 24 different phone numbers, although only 16 of them could use the DSL connection at any one time. The customer receives the benefit of increased flexibility while the provider enjoys the advantage of an expanded service offering. Limited bit-based signaling cannot support such flexibility for over-subscription because its channel-associated focus lacks the ability to manage overall bandwidth usage for multiple channels. On the other hand, message-based systems give the provider the option to provision additional lines without any risk of ever exceeding the DSL connection’s actual capacity.
Laying a Strategic Foundation of Extensibility
In the long run, the VoDSL signaling system also must support sustained growth in terms of both network-wide scalability as well as in the capacity to add new services and functionality. As the state-of-the-art in signaling evolution, message-based methods are currently deployed in some form within the majority of existing PSTN telephony systems. The inherent flexibility of message-based signaling is already accelerating the deployment of advanced services, such as caller I.D., within the PSTN environment and therefore is constantly “raising the bar” for user expectations.
For providers of VoDSL services, message-based signaling therefore represents a fundamentally strategic imperative: providing a solid foundation for delivering both today’s required quality levels and for keeping up with tomorrow’s escalating traffic and service demands.
