SDSL (Symmetric DSL) and S-HDSL (Single-pair High-data-rate DSL) are the few of the several DSL options over which VoDSL runs. The former is a perfectly symmetrical version, with equal amounts of bandwidth both downstream and upstream and runs at signaling rates as high as 1.544 Mbps, which is equivalent to T1 speed, over distances up to 2 miles or so. SDSL also can be adjusted downward to run at rates of 384 Kbps and 128 Kbps. The latter is standardized as G.991.2, runs at rates up to 2.3 Mbps over distances well beyond the typical DSL limit of 18,000 feet, and it does so over a single UTP copper pair.
The four basic elements that make up the VoDSL architecture are the IAD (Integrated Access Device) the DSLAM (Digital Subscriber Line Access Multi-plexer), the data switch and, the voice gateway. The IAD will reside at the subscriber’s location and most frequently consists of two to sixteen analog POTS (Plain Old Telephone Services) ports. The POTS lines can connect any analog telephones, fax machines,modems, KTS (Key Telephone Systems), or PBX systems. The IAD will also have a data connection of some type such as Ethernet, USB, ATM25, or V.35.The DSLAM usually resides at the ILEC’s central office. The DSLAM’s main purpose is to terminate DSL lines from the customer premises and act as a backhaul facility for data and voice traffic from thesubscriber. The role of the DSLAM in the VoDSLarchitecture is vital because it provides the perfectphysical pathway for voice and data—enabling them tocoexist on a single copper pair.The data switch uses an ATM, Frame Relay, or IPprotocol that receives traffic from each DSLAM andredirects it to an appropriate termination point. Thisswitch is an efficient way to link data traffic to the ISPinfrastructures and direct voice traffic to the finalcomponent of the VoDSL architecture—the voicegateway.The voice gateway receives voice traffic from thesubscriber in packet format and efficiently reconstructsit to be received by the service provider’s Class 5 voiceswitch through standard TDM (Time Division Multi-plexing)trunks using a GR-303 interface. The GR-303is a Bellcore or now Telcordia-defined interface27 between a Class 5 switch and remote digital terminalequipment that provides PSTN access to analog loops.VoDSL Protocols and Network ProductsADSL (Asymmetric Digital Subscriber Line) allowsthe subscriber to use a standard telephone and connecttheir PC via an ATU-R (ADSL Transmission Unit-Remote).Within the recent G.lite standard, the sub-scribersplitter was eliminated except for special linefilters required on some telephones. VoDSL eliminatesthe splitter component required for ADSL and any typeof DSL can now be used such as SDSL, HDSL, and VDSL.There are multiple choices of VoDSL products todayand each uses a different protocol such as ATM, FrameRelay, or IP for connection to the network. Jetstream isa company that offers ATM and Frame Relay VoDSLproducts. Coppercom and Accelerated Networks offeronly ATM products. Tollbridge and Lucent Technolo-giesoffer IP VoDSL products. Sprint ION will go withATM as its packetized voice solution (Fred Dawson,"FocusOn: A Work in Progress," Inter@ctive Week,July 1999) and Covad has announced plans to provideVoDSL to consumers and businesses also utilizingATM transport technology.VoDSL with Frame Relay is required by someDSLAMs, such as Lucent’s DSLTNT and CopperMountain’s Copper Edge which can only use FrameRelay on the network between the IAD and theDSLAM. While standard development is underway,most Frame Relay networks don’t currently have built-inQoS (Quality of Service) that is needed to prioritizethe voice and data traffic out to the network. As aresult, a proprietary priority mechanism needs to beimplemented in the DSLAM to support the traffic.This mechanism is needed because the one VC (VirtualCircuit) that carries voice and the other VC that carriesdata will compete to gain access to the network. Sincevoice is time-sensitive and any delays can jeopardizevoice quality, it must be given the highest priority.Both Lucent and Tollbridge’s IP-based VoDSL productsare flexible because they are independent of theDSLAM subscriber technologies. However, it is stillnecessary to maintain priority of the voice and datatraffic.The best way to insure the integrity of voice traffic is toutilize the QoS that ATM protocols provide. ATM hasa built-in mechanism to handle time-sensitive data,such as voice, and ATM allows voice to transmit out ofthe DSLAM uninterrupted. In this protocol the voiceVC is given priority over the data VC. The data VC istypically set up as UBR (Unspecified Bit Rate), whilethe voice VC is configured as VBR (Variable Bit Rate)in real time.Most of the solutions mentioned require ATM connec-tionfrom the DSLAM to the Voice Gateway, typicallyusing a DS3 or OC3 uplink for connection to the ATMnetwork.
