VDSL systems are defined according to the following criteria:
E1/T1 technology has been with us since the early 1970's and is the basic technological building block in present day PSTN networks. The technology was originally voice oriented and enables the transmission of 30/24 voice/data channels (Time Slots) over two twisted pairs or two coaxial cables. It is used today for high count voice channel consumers like call centers & ISP's. Other applications include PTT premise connectivity. WAN/MAN (Frame-relay, IMA), Cellular base station connectivity and more. Being such a widely used solution in today's networks and the growing demand for higher data throughput, increases the number of E1/T1 connections over existing copper infrastructure. HDSL technology is widely used toady for the transport of E1/T1 channels over copper wires.
E1 is a European based standard and commonly used all over the world. T1 is a North American standard and is used also in Japan (with minor changes-J1). Fractional E1/T1 services use a 64 kbps channel as the basic unit of transport and enable aggregation of several channels into a high speed channel.
This document describes how the mature E1/T1 and the new VDSL technologies can be joined in order to get a technological advantage
The Bandwidth Demand
The fast development of data networks, Internet applications and multimedia capable computers creates a large demand for high-throughput connectivity over the traditional wide area networks. New cellular network services and the introduction of small area coverage micro and pace cells create a demand for cell-to-cell connectivity using microwave wireless connections or twisted-pair connections. Many of the services mentioned before use E1/T1 as the basic transport standard. The demand for E1/T1 connections is served today by PTT's using technologies like HDSL. The HDSL technology uses four or six wires and enables the transmission of up to 2 Mbps (A throughput of a single E1) over a few kilometers length of wire. With the rising demand for bandwidth, other technologies like SDSL and VDSL can be deployed for the transport of E1/T1 channels.
The new and exciting VDSL technology is a major part of the solution to the high bandwidth demand. VDSL provides high bandwidth over the existing copper infrastructure that is available to the network operators. VDSL will be able to provide 4 full-duplex E1/T1 channels over a single copper wire for a distance of up to 1.3 Km. The distance can be extended when a smaller number of channels are being transported, or if the environment of the twisted-pair experiences less background noise. VDSL technology can be operated in conjunction with fiber-optic technologies to provide high-speed network access to all the telco customers.
The new data services that are being provided and developed over the emerging data networks include Internet connectivity, electronic commerce, WAN, Intranet and branch connectivity, providing of multimedia content and Cellular network connectivity.
Telco Access Networks
The traditional access network is based on an infrastructure of bundles twisted pairs that are connected on one end to the local exchange and on the other end to the customer's premises. Each group of customers is connected to a junction box or street cabinet and the bundle of twisted pairs is routed from the local exchange through a number of junction boxes. The traditional services that are supplied by the PSTN are supplied from the local exchange, to the customer's twisted pair. These services include POTS and ISDN services.
The demand for high speed, always on, data services, and the lack of fiber infrastructure, has created a situation in which subscribers are provided with dedicated data services with point to point technologies that operate from the local exchange over a dedicated twisted pair. Dedicated data services can be supplied from the area of the cabinet if a high-speed fiber reaches the cabinet and if the structure of the cabinet supports the deployment of active electrical equipment.
The extensive deployment and the experience in managing large networks places the telephone network operators in a position from which they can deploy high-speed data networks. These data networks can supply all of the emerging services that are required by the networked world.
FSAN Network View
A group of operators have collaborated to define a global and standard vision of a Full Service Access Network (FSAN). This network uses a combination of fiber and copper to provide switched ATM transport to the home. A passive optical network (APON) connects the telco exchange to the equipment in the cabinets and VDSL is used for the final drop from the cabinet to the customer's premises. Customers that are close to the local exchange can be directly to it with VDSL.
Using the same offered infrastructure enables the transport of E1/T1 channels using the ONU/VDSL model or the direct VDSL connection to the CO instead of the proposed ATM transport.
Existing Frame-Relay services, Inverse Multiplexing over ATM (IMA) and other services such as cellular networks using E1/T1 transport can be expanded using existing copper infrastructure, together with the PSTN network, to provide high-speed data services. The Frame-Relay or IMA network will use hierarchies and switching (routing) procedures that resemble the ones used in the digital PSNT network.
The FSAN network is viewed as a long-term infrastructure that must be able to support future applications. Symmetric operation and high bandwidth connections, from the user to the network, must be supported. The components are designed so that the complex and energy consuming ones are located in the exchange and not in the access network and cabinets.
DSLAM Oriented Network
Digital Subscriber Loop Access Multiplexers (DSLAM) are used in hybrid networks that supply multiple types of xDSL technologies and services from one type of equipment that is located in the local exchange. This type of equipment can provide different types of transport service to the customer's premises (ATM, IP, TDM) while providing protocol conversion and connection to the operator's switching network.
The DSLAM is part of an overlay network that routes the data traffic to a dedicated data network while the traditional services continues to be provided by the PSTN network.
Alternate Service Network
The existing access infrastructure can be used to provide alternative types of service by alternative service providers. The protocols and service types can differ but the basic transport media will continue to be VDSL or other types of xDSL.
Alternative Non-Telco Networks
Data communications can be provided to customers over non-telco networks. These networks include cable networks, satellite networks and wireless networks.
Cable modems provide Internet access to customers. The "tree" topology of the network creates a situation in which the upstream direction is shared between multiple users and is, thus, bandwidth limited. Ethernet packets are the mode of transport over these modems.
Satellite and wireless technologies are limited in bandwidth and van be used for Internet access with a limited customer base.
The family of xDSL technologies includes a number of technologies that can be used to provide service from the network operator to the customer. All the technologies are designed so that they can co-exist in the same bundle and that each customer can be serviced with the required type of service. A trade-off between reach and data rate exists. Low rates and long loop lengths are achievable with one type of xDSL while shorter ranges and higher bit rates are achievable with a second type of xDSL. We will discuss only symmetrical xDSL standards being suitable for E1/T1 applications.
HDSL - "High-bite-Rate Digital Subscriber Line"
SDSL - "Symmetrical high-bit-rate Digital Subscriber Line"
HDSL serves as a full or fractional T1 or E1 modem over twisted pair copper lines and does not require a COAX or fiber link. It uses a relatively small amount of bandwidth and requires repeaters to reach all customers with full rate. SDSL, a variant of HDSL, uses more advanced modulation techniques, supports both voice and data, and transmits up to 1.544 Mbps or 2.048 Mbps over a single twisted pair compared to 2 pairs with the HDSL. The distances supported by HDSL/SDSL over a 24-guage line reach up to 12,000 feet. Typical applications include PBX network connections, digital loop carrier systems, Internet POPs (Point of Presence), servers, and private data networks. HDSL is the most mature of the xDSL technologies and can be used today for applications suck as Internet access and remote LAN access.
VDSL - "Very-high-data-rate Digital Subscriber Line"
While no approved standard exists yet for VDSL, several groups including ANSI, ETSI, ITU and FSAN are currently working on this issue. VDSL is targeted at supporting shorter distances than ADSL with higher bit-rates: Up to 51 Mbps at 300 meters (1,000 ft.), and up to 13 Mbps at 1.5 km (4,500 feet). It can be either symmetric or asymmetric.
The ETSI, ITU and FSAN organizations are standardizing QAM modulation for VDSL applications due to the fact that VDSL is directed at shorter distances. In many ways, VDSL QAM is simpler than ADSL DMT is, even though it is ten times faster. VDSL supports POTS or ISDN on the same line, and can work in an environment with heavy interference from ISDN, HDSL and ADSL. The splitter is implemented with passive filtering, and it can be easily implemented from certain designs. VDSL is suitable for the transport of up to E1/T1 channels over a single twisted-pair.
The following issues must be considered when designing and comparing VDSL solutions.
The VDSL systems must be spectrally compatible with all the existing systems in the network. Cross talk from one system to another can reduce the reach of an xDSL system. Frequency separation between xDSL systems and between downstream and upstream transmission is used to ensure spectral compatibility. The maximum transmission level must be defined so that the cross talk will be controlled.
The VDSL system must be flexible so that different spectral allocation can be used for different regional environment. The noise model and available frequency range can differ from one location to another. The noise models and the PSD masks have been defined by the different regional standardization bodies.
VDSL subscribers may be located at different distances from the exchange or the cabinet. All of the subscribers must be guaranteed a certain bandwidth over which the subscribed services will be delivered. Cross talk from a VDSL system in the access network must be able to perform a power back off algorithm so that systems that are close to the cabinet will reduce the transmission power allowing remote systems to function at the guaranteed bandwidth. VDSL is seen ads an infrastructure for future services and it cannot be a best effort service based on a temporary status of a copper bundle. This function is increasingly important in an unbundled environment.
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VDSL is an always -on service that must be available to the subscriber as much as possible. The noisy environment in which VDSL is deployed, and the limited transmission power, creates a situation in which VDSL must be able to adapt to changing noises and in some cases even perform a restart of a link. This process must be robust and fast so that the user applications will continue to function.
VDSL will primarily be deployed from the cabinet. The electronic equipment that is located in the cabinet must have minimal power consumption and size so that the operator's investment in the infrastructure will be minimized.
Many businesses and city customers are located within a short distance from a local exchange. These customers can be serviced by equipment that is located within the exchange. In many European countries, 50% of the customers are located within 2 km. from the local exchange. In the larger cities, a higher percentage of customers can be reached from the exchange.
The total loop length and average loop length differs from country to country. In North America the loop lengths are especially long and there may be loading coils and bridge taps that limit the availability of service.
Most of the customers are located within about 700 meters from a junction box or cabinet. The distance is usually much less. Two issues must be addresses before service can be offered from the cabinet. The first issue is the extent of the fiber deployment. Fiber must be available at the cabinet for electronic equipment. Most cabinets are designed for passive connections of wires. The cabinet must provide enough space and sufficient environmental conditions so that electronic equipment will function.
Campus connectivity solutions like LAN extensions and cellular network Macro cell to Micro cell connectivity may use private copper infrastructure for the transport of data between remote sites. Providing site connection using two simple stand-alone modems and existing E1/T1 routers may prove to be a cheap and simple LAN extension solution. The noise model for private wiring is less demanding than public bundles noise models and may provide service for longer distances. High power signals can also be transmitted over a private service for longer distances. High power signals can also be transmitted over a private twisted-pair infrastructure and provide longer ranges of operation. A Point-to-point connection may be also less demanding on power consumption specification.