Cellular Backhaul and ATM Transport

The second generation of cellular services (2G), as characterized by the GSM/CDMA/TDMA standard, is a TDM-based technology. The main applications, switched voice services and simple messaging services, run over PDH/SDH access networks, relying on a wide variety of wireline and wireless transport solutions. The relatively new 2.5G (GPRS) data service is still very much a TDM-based technology, providing an interim solution until wideband 3G services take hold. The aim of the emerging third cellular generation, characterized by the UMTS/CDMA2000 (or W-CDMA) standard, is to provide a wireless and mobile means to support wideband voice and data services, such as multimedia applications, interactive Internet surfing and videoconferencing. This third generation of mobile technology is expected to provide up to 2 Mbps (384 kbps of net bandwidth to the user) of wireless service to each subscriber. A substantial leap from 64 kbps GSM/CDMA/TDMA bandwidth, 3G requires a broadband transport technology solution based on a cell- or packet-switched access network.

3GPP and 3GPP-2 standards for W-CDMA define that all the elements of the terrestrial radio access network (known as UTRAN in UMTS systems) be interconnected over an ATM transport network. Following this requirement, Japan's NTT DoCoMo, the first cellular operator to migrate to 3G, employs a cell-switched network for transport of its new services. W-CDMA equipment manufacturers, including Ericsson, Nokia, Siemens, Alcatel and Nortel, have followed suit with the standards recommendation, incorporating ATM access ports in their base station switches to comply with the 3GPP standards. Complicating matters, however, Revisions 4 and 5 of the 3GPP standards discuss the adoption of IP transport some time in the future. Although the adoption of an all-IP transport network will simplify network design, as well as reduce infrastructure expenses, actual deployment of a packet-switched solution is not likely in the near future. The new standards to support this development are not yet finalized, and mature IP products with the means to assure Quality of Service (QoS) for both voice and data do not exist.

Cellular operators have invested extraordinary sums to acquire 3G licenses. Nevertheless, the rollout of new mobile services will happen incrementally. Cellular operators are expected to take a step-by-step approach. Most likely, 3G coverage will initially encompass major urban areas to address the needs of business customers. Once clear patterns of demand arise, cellular operators will begin a carefully controlled build-out of UMTS services, cost-justifying expenditures every inch of the way.

The transport network mentioned above refers to cellular backhaul or the delivery of traffic among radio sites in a cellular operator's network.

The operator's transport network includes the following sites:

- Base stations (2G radio sites - BTS) and Node Bs (equivalent to the 2G base stations)

- Base station controllers (2G BSC) and Radio Network Controller (RNC, equivalent to the 2G base station controller)

- Mobile Switching Center (MSC)

- Serving GPRS Support Node (SGSN)

- Operation and Management Center (OMC)

Cellular ATM-based traffic in the radio access network can travel over E1 UNI ATM or E1 IMA (inverse multiplexing) connections, or over one or multiple E1 lines, depending on the amount of traffic at each Node-B or 3G BTS. The RNC will have STM-1 links to interconnect over the transport networks. Traffic to and from the MSC will require larger pipes, such as STM-1 and STM-4. The IMA option allows the operator to expand its service as demand and the number of subscribers grow and as the number of W-CDMA users increases. The ATM cells will be tagged accordingly with a service class based on the type of traffic, e.g., AAL2 for time-sensitive applications such as voice and video streaming; AAL5 for data services such as Internet access and management of the cellular infrastructure.

The transport network consists of several types of media:

- Fiber optics

- E1/T1 copper leased lines

- Microwave

- DSL

The decision as to which media to use depends primarily on cost, availability and capacity. Each media has its advantages, as discussed below. Several media types can be combined in the same radio access network.

Fiber optics

Fiber is the best transport option due to its speed and capacity, and because it is widely deployed in metropolitan areas. Where a fiber optic infrastructure does not exist, however, the cost of laying new fiber may be prohibitive for this purpose.

E1/T1 copper leased lines

Node-B and 3G BTS ATM cells travel transparently over E1/T1 lines. ATM switches are employed to groom the traffic for connection to the RNC or 3G BSC. The appeal of the G.703 E1/T1 network for 3G cellular transport is its widespread deployment, capable of handling the enormous number of links required for nationwide coverage. A typical UMTS network, for example, will consist of several hundred (or thousand) Node Bs and tens (or hundreds) of RNCs. The result: UTRAN will comprise tens of thousands of E1/T1 links and ports for connecting the myriad elements of the UTRAN.

Microwave

Many operators run their own microwave-based transport network. Microwave affords a low rate (G.703 E1/T1) connection - up to 8 E1/T1s -- to distant Node Bs or BTS. Microwave has been widely implemented for GSM, CDMA and TDMA, and it will prove useful in the initial migration to W-CDMA, where few links are required. Microwave is also easy to upgrade. As demand for wideband increases, however, network extension will require a broadband solution.

DSL

Many carriers have expanded their DSL networks to encompass business centers as well as residential areas. Much of the impetus for this build-out came from an initial attempt to deploy converged networks, combining higher-speed data services with VoDSL telephony. VoDSL did not take off as expected, however, leaving in place a large footprint of relatively underpopulated DSL networks and associated links. Using SHDSL technology, carriers can aggregate cellular traffic to existing DSLAMs and offer a very attractive, low-cost solution for cellular backhauling. This is similar to the way GSM/CDMA/TDMA traffic is backhauled over E1/T1 leased lines.

The rollout of 3G networks providing W-CDMA services is expected to start in 2003. Nokia, for example, has announced that its 3G handsets will debut in 2003. Operators that have invested in 3G licenses are certainly eager to recoup their investments; however, the high initial cost of 3G services will preclude many users from switching immediately to 3G. For this reason, GSM/CDMA/TDMA will not be replaced overnight, but will exist side-by-side with the 3G network for many years to come.

To keep costs down, operators will elect to use the same infrastructure for all cellular services. This means establishing Node Bs and 3G base stations alongside 2G base stations. Sharing the same wide area network (WAN) transport facilities for the Node Bs or 3G base stations and the 2G base stations would make this colocation more efficient and easier to manage. Since ATM infrastructure is needed for the W-CDMA traffic, carriers will have to provide CES services over AAL1 to connect the GSM/CDMA/TDMA equipment over ATM.

Thus, the transport solutions for the emerging 3G networks will have to do double duty: to cope with the deployment of new broadband services and ensure a smooth and secure path for 3G investments, while keeping legacy GSM/CDMA/TDMA alive for an unspecified period of time.

Quality of Service

In many instances, cellular backhaul involves transporting radio traffic from one carrier network to another - cellular operator to local access provider and back to the cellular provider's own network. As long as TDM technology is involved, quality of service issues are less complex because base station traffic typically has a dedicated path across the transport network's service span. ATM, on the other hand, is cell-based and statistical in nature, implying shared network resources. For this reason, a clear demarcation between the cellular operator's facilities and the local access provider's network is advisable to ensure quality of service. RAD's ACE-202 ATM network termination unit (NTU), for example, owned and operated by the carrier and installed at the customer premises (in this case at the Node B or base station site) defines a precise border between the carrier network and the cellular operator's customer located equipment. This device enables the ATM carrier to manage services end-to-end across the Last Mile, providing Quality of Service (QoS) and meeting service level agreements (SLAs) with the cellular operator.

In addition, the ACE-202 performs sophisticated traffic policing and shaping, for more efficient use of ATM technology. Although W-CDMA equipment is designed for ATM transport, it does not take advantage of ATM's traffic management capabilities.

The NTU is also engineered to integrate ATM W-CDMA cells and TDM GSM/CDMA/TDMA timeslots at each site. Using one unit to deal with both types of traffic reduces the number of WAN links and simplifies network management. As a modular device that supports a wide variety of interfaces, the NTU enables the carrier to connect the radio site over multiple E1 links, IMA, or even microwave links.

ATM Traffic Aggregation

Connecting individual base stations and Node Bs directly to BSC/RNCs in a large regional or nationwide coverage area would require an enormous amount of copper, fiber and microwave links. Not only would this network configuration entail significant operating expenses, but it would also be difficult to monitor and manage. A better alternative is to reduce the cost of transport network operations by aggregating traffic onto fewer ATM links in a star or ring topology. RAD's ACE-2002 ATM multiservice access node, installed in a dedicated point-of-presence (POP) owned by the ATM service provider, can groom both 3G and 2G traffic onto a single link. The ACE-2002 is a fully modular device supporting E1 links over STM-1, E1 UNI and IMA, and CES for co-location of W-CDMA and GSM/CDMA/TDMA base stations, over copper or fiber networks. By applying sophisticated traffic policing, scheduling and shaping capabilities, RAD's ACE-2002 enhances the efficiency of the ATM transport network.

Serving Several Cellular Operators over a Shared Infrastructure

Keeping the lid on network operating expenses will be even more crucial in the 3G era than it was during GSM/CDMA/TDMA deployment. For this reason, competing cellular operators should not object to using a shared network transport infrastructure from a common carrier, providing that carrier will be able to guarantee individual Service Level Agreements and security. Technically, this is possible by aggregating cellular traffic coming out of several operators radios, either co-located at the same "radio/BTS hotels" or residing in the same vicinity, and integrating it over a higher bandwidth pipeline. In such a case, a point of demarcation is required to define physical and/or logical boundaries at which the services will be monitored. The carrier can assign different virtual paths (VPs) or virtual channels (VCs) at a variety of data rates to each cellular operator. The service levels (CBR, VBR, etc.) can also vary among the operators. This aggregation functionality, together with the ability to separately handle each dedicated cellular operator's traffic, may be accomplished by deploying a multiservice ATM access concentrator such as RAD's ACE-2002. This solution would be installed at each carrier point-of-presence and connected to each customer's point of entry to the network. In this way, it is possible to manage the connections separately and uphold each operator's SLA. The ACE device provides end-to-end management, supporting full OAM capabilities of several services using a single box.

Cellular Transport over Deployed DSL Networks

The demand for high speed Internet access has led to a large-scale rollout of DSL networks in many parts of the world. For various reasons, a large proportion of these networks are underutilized. As a result, DSL operators have an opportunity to leverage their ATM network coverage and offer cellular backhaul services over existing copper plant. This can be achieved by installing a dedicated CPE at each operator's radio site. Such a device can connect the TDM traffic of the 2G base station or the ATM cells from the Node B or 3G base station over SHDSL links to previously deployed DSLAMs. Installing RAD's LA-140 or LA-110 ATM-based integrated access devices (IAD) at each 3G Node B/base station or 2G base station enables the carrier to provide an alternative means of connectivity over copper with the ability to control and manage the cellular traffic over the Last Mile. RAD IADs encapsulate the traffic over ATM to the DSLAM using standard SHDSL technology. They use ATM management features to control and monitor the traffic, thereby guaranteeing quality of service and SLAs.

Management of Cellular Base Stations

Carriers can use the LAN module of the ACE-202 NTU to manage auxiliary base station/Node B equipment, such as the microwave antenna or SCADA devices, and to perform remote configuration and diagnostics. The management information is sent to the operations and maintenance center (OMC) over the same link that is used to send the cellular payload.

Ethernet/IP transport services

The carrier may consider Ethernet access for additional transport services, such as IP networking to ISPs over the same ATM network. ISPs located in the vicinity of the carrier POP may be connected to the ATM network by implementing an Ethernet/Fast Ethernet connection to the ACE-2002, utilizing the LAN module.