Facing the challenges in building a next-generation transmission network

In recent years, the mobile industry has faced a serious downturn. Excessive costs of 3G licenses in Europe and unrealistic expectations of industry growth have led mobile operators to change their business model. While still investing in the development of new access networks in emerging countries, operators focused on reducing their capital expenditures (CAPEX) and operational expenditures (OPEX) to improve their balance sheet at home. This led to delays in the deployment of next-generation mobile systems. However, analysts agree that the industry is bound to recover and rebound as pressure builds to reduce network costs, add new subscribers and increase average revenue per subscriber through the introduction of new data-oriented services.

Today, the transmission portion of a mobile network represents a significant portion of the operating costs of the network. Operators often use microwave links as the solution of choice to connect base stations (BTS or Node B in 3G terminology), particularly for new deployments. This paper outlines the challenges and issues that the mobile transmission group faces when attempting to upgrade their current network to meet the demands of next generation technologies, whether that may be W-CDMA, EDGE, CDMA-1X EVDO or CDMA-1X EVDV.

One of the benefits of the industry crisis has been the steep decline of the network elements required for the deployment of the next generation mobile solution. The cost of 3G Node Bs has been cut by as much as 75%. This means that the cost per user of a 3G system is now below the cost of a 2G (GSM/CDMA) system.

Gone are the years where operators were wondering about the elusive "killer app"--it might now simply be voice calls. With the frequency bands of operation now established and the cost of licenses written off, 3G systems must be deployed, most likely in a gradual fashion. Today, and for the next three or four years, existing 2G and new 3G base stations will be collocated in urban areas. Handover between the 2G and 3G networks is now supported by most new handsets, allowing operators to start off with good service coverage. As a minimum, collocation allows the operator to offload congested areas in the urban environment while keeping their traditional system operational in the less populated places.

Access networks will face a gradual increase in capacity over the next five to seven years. Initial deployment of the 3G Node Bs can be done using single E1/DS1 (2/1.5 Mbit/s) per site. As usage increases, the capacity should reach a capacity of 3xE1/DS1. The downlink capacity, which is the data speed of the connections, depends on the number of Node Bs deployed. It is estimated that when 3G is used as an overlay of the current 2G network, 384Kbit/s capacity will be achieved.

The following chart outlines the capacity required per combined (2G & 3G) site. When additional micro base stations are added for small site coverage, an additional increase in backhaul capacity is assumed.

To keep pace with this increased demand, the capacity in the access network will have to double over the next two years, and double again within the next five years. The optimum network design should be able to follow this capacity upgrade path, without necessitating a wholesale change-out of equipment. 

In today's wireless transmission technology, there is still a large gap between the PDH transmission systems with capacities ranging from 4x to 16xE1/DS1, and the SDH systems with capacities starting at STM-1/OC-3 or 63xE1/84xDS1. This gap is even more significant when looking at the equipment costs. Originally conceived for optical fiber transmission, SDH/SONET technology is a mature technology, but it is inherently more expensive than PDH technology. The mobile operators are using the SDH/SONET technology as purely a transport technology, essentially carrying E1/DS1 tributaries. While looking at new technology for their access network, the operators need to fill the gap between SDH/SONET and PDH capacities with a scalable transmission solution, which will follow market demand realistically.

When considering upgrading the capacity of their access networks, operators also need to look at the reliability of their current design. The first mobile access systems were deployed using tree-type architectures to connect the base stations. As additional base stations and capacities were needed, more branches of the tree were added. By adding more branches however, the network became less reliable, hence increasing the maintenance costs and the OPEX. Any single failure could bring down an entire section of the network.

The reliability of the network design can be improved significantly in the future by using ring architectures. Within this ring structure, one ring should connect a single link of microwave connection or, at a maximum, two consecutive links. The ring can be closed using a mixture of radio and fiber connections. In case of a link failure, a protection scheme also has to be provided in order to automatically wrap the data in the other direction.

For optimum performance and future-proof architecture, the ring should be considered as a flexible access network, able to grow in capacity as the number of users on the network increases, and essentially acting as a buffer network between the base station nodes and the high-capacity core network.

Sitting at the edge between the core network and the last connection link, this buffering network should use high-speed PDH capacities ranging from 16E1 to 64E1/16xDS1 to 84xDS1. This buffer network will ensure a reliable transport for the infrastructure and can be easily upgraded as the traffic demand increases.

Figure 2 shows the design of a typical mobile access network. This network would be upgraded to UMTS by adding a UMTS Node B to the existing 2G site. Initially the capacity assigned to each Node B is limited to a single E1/DS1.

A high-speed ring with capacity of 16xE1/DS1 would be used initially, but it should be incrementally upgradeable to 32xE1/28xDS1, and then to 64xE1/84xDS1. At the collector site, this high-speed PDH ring connects to the SDH/SONET backbone via an add/drop multiplexer (ADM). An Asynchronous Transfer Mode (ATM) switch can be collocated at this collector site, which will terminate the E1/DS1 lines and concentrate the ATM cells inside a VC4 container transported over an STM-1/OC-3 (155Mb/s).

GSM base stations are connected via E1/DS1 lines. For the Node Bs, E1/DS1 is still the preferred interface. ATM is the transport protocol of choice in the core as well as in the access network. Data and voice traffic are carried by AAL2 ATM cells back to the core network, where an Inverse Multiplexer Protocol (IMA) provides the mechanism to carry the ATM traffic on separate E1/DS1 lines. 

Revision 5 of the UMTS specification supports IP as a transport protocol. This means that the new base stations, generally designed around a network processor architecture, will support the traditional circuit switch transport, the cell switched ATM protocol stack and the packet switched IP network. Thus, multi-standard Node Bs will also support both the traditional 2G base stations and the UMTS standard. Figure 3 shows the generic network interface architecture for this new generation of multi-standard base stations.

The Ethernet 10/100 Base-T interface will ultimately become a standard interface for the transport of IP traffic, with the introduction of a DiffServ-based QoS mechanism. The Ethernet interface will allow deployment of gradual cost-effective edge switches and routers in parallel with the ATM switches.

Operators need to anticipate the need for the transport of Ethernet traffic in their access network to ensure a smooth migration of the network when the standards are implemented. Ideally, transmission equipment deployed today should provide a smooth upgrade path to transport Ethernet as well as TDM traffic, without significant impacts in terms of upgrade cost or network disruption.

Handover between UMTS and 2G allows the latter to be used as a fallback coverage for the initial UMTS deployment. This means that the subscriber can experience seamless service during the initial phase of the deployment.

Dual mode 2G/3G handsets require an interoperability mechanism between the UMTS and the 2G technologies. For instance, if the user has established a voice call with the UMTS network, he needs to be able to switch to 2G when he gets out of the UMTS coverage zone. This handover mechanism is based on the compressed mode of operation. While operating in the UMTS mode, the handset uses the gaps opened by the compressed mode transmission to listen and measure the 2G network. Based on average signal quality and strength, the handset makes a decision for the handover.

This compressed mode channel operation forces synchronization between the 2G and the UMTS network to operate correctly in the assigned time slot. Traditionally, the base stations are synchronized either via a separate synchronization network, via GPS receiver or via the transport network.

E1/DS1 lines carried via a PDH transport access network have no wander, and are typically good synchronization sources. However, when the E1/DS1 tributaries are transported via an SDH/SONET network, the pointer movement generated by the resynchronization causes low frequency wanders of the signals. An SDH/SONET network cannot carry synchronization signals via its tributaries. In contrast, a high-speed PDH buffer network (up to 64xE1/84xDS1) will transport synchronization signals while also giving the operators the room to extend the transmission capacity--all the advantages of a high-capacity SDH/SONET network, but without the cost and network complexity. Thus, when introducing UTMS into the network, by adding to or extending the capacity of the PDH network operators can avoid the cost of expensive network synchronization solutions.

When looking at upgrading access networks to cope with the challenges of next-generation mobile networks, operators must anticipate the demand for capacity, improve the network reliability, and anticipate a migration to all-IP transport.

An ideal solution to support a smooth migration from today's low-medium capacity, TDM-based access networks is the introduction of a high-speed PDH buffer network. Capacity upgrades can then be easily implemented with minimum cost and network disruption.

Ring and star network topologies can be designed allowing for gradual capacity upgrade and network expansion. Equipment should provide an optimized solution for building network nodes to reduce the amount of equipment within the network, thereby reducing costs and improving network reliability. An all-PDH access network also provides a much more suitable solution for the distribution of synchronization signals via tributaries in the access network to ensure proper timing and smooth handovers between current generation and 3G base stations.

New and innovative equipment is now becoming available that provides these features. These products are designed to specifically address the needs of mobile operators to save on capital costs today and reduce ongoing operational costs tomorrow, while also ensuring that equipment deployed now will accommodate a smooth upgrade path to support access network requirements in the future.

Alain Hourtane is the director of strategic marketing at Stratex Networks. He can be reached at alain_hourtane@stratexnet.com.

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