2.5 Making Transition to Packet Technology Networks

The new packet-based MBH networks will be different: main changes are in network capacities and implementation technologies. MBH capacity changes are directly linked to the mobile traffic volume increases, as well as to changes in traffic characteristics. Technology changes in MBH networks are related to different cost-efficiency of the legacy and packet-based transport technologies in handling high traffic volumes and bursty traffic, but they are driven also by the change of interfaces in mobile network elements – more and more of the mobile traffic (at the transport interfaces) is natively packet based. These factors together mean a very strong push for packet-based MBH network solutions, and in the longer term towards fully packet-based MBH networks.

The change in many networks already going on, and according to the forecasts it will even accelerate in the near future. This means that more and more of base station connections will be packet based, often using Ethernet as the lower layer solution. One view about the global change in MBH technical implementations is shown in Figure 2.10.

However, it is important to note also that the different geographical regions and countries and even mobile operators are in different situations: they have different technological starting points, differences in traffic growth as well as significant differences in available funds for change, and that all results in very different speeds in the MBH technology change.

Figure 2.10 Forecast about change to packet-based technologies in MBH networks (from Infonetics Research Inc, 2011).

2.5.1 Transition Strategies for Packet-Based Backhaul

There are several possible paths to follow when moving from a small (or medium) capacity MBH network to the higher capacity one needed with extensive use of mobile data and other broadband services. A general backhaul transition strategy is helpful for making detailed transition plans.

An obvious starting point for the MBH transition strategy is the existing backhaul network, including own facilities and existing leasing agreements and other out-sourced services. The first things to look at when formulating the MBH transition approach is the mobile operator business targets and the mobile network development plans based on those. The MBH network needs to be able to serve the mobile network in all its development phases and in each phase to provide enough capacity for the mobile traffic that is planned for.

In all MBH development plans one has also to note how costs of a wholly different magnitude are related to building new physical links compared to changing the technology (i.e. typically transport equipment) on existing physical links. This applies especially for links based on cables, i.e. on fiber-based transport links (as well as for still used copper links).

Building new physical links usually means not just new cables, but above all digging the ground, and related costs are often very high. Digging costs can easily be higher than the transport equipment costs already for a distance of one kilometer (in rural areas) or even for a link of 100...200 meters (dense urban area). Thus new physical links (e.g. optical cable links) are added into a MBH network only based on careful economic consideration; obviously, if there are possibilities to share the new links with fixed services or with another operator, new physical links can more easily be justified.

2.5.1.1 A Target MBH Network

The transition strategy can be formulated starting from the target network: what kind of MBH network is the longer term (or medium term) goal, what technologies it uses, what kind of topology it has and how is the connectivity arranged. This target network will take into account the above discussed issues of cost optimization, especially operational cost optimization, but it also needs to have enough flexibility, as the business and mobile network plans often change during the implementation phases.

Based on the target MBH network, mobile network development phases can be drafted, starting from the present existing MBH network.

In most cases the MBH target network, in addition to being of higher capacity than the existing one, will use different technologies, and the technology transition(s) needs to be considered in the evolution steps together with the capacity expansions.

2.5.1.2 Serving Existing and New Mobile Systems

A challenge in many (or most) areas is that the MBH network needs, in all its development phases, to support both base stations of older generations (still kept in service for years) and new base stations of the very latest generation, and often also some base stations between these generations.

For example, there will be many base station sites with 2nd and 3rd generation (e.g. GSM and WCDMA) base stations operational for several years where new high capacity base stations of the latest generation (e.g. LTE) are added. The former ones have TDM or ATM interfaces and may not be upgradable for a packet interface, while the newer ones often only have packet-based interfaces (e.g. Ethernet at lower layers). A MBH network obviously needs to provide simultaneously transport connections for all of these base stations.

2.5.1.3 Different Type of Areas

The MBH evolution and transition strategy can be, and in practice very often needs to be, different for different types of areas.

Capacity expansion needs will be highest and occur first in city centers and business districts and in those areas major MBH expansions are needed first. In these areas the likelihood of new filling base station sites (with only new technology base station, e.g. LTE) is highest, and these new sites require new high capacity and low cost MBH links to connect to the rest of the network. These links are obvious candidates for immediately using the packet technology of the planned target network. Thus the transition in these areas starts at the new sites; the high capacity needed may also justify changing all MBH links in that area for the new technology. As change in the upper tiers is often more easily done, and the traffic increase anyway requires new transport solutions, transition in this kind of area may be economically justified more or less simultaneously in the whole MBH network.

On the other hand, in areas with expected smaller traffic increase (and only few if any new base station sites), new technology base stations are coming later. Then the change of base station interface types and correspondingly the MBH technology is purely a question of cost optimization, and the MBH access network transition may be done in a later phase, especially if big investments are required in early phases in other areas. In such low-growth areas MBH technology transition typically happens first in the backbone and then in the aggregation tiers, and later in access tier.

2.5.1.4 Overlay or Replacement

For each area where mobile network is upgraded and expanded, a major decision in the MBH strategy is whether to build a (temporary) MBH overlay network for that area, or to replace the existing MBH equipment in that area wholly by new equipment. Generally speaking, when a new transport overlay is built, it is easier to optimize it for the new base stations and their requirements and traffic profile, but the whole MBH network becomes more complex and there is more equipment to manage and maintain. On the other hand, the replacement approach often leads to a simpler MBH network, but the challenges are: timing of change-over on various sites, managing the (moving) interface between the new and old MBH domains and support of older generation base stations. Economically, selection depends on the case and local circumstances; either of these approaches can lead to a lower investment cost solution, but the overlay tends to result in higher operational costs, at least for a period of time.

Overlay MBH network is usually based on the technology selected for the target network, and its capacity is dimensioned to accommodate traffic from the new base stations, at least in the first phase, in addition to all the traffic of the existing base stations. Actual moving the traffic of the older generation base stations over to the new transport network is postponed until the interfacing and other requirements are fulfilled. It may also be necessary to keep the existing MBH network in operation until all the backhaul requirements, e.g. provision of base station synchronization (see Chapter 06), are satisfactorily fulfilled within the new network. An additional benefit of keeping the legacy network up and running for longer than absolutely necessary is added resiliency during the extended parallel operation – the legacy MBH network together with the older generation base station form a back-up network which can carry at least some traffic in case of problems with the new network; this increased resilience obviously comes with a cost – higher OPEX of operating and maintaining parallel networks.

In the replacement strategy all transport in a certain area is changed, and all existing and new base stations are immediately supported by the new MBH network. Thus all the backhaul requirements, including synchronization, need to be fulfilled immediately from the change-over day, and some additional equipment may be required at the base station sites to connect the older generation base stations (e.g. layer 2 or 3 devices supporting pseudo-wire emulation for TDM and/or ATM interfaces). Replacement areas and dates need to be carefully selected so that interface between the existing technology MBH network and the new MBH network can be managed without too high costs. A possible network evolution scenario is shown in Figure 2.11.

2.5.1.5 Leased Lines and Out-Sourced Transport Services

In all transition strategies temporary or more permanent use of leased connections and out-sourced transport services need to be considered, as it may help in managing the transition phase. Conventional TDM leased lines may be used to keep the TDM part of the MBH network as a working entity during a transition period. And new leased packet-based connections (or out-sourced transport services) can be used to create a full coverage for the packet-based MBH network faster than would be possible by relying only on one's own facilities.

As always, use of other operators' services in the MBH network is more likely in the backbone and the aggregation tiers where the availability of these services is much better than in the access links, and also cost level is often lower due to more competition in offerings; a typical case is shown in Figure 2.12.

Figure 2.12 An example of a combined MBH solution: the MBH access tier is based on own transport facilities (e.g. on microwave radio links) and the aggregation and backbone tiers are based on leased lines (Note: RNC's and ‘intermediate gateways’ are here located on the core site, as often is the case when both aggregation and backbone tiers are leased).

2.5.2 Implementing Transition and Network Evolution

Practical implementation of the MBH network transition and evolution starts from the selected network strategy and from the existing MBH network. Good planning is an important first step, which to a large extent is responsible for smooth and successful network transitions.

2.5.2.1 Planning the Changes

Proper plans are needed for each network expansion phase, and detailed plans are needed for each area and each site, to make major MBH changes doable and to guarantee proper network operation during the transition steps and after them.

All plans have to be detailed enough and include accurate timing of various steps: addition of new equipment, creating connections over the new links (possibly first for new traffic only), then taking them into service, (later) moving all traffic away from the old equipment, and finally taking old equipment out of service and removing it from the sites.

2.5.2.2 Addition of New Equipment and Links

Addition of new equipment needs careful consideration on small sites, especially on small base station sites, and on leased sites. In those cases the available space is often limited (or additional space is expensive), and thus careful planning of equipment lay-out and installation is needed. In all sites practical things, like cabling, power supplies and temperature control, need to get enough attention. Even if a fast transition is planned, the existing and new equipment often have to be operational in parallel for some time, and e.g. power supply and heat removal must be sufficient also during this period.

Before making the new equipment and links operational, they are taken under the network control and supervision, either within the existing network management system (if the new equipment is compatible with it) or a new management system is taken into service together with the new equipment. In both cases the network management links also need to be planned, and implemented in an early phase.

Finally, the new MBH connections, before they are taken into service, are tested and proper operation is verified according to the test plans.

2.5.2.3 Parallel Working of Old and New

When the new links are in service, the MBH networks often include different types of connections, also between the same sites, and network operations may for a period need more attention. Also, a different kind of know-how is often needed, especially when the older equipment and links are TDM (or ATM) based and the new parallel equipment and links are based on packet technologies.

When the new and existing equipment are under a different network management system, some additional care and administrative work is needed to keep the overall picture of the MBH network up-to-date and to consider possible mutual influences of changes in the parallel systems.

2.5.2.4 Simplifying the Network

The final step in the transition is typically a simplification of the MBH network. In this phase all the traffic is changed over the new links; sometimes interworking units (e.g. pseudo-wire units for TDM over packet) are needed to enable this. This step usually means significant complexity reduction in network management and in network operations, and thus helps to reduce the MBH operational expenses, as discussed earlier.

Note

1. Therefore the backhaul operational costs between mobile operators and networks cannot be compared at all without knowing how much of the backhaul is based on leased lines and outsourcing in general.