10.4 MBH Solution Alternatives
When the hard facts about the environment and existing networks are known, as well as the goals of the new networks or network expansion, it is time to look at available solution options. A MBH network for a given mobile network can usually be designed in many different ways, even using quite different technologies and network strategies.
Therefore the first task is to limit the number of options by defining the main lines of the MBH solution – for example, whether an existing network is expanded with some functionality enhancements (minimizing disruption), or whether it is time for a wholly new network based on new technologies and equipment (minimizing ‘burden of past’).
Often the new packet based MBH network needs to be planned for significantly higher capacity than the existing MBH network, due to high mobile traffic forecasts already for the life-time of the first phase of network. For example, mobile traffic forecasts for an area for the next 2...5 years may mean that the MBH capacity should be 4...10 times higher than the present network. In such cases the present network offers mostly the infrastructure (sites and routes and possibly cables/fibers) for the new network, but otherwise the new packet based MBH network needs to be built more or less independently of the equipment presently used in the MBH network.
Another very important consideration is whether the new MBH network will be wholly based on the mobile operator's own facilities and equipment, or smaller or bigger parts of the network will be based on leased connections/outsourced transport services. Whether or not to use outsourced transport obviously strongly depends on what the offerings are in the area in question, what their present and forecasted future price levels are and how reliable those services are estimated to be. Selections between in-house facilities and outsourced transport services are among the most important decisions to be made in the MBH solution creation, as these decisions have not only immediately big economic impact (e.g. capex or opex weighted cost structure), but also very significant long term influence on operator organization, and on strategic options available for the following network building phases.
In the next subsections different approaches for building packet based MBH based on in-house equipment are discussed, with the main division between enhancing existing MBH to better handle packet traffic (Section 10.4.1 and 10.4.2) or taking a more disruptive approach and starting to build fully packet based and packet optimized network parts (Sections 10.4.3...10.4.6); in practice a good solution may also be something between these cases. Then the following section 10.5 discusses the important question about the role of leasing and outsourcing as a part of the packet based MBH solution.
10.4.1 Enhancing SDH/Sonet Networks with NG-SDH/MSPP Equipment
Perhaps the least disruptive and easiest way to improve packet traffic carrying capability of the existing MBH network is to add new nodes and replace some existing ones based on existing technology and product family, but with added packet switching capabilities. Then in the first phase for example only some of the existing SDH nodes need to be replaced or upgraded to be NG-SDH/MSPP nodes – this type of equipment is described in Chapter 5 (Section 5.2.5).
The MBH network efficiency for packet traffic can be greatly improved in this way, especially in the network parts where statistical multiplexing can work effectively, i.e. in network parts where traffic flows from several base stations are already combined; thus the main application area could be the MBH aggregation networks and upper parts of the MBH access networks.
In this solution the SDH network management (with upgrades for NG-SDH/MSPP nodes) can still be used for the upgraded network, and thus it can remain under a single NMS system. Also one of the benefits of SDH network, easy distribution of synchronization towards the base stations, is kept in this phase and consideration of the packet based schemes can be postponed to a later phase.
The main questions to be considered here are whether this improvement is big enough (compared to mobile traffic forecasts), and after this first step, how packet traffic capacity extension continues in the future. When new transport nodes with packet switching capabilities are added (or present ones upgraded with new units) and this is done in the majority of the nodes, it may also be possible to increase the trunk line bit rates and in that way increase the overall capacity, and so significantly extend the expected life-time of this solution. However, this kind of solution is always more rigid in the longer term than a pure packet network, and this disadvantage needs to weighted against short term benefits.
Another limitation, compared to the next example, is that the packet switching technology and solutions are limited to those alternatives available in the NG-SDH/MSPP nodes belonging to the same family as the existing SDH nodes – for example, only some of the Layer 2 packet switching solutions may be possible.
The solution example shown in Figure 10.2 is for a case where new high capacity base stations generating a lot of packet based traffic are added first to one area; so in this area the packet transport efficiency needs now improvement. Here it is done by upgrading part of the SDH nodes to MSPP nodes with packet switching capability, so that the statistical multiplexing benefits of the packet traffic can be utilized. First upgrades are where the packet traffic enters the SDH domain and where it leaves it; the SDH nodes in the middle (shown by an asterisk) can be upgraded to MSPP when there is packet traffic coming from several nodes below it (e.g. when another area is also equipment with new high capacity base stations).
Figure 10.2 MBH network upgraded for packet traffic using MSPP nodes.
The new MWR links shown with dotted lines can be packet based from day one. The situation for the two MWR links marked with two asterisks depends on the capacity and type of the existing links: they can be used, if their capacity is high enough and if they can be provided with packet (Ethernet) interfaces to work in hybrid mode – Ethernet interfaces are needed, as carrying the packet traffic over a number of (bundled) E1 interfaces would be cumbersome and expensive. If this upgrade is not possible with the used equipment or if their capacity is not high enough, new high capacity radio links need to be installed – either packet based radio links in parallel to existing ones, or new links replacing the existing ones. In the latter case the new links also need to support the existing base stations, i.e. their TDM (or ATM) traffic and satisfy their synchronization requirements.
10.4.2 Enhancing SDH/Sonet Networks with a Packet Overlay
This approach is similar to the previous one except that the packet switching capability is added using separate nodes which are then connected to the existing transport equipment. Thus starting packet based MBH network in this way is also relatively easy and flexible if the new packet nodes can be accommodated in the existing sites. Existing synchronization schemes can be kept to a large extent; in addition here the packet switching technology can be selected independently of the existing nodes (e.g. Ethernet Layer 2 solution or partly IP/MPLS solution). A limitation can be (or some extra costs may be caused) because of the packet nodes may in this solution need SDH type interfaces for the interconnection to existing nodes.
The existing trunk capacity is thus also shared in this approach, which can be the main limitation of the solution; if the existing links still have a significant amount of unused capacity or if the mobile packet traffic growth in this area is moderate, this approach can provide a solution for several years.
This solution has the benefit, compared to the previous one, that the new packet nodes can be immediately of significantly higher capacity (even much higher than present trunks can handle), to be ready for the future network evolution steps when the trunk lines will be replaced with new higher capacity ones.
The solution example shown in Figure 10.3 is also for a case where new high capacity base stations generating a lot of packet based traffic are added first to one area which then needs packet transport efficiency improvement. Here it is done by adding packet switching capability using new nodes but still utilizing the existing trunk line capacity. These new nodes are first added only to the sites where the statistical multiplexing benefits of the packet traffic can be best utilized, i.e. where the packet traffic enters the shared transport domain and where it leaves it (dark nodes in the figure), and possibly also to the sites in the middle where several packet based connections merge (light nodes in the figure). These nodes can also be added later when the packet traffic starts to grow faster.
Figure 10.3 MBH network upgraded for packet traffic using packet switch nodes.
The new MWR links shown with dotted lines can be packet based from day one. The situation for the two MWR links marked with two asterisks is similar to that in the previous solution: the existing links can be used, if their capacity is high enough and if they can be provided with packet (Ethernet) interfaces to work in hybrid mode. Here the need for Ethernet interfaces is even more important, if the packet switches are to be connected to these links. If Ethernet interfaces cannot be added or the capacities are not high enough, new high capacity radio links need to be installed – either packet based radio links in parallel to existing ones, or new links wholly replacing the existing ones. In the latter case the new links also need to support the existing base stations, i.e. their TDM (or ATM) traffic and satisfy their synchronization requirements.
10.4.3 Fully Packet Based Networks for MBH Backbone and Aggregation
A common way to start building a fully packet based backhaul network is to build a packet based transport for backbone and aggregation networks first, and to extend packet networks at a later phase to MBH access networks.
The backbone network used for MBH traffic – also in those cases when connections are not leased but based on in-house facilities – is usually shared with other traffic types (fixed network), and therefore the packet network in this tier is a multiservice network.
In general there is a lot of fiber pairs between the major sites, and then the packet network can use its own fiber pair; in other cases wavelength division multiplexing is used and the packet network can have own wavelengths. In both cases the packet network is thus parallel to the ‘legacy’ transport network and mainly independent from that. The backbone packet networks are optimized for total traffic carried over those, and therefore some care needs to be taken that all mobile specific requirements are also fulfilled. Most importantly, when at a later phase of the evolution the legacy backbone is taken out of service, synchronization of the mobile networks must then be good enough over the packet network (unless external means like GPS is used; see closer in Chapter 6).
Aggregation networks are more local, and building of packet based aggregation networks can be done area by area. Often also here the packet network is first built to be in parallel (as an overlay) to the existing legacy transport network. When the packet based MBH aggregation network requires very high capacities from the early phases on, sharing of the infrastructure with the existing transport is best done on the basis of own fibers or own wavelengths.
Aggregation networks can be mobile specific (for MBH traffic only) or shared with other traffic; in the latter case mobile specific requirements again need special attention. And when the legacy transport network is planned to be taken out of service in an aggregation area, mobile network synchronization also needs a new solution in the aggregation tier.
10.4.4 Building Fully Packet Based MBH Access Network for New Base Stations
One pragmatic way to move towards a packet based MBH network in the access tier is to build a packet transport infrastructure first for new high-capacity base stations – these base stations are anyway likely to carry a lot of packet based traffic which could heavily load the existing (smaller capacity) MBH network.
10.4.4.1 Green-Field Case
If the new high-capacity base stations are coming to an area where there is no MBH network yet, this is a pure ‘green-field’ case, and the MBH solution starts from finding the most cost-efficient physical (‘layer 0’) connections. The fastest and also most cost-efficient solution in this case may be a wireless one, i.e. transport from base station sites based on microwave radios. Installing fiber cabling to the new sites requires time and permissions, may be very expensive but provides for the highest capacity, especially in the longer term. Leasing or outsourcing of connections is an option if another operator (mobile or transport operator) has already built a transport network in the area, or is ready to quickly build one at a reasonable cost.
When the feasible ‘layer 0’ solution alternatives are known, design of the packet networks itself can be made. Newest available technologies and equipment can be considered for such a new network, obviously with the optimization criteria of Section 2 in mind. Usually there are many technical alternatives, and just as an example a possible solution is described below; in practice other options may be considered as well.
An example of the mobile network coverage area extension is shown in Figure 10.4. Here the existing part of the network is left as it is for the time being; new coverage area is built with new higher capacity base stations. The new base stations are assumed to have native packet interfaces (Ethernet at the lower levels), and thus the MBH solution can be fully packet based; also it is assumed that the new base stations have integrated packet switches (typically Ethernet switches) so that the intermediate base station sites do not need external switching equipment.
Figure 10.4 A fully packet based MBH network for a new coverage area.
Here the MBH access network is built with high capacity packet based microwave radios – in other areas fiber based solutions may be considered as well. In this example the new packet switch (at the site B) is using different fiber pairs (or different wavelengths of a WDM system) than the existing SDH nodes, making the new packet network an overlay network and thus independent of the existing TDM transport – benefits include independent network design and e.g. free selection of trunk capacities, but on the other hand a packet based synchronization scheme must be in use from the day one.
10.4.4.2 Overlay Case
When the new high-capacity base stations are coming to an area where there are already base stations and sites, there is also an existing MBH network, and the new packet based network will form some kind of an overlay structure. Existing physical links may be utilized for the packet connections by sharing the capacity; however, if the new base stations require much higher capacity per site than the existing ones, it is likely that many existing transport links do not have sufficient capacity for this and new links need to be considered. Then again there are two options: either build new links in parallel to existing ones, or to replace them with new high-capacity links and then share capacity from the new links for the existing system. In the latter case the new links need to be interoperable with the existing transport (e.g. support the existing connection types and provide suitable interfaces).
In both cases the packet network itself will most likely be based on its own new nodes, and its design can be similar to the case above. However, in this case, the possibility of carrying all the traffic of these base station sites over the packet network at a later date needs to be considered, and thus options for supporting ‘legacy’ traffic can be important here.
An example of mobile network capacity expansion is shown in Figure 10.5. Here the existing base stations are left operational as they are for a significant period of time (for several years) and more capacity is provided by newer generation base stations installed mainly on the same sites as the existing ones, and some new sites added where capacity targets cannot be met otherwise (in the example the site F).
Figure 10.5 An example of the packet based overlay MBH network.
In the site A the same basic solution is used here as above, i.e. different fiber pairs (or different wavelengths in a WDM system) are used for the new packet based network, making it independent of the existing one. The MBH access solution is also here based on microwave radios, with two different approaches shown in the picture. For the sites B and C at left the existing microwave radios are replaced with new hybrid MWR systems, providing TDM capacity for the existing base stations and packet connections for the new base stations (only for these hybrid links the MWR terminals are shown, just to make the connections clearer). On sites D and E the existing MWR are left as they are, and in parallel to them new high-capacity packet based radios are built. And the new site F obviously only needs a packet MWR connection (if the packet based synchronization is ready in use in this network – if not, a hybrid MWR could also be considered to this site).
10.4.4.3 ‘Fill-in’ Case
A combination of the cases above is a case – likely to be very common in the near future – where new high capacity base stations are coming to an area with existing base station sites and with an existing MBH network, but also a significant number of new base stations are located between the existing ones. Thus the existing MBH network needs to be strengthened significantly (as in the overlay case above), and in addition a significant number of new links needs to added to connect the new base station locations (number of F type sites is much higher than in Figure 10.5).
A special case of this is when the new base stations are very small physically and lower cost location solutions can be used – for example, outdoor base stations connected on building walls or even to street lamps. In such cases the cost pressure for the backhaul connection is very strong, and new ‘light-weight’ MBH solutions may be needed, as the cost of a conventional connection link may be un-proportional to the cost of the base station itself (see also the example later in Section 10.5.1).
10.4.5 Building Fully Packet Based MBH Access Networks Area by Area
Another way to start moving to the packet based MBH network (in the access tier and possibly also in the aggregation tier) is to build a packet based transport area by area, simultaneously for both new and existing base stations. Then the legacy MBH network(s) in those areas will be quickly and wholly taken out of service, and the cost of continuous operation and maintenance of parallel networks can be avoided.
The first planning questions in this approach are how big areas are taken in each step, in which order those network areas are handled and how the moving boundary between the new packet based MBH network and the legacy network is taken care of. If the aggregation tier has already been built earlier for packet traffic, then interfacing problems are greatly reduced, as the access areas can work without direct interconnections, at least temporarily.
The technical solution for the packet based access MBH here is similar to the overlay case in Section 10.4.4 ‘Building packet MBH network for new base stations’, except that the support for all the base stations, including the oldest ones, is more important: all base stations must be fully supported by the packet based MBH from day one, as the legacy MBH network is going to be dismantled very soon after the traffic change-over. And this full support obviously needs to include synchronization transfer for all different base station with their different requirements, if synchronization is based on transport network (i.e. not based on GPS or other network external means).
10.4.6 Other Possible Approaches/Strategies
In practice transition to a (high-capacity) packet based MBH network is often some combination of the above cases, as conditions and targets within a large network area can vary considerably. Then different approaches may be applied within the same mobile network for the transition to the MBH packet network; however, still a clear change-over strategy is usually preferable (if needed, separately for each major network area).
Various combinations of the solutions are also possible when part of the packet based MBH network is leased or out-sourced in certain areas or network tiers – those cases are discussed in the next section.