Stephen Mallinson, CEO, ip.access, looks at the potential for providing mobile services over an IP network
Most people are aware of the prevalence of IP networks, which surround them at work and, for some, even in the home. These networks are primarily regarded as the method for delivering data to their computers, and giving them access to the Internet and email. Due to the ubiquity of IP networks, companies are deploying ever more applications to leverage the infrastructure — voice-over-IP, video-over-IP; and now, GSM-over-IP.
It was noted a few years back that indoor or in-building coverage for mobile phones was pretty poor. Considering that some of the calls attempting to be connected originated from the lucrative enterprise market, this seemed especially surprising. As a result, many in the industry began asking: why not bring coverage indoors? Well, it was initially tried with leaky feeders and RF cabling — cumbersome, disruptive and, at the time, expensive. The next question to be posed was: why not create basestations small enough to be deployed inside the actual building? Again, this was considered but issues quickly arose concerning powering and interconnecting with the core network (lots of little basestations meant lots of leased lines to backhaul traffic and signalling). With today’s widespread availability of IP networks, there are many ways in which their use can now solve these issues and provide even more benefits to the mobile network operator.
With the ability to place GSM basestations exactly where they are needed, mobile network operators can offer new and innovative services such as Wireless Office to enterprises, and even location-based applications to ‘mobile marketing’ providers. The benefits of deploying GSM basestations utilising IP networks are closely linked to cost — installation cost, the cost of disruption and the cost in maintenance. Using IP networks, all are dramatically reduced.
There are several main elements that a mobile network operator would need in order to deploy a service such as Wireless Office. Importantly, the basestation needs power and also interconnectivity to the core network. Using IP networks, power can be delivered to the basestation and various methods can be employed to implement less expensive backhaul options.
Delivering PoE
To illustrate one benefit of offering a device that can be powered over the Ethernet, with this solution there is no need to run power cables to each device. By simply using the spare capacity of the LAN, no disruption is caused when installing or adding new devices. Power-over-Ethernet (PoE) eliminates the need to run electrical leads to access points for devices powered up from 110V/240V. Using PoE, system installers need only run a single CAT5 Ethernet cable to carry both power and data to the device.
There is a plethora of innovative applications that could spring up from powering devices over the LAN but all have one factor in common: reduced cost. The simplicity comes from the implementation. The power sourcing equipment is usually located in the wiring closet of a building where the standard local area networking equipment is kept. The power is delivered over two of the four twisted pairs of wires in a CAT5 cable. This usually allows up to 13W of power to be transmitted to the powered device.
Delivering GSM-over-IP
In deploying or expanding coverage in a GSM network, a major factor is the cost associated with backhauling between the radio access points and the core network. By using IP as the interface presented to the core network, the use of various existing and emerging IP or packet-based backhaul technologies can be evaluated. The choice of backhaul medium depends on the cost, availability, required payback timeframe, the potential number of sites to be covered, and the bandwidth required (i.e. number of GSM transceivers) per site. There are a number of different methods with the potential to be used as a backhaul medium for GSM networks.
The first is traditional E1/T1 Leased Lines. This common backhaul link used for data transmission and GSM backhaul networks is a dedicated private link and is usually provided by the incumbent fixed line network operator. In many cases, the incumbent is the only operator with the national footprint required by mobile operators and, as such, the pricing can reflect this monopoly position. In many countries cost does not reduce as the number of sites served goes up, and there is no economy of scale, as is the case with shared IP networks. The bandwidth of the link required is dependent on the number of GSM transceivers per site.
DSL is a service that is expanding rapidly in coverage, especially in certain countries, and is a very cost-effective IP backhaul method for GSM basestations. ADSL, limited in uplink bandwidth, can support one transceiver with five active voice channels per link, and with compression applied it can support two transceivers (one full and one partial). The emerging symmetric services such as SHSDL in the UK will be able to support several transceivers depending on the bandwidths offered.
Extended Ethernet is a very cost-effective form of backhaul for short distances, and is often used to provide the link to an IP network. The public Internet can also be a low cost backhaul solution for IP GSM traffic from the transceivers; it is especially attractive if there is an existing link from the service site (usually providing internet services) that has sufficient bandwidth to allow the use of spare capacity for GSM traffic. Even if the site does not have existing Internet connectivity provision, this can still be a cost-effective way of providing backhaul. However, in either case, a VPN over the Internet connection provides privacy and security.
Corporate data networks, available mainly in large cities and quickly expanding, are offered by service providers, with their own (usually fibre) networks. A pipe (often Extended Ethernet) is used to connect to the nearest Point of Presence. They are competitive in pricing compared with end-to-end leased lines, provide Ethernet presentation, and offer QoS on various size, uncontended bandwidths.
Another option is the use of microwave links. The advantage of using microwave backhaul is that there is no running cost element, merely the cost of the equipment for providing the link. However, operation in most frequencies requires a license. The limitation of this method is that line of sight is required, and often also a building permit for mounting the microwave antennas. Operation in the unlicensed bands, such as 2.4GHz, may require QoS provision on the air interface, since the frequency may be used by other users within the area.
Satellite backhaul is the only way to reach coverage in marine and aerospace locations. Moreover, it is also often very economical to enable IP traffic to reach remote rural areas.
A satellite service can be provided as either a dedicated link or part of a service such as broadband satellite. In a dedicated link, a VSAT terminal is used at both the service and the BSC/switch site. This is appropriate for applications such as rural coverage, where a large number of subscribers are served in one location. In a broadband satellite type service, the satellite terminal is only on the service site and is linked back to satellite service hub where the provider manages the service and backhaul to the operator’s BSC/switch site via the Internet or a dedicated link. This is appropriate for a situation where several sites with a small number of transceivers each need to be backhauled, such as coverage on ferries.
Quality of service (QoS) provision is recommended on any shared backhaul link in order to give voice traffic priority over other non-time sensitive traffic; otherwise, voice quality will be adversely affected. Even if an E1 leased line link is shared, it is recommended to use some form of QoS on the router, such as ‘low latency queueing’. In shared links without QoS provision, jitter is the critical parameter affecting voice quality, and is usually due to congestion. If the backhaul used is an IP network, such as ADSL or some form of VPN, QoS should be offered by the service provider as part of the package and usually is. This is often offered as guaranteed presentation bandwidth.
There is a significant cost advantage in using an IP backhaul network for backhauling the GSM traffic from a service site to a central site comparing a leased line to ADSL. This is particularly apparent when looking at the cost based on one GSM transceiver site and UK prices for a 256 kbps leased line for a distance of less than 10 km. The cost of using ADSL as backhaul starts to look competitive after a small number of sites (just over 20) and decreases rapidly with the number of sites served, while the cost of E1 leased lines is flat, so the operator can expect payback in a shorter period of time. In using ADSL, the Opex cost is much lower, so if the service is signed for over one year, it starts to look even better.
The benefits of using IP networks for deploying GSM infrastructure are immense: the ease of installation, the reduced issues of maintenance and the associated reduction in costs in all aspects — but especially the major overhead: the backhaul. The effectiveness of this solution alone can convert a weak business case into a compelling reason to begin offering new services, direct to the subscriber — whether a retail, consumer or enterprise customer.
