Deploying a low-cost data service in rural areas using WiMax over GSM/GPRS by Nicolas Chauville, Damien Chatelain and Prof. Barend van Wyk, Tshwane University of Technology

In some South African rural areas, GSM networks have spare capacity. The available capacity is usually not fully utilized and other services can be developed to use capacity more effectively. One possibility is to try to use this spare capacity to provide a low speed data service to rural areas.

When we speak about data access through a mobile switching centre (MSC). The PC is Modifications to the Linux kernel public land mobile network (PLMN), general equipped with a Linux kernel modified and To communicate with the IWU, a new protocol packet radio service (GPRS) comes to mind, adapted to the new protocol in order to offer was developed. Its function is to establish a but GPRS access is not well adapted to our a classical network interface to the application connection to the IWU and to organise IP problem. One of the reasons is that we do not layer. Classical quality of service or load data in order to make it understandable to need to cover an entire area. In many rural areas, sharing applications can therefore be used the IWU. As shown in Fig. 3, this protocol is requests to access a data network are extremely transparently. localised between IP and Ethernet on the OSI localized (town hall, administration center, layer scheme, where Ethernet is the hospital, backpacker hotel, internet data link protocol. Our interface is café, etc. [1]). A point-to-point access connected to the DXX using a single to the PLMN is therefore more suitable Ethernet card. than GPRS. Since subscribers needing data access in these areas are not only The implementation of this new extremely localized, but can also be protocol, called the IWU protocol distances of up than fifty kilometers in the sequel, is divided into several apart from each other, WiMax can be parts. First, main functions of this used to extend services and increase protocol have to be implemented in range. It is therefore interesting to the kernel domain. Then, in the user investigate a way to access the PLMN domain, a daemon called iwud is over the GSM classical core network implemented in order to take care of Fig. 1: A WiMax network interfaced with GSM. using WiMax technology. signaling to establish or to terminate a connection. Finally, this protocol In the first part of the paper ideas to build a Modifying GSM network should be transparent to applications such as WiMax network interfaced with a GSM network To interface the PC with the GSM network, quality of service or load sharing software. are introduced (refer to Fig. 1). This solution, several modifications were implemented. We introduced in section II, is certainly an option, interface with the DXX, therefore the base On a Linux system, the only way applications but a 64 kbps link is permanently used. Since station transceiver (BTS) configuration does can communicate with the kernel is by using the provision of a permanent 64 kbps link is not need to be modified. Minimal changes the system call socketcall. Although this not a perfect solution, a best-effort service over have been made on the BSC and on the system call is almost never used directly by a classical GPRS core network is introduced in MSC. As shown in Fig. 2, a semi-permanent programmers, libraries using this call were the second part of the paper. connection is established through the BSC developed. and through the MSC. The idea is to establish Wimax over GSM So, the point is not that applications reach the a direct connection between our interface and IWU protocol (because user applications only This section is divided into two parts. First we the interworking unit (IWU) in the MSC. The access a upper layer) but that the IWU is called discuss some ideas about interfacing WiMax IWU can be connected to the net through the by layer 3 (IP in this case). The classical way to with GSM and then the reasons for building PSTN or through ISDN. Its job is to convert do this is to use a structure called net_device. a network based on WiMax point-to-point data from PLNM to analogue data, for instance, connections are presented. In section II.B if the IWU is connected to the net through The Linux kernel provides an interface to the the interface between WiMax and GSM is the PSTN [2]. Finally, the WiMax interface is middle of OSI layer 2 (see Fig. 4) and since presented. This interface consists of a PC directly connected to the IWU by reserved programming under this layer depends on (configured as a router) connected to the time slots to establish a connection to the net. the hardware used, kernel developers have digital cross connecter (DXX) using a LAN Instead of having to respect GSM protocols to defined a software interface in order to allow module. Minimal modifications have to be communicate with BTS, the interface has just designers to easily create kernel drivers. This made to the base station controller (BSC) and to communicate with the IWU. interface is a C structure called net_device.

38 Elektron February 2006 Fig. 2: Modifying the GSM network.

Fig. 3: Adding a new protocol. that this kind of packet must be forwarded to the iwu interface - more information can be By definition, best effort traffic should not found in [3]). At this layer, routing facilities of have any influence on classical GPRS traffic. Linux and classical applications can be used. On the GPRS core network this implies that Although IWU is transparent, the iwu interface GPRS equipment supporting GPRS support is not the final destination. The iwu interface is node (SGSN), gateway GPRS support node implemented as a tunnel interface, since it is (GGSN) and other network resources have to not linked to specific hardware. After a packet treat WiMax data as a second priority. has been processed by the IWU protocol it is In section III.A a way is described to create a forwarded using the classical dev_queue_xmit best effort service within an SGSN. In section call of the Ethernet interface. III.B it is explained how to configure the Gb interface on the SGSN and BSC to have best Fig. 4: OSI layer and Linux kernel. Creating a best effort service using GPRS core network effort traffic in the network even if the GPRS When a packet (the skb structure in Linux operator uses a frame relay provider for the The GSM core network is circuit switched and is associated with a packet and contains all interface. so it can not be easily used for the purpose of packet data including headers and functions) is providing a best effort service to the WiMax Using GPRS to create a quasi best-effort service processed by the IP layer, the IP protocol calls sub-network. The GPRS core network is more within SGSN skb->dev->dev_queue_xmit where dev is the suited since it is packet switched. The question net_device structure. This means that, if a new A software solution (which is possible since of using the GPRS core network as a backbone, interface (called iwu) is set up, the IP protocol some GPRS platforms use open source system is therefore worth investigating. will call it (obviously if a routing table shows like Open Telecom Platform [4], [5]) allows

39 Elektron February 2006 Fig. 5: Using SNDCP NSAPI to create a best effort service. Fig. 6: Operators reserve bandwidth on classical GSM trunks for frame relay traffic. for a real best effort treatment, but will require relay equipment used for tests should allow for and for his time and the Centre of Excellence extensive testing before deployment. It is powerful QoS mechanisms (priority queuing in Modeling and Planning (Alcatel, Telkom, possible to have an easier less costly solution for instance) and can be used as routers for Molapo and the National Research Fundation) which provides a quasi best-effort treatment our purposes [11]. With such equipment and for its support. by using available QoS mechanisms. The with the proposed architecture, the network References GPRS standard provides different classes can provide priority to classical GPRS traffic [1] M.L. Best, C.M. Maclay, “Community Internet Access of service by defining some QoS attributes on the Gb interface. such as precedence class [6], delay class [7], in Rural Areas Solving the economic sustainability puzzle”, 2002, available at http://www.inta.gatech. reliability class [7], etc. SNDCP protocol [8] Best effort GPRS access edu/michael_best/vita.pdf provides different classes of services to the In order to use the architecture described in [2] Ericsson Telecom, Telia and Studentlitteratur, upper layer. the previous sections, GPRS protocols have to “Understanding Telecommunications”, 1998, ISBN 9144002149, Best effort traffic on Gb interface be strictly adhered to for access provided by WiMax equipment. As shown in Fig. 6, a WI- [3] K. Wehrle, F. Pahlke, H. Ritter, D. Muller, M. Bechler, Sometimes layer 2 of the Gb interface is MAX interface emulates a simple GPRS call “Linux Network Architecture”, 2001, Chapter 14 provided by a frame relay operator. This is to the BSC and to the GPRS core network. This (p263-314), ISBN: 0131777203; the reason why acting on the traffic has to be interface simulates a mobile station requesting [4] Seved Torstendahl, “Open Telecom Platform, Ericsson Review No. 1”, 1997, available at http:// done just after SGSN or BSC. To do this very a GPRS call, creating a PDP context. A best www.ericsson.com/about/publications/review/1997_ few modifications have to be made to the effort access is provided on the PCM link 01/files/1997012.pdf BSC and SGSN in the network service layer between BTS and BSC by using empty time [5] Lars Ekeroth and Per-Martin Hedström, “GPRS [9]. From the network service layer point of slots reserved for a BTS if some are available. support nodes”, 2003, http://www.ericsson.com/ view, the connection between a BSC and If not, data are buffered on the interface. about/publications/ review/2000_03/114.shtml SGSN is identified by a network service virtual [6] ETSI, “Digital Cellular telecommunications system Conclusion connection identifier (NSVCI) which maps (Phase 2+); General Packet Radio Service (GPRS); a frame relay permanent virtual connection In this article, we have demonstrated that Service description, Stage 2” ETSI standard GSM (PVC). The idea is to multiplex two NSVC on WiMax technology can be used to provide 03.60. the same PVC. Two classical routers are used access to localised rural points of demand, [7] ETSI, “Digital Cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); as multiplexers. These multiplexers allow the using GSM/GPRS as a core network. Since the Abis interface is used, no significant operator Service description, Stage 1”ETSI standard GSM one NSVC to have priority over the second costs are incurred. Very few modifications 02.60 one, which becomes best effort from the on the GPRS core network (done just by [8] ETSI, “Digital Cellular telecommunications system network point of view (Fig. 5). Between the properly configuring the SGSN, GGSN and (Phase 2+); General Packet Radio Service (GPRS); BSC and SGSN a new NSVC is defined. Data BSC) have to be made to implement a quasi Subnetwork Dependant Convergence Protocol from our service is routed to this new NSVC. best-effort service which has no influence on (SNDCP)”, ETSI standard GSM/GPRS 04.65 This NSCV maps to a new PVC connected to classical traffic. The only costly components [9] ETSI, “Digital Cellular telecommunications system a frame relay router. The new PVC between of the proposed system are related to WiMax (Phase 2+); General Packet Radio Service (GPRS); SGSN and router and between BSC and router deployment making the described solution Base Station System (BSS) – Serving GPRS Support Node (SGSN) interface; Network service”, ETSI are set up and on this part of the network extremely cost effective. WiMax for rural GSM/GPRS standard 08.16 GPRS there is therefore reservation of resources. access over GSM/GPRS could become a cost-effective solution for providing data and [10] Vesa Kosonen, “QoS and Frame Relay”, 1999, As the routers are located close to the SGSN Presentation at Helsinki University of Technology, internet access to underserved rural areas in and BSC (in the same room), there is no over- Laboratory of Telecommunications Technology, years to come. dimensioning of the Gb interface. The router P.O.Box 3000, 02015 HUT;Finland; can then differentiate between the two classes Acknowledgment [11] Cisco, “Configuring Frame Relay - Cisco IOS Wide- of traffic and can act on them. Area Networking Configuration Guide”, 2000 Cisco The authors would like to thank MTN (South systems, pp 113 to pp 170 Frame relay standards offer some QoS Africa), who allowed them to perform tests Contact Damien Chatelain, Tshwane mechanisms but these are too basic for in their laboratory and especially to Robin University of Technology, Tel (012) 318-5706, creating our best effort service [10]. Frame West, the laboratory manager, for his help [email protected]

40 Elektron February 2006