Ch.16 - Wireless WAN System Architectures 1 Wireless WAN 2 GSM via PSTN 3 GSM via ISDN 4 GPRS 5 Mobitex 6 CDPD 7 PPDC 8 UMTS 9 Future Systems 10 Systems Summary 1 11 Systems Summary 2 1
This section will give a closer description of the wireless WAN systems mentioned in the earlier parts. The system's architecture, time-plans and characteristics are described here. As a comparison to the packet switching systems, Ericsson's solutions for GSM-data and HSCSD are included. The terms first, second and third generations cellular systems are introduced. The greatest importance is given to the systems of the second generation, to give a feeling of how far the standardization work has come. The section ends with a summary of the situation. 2
Ericsson's solution for GSM connection to a company's Intranet or to the Internet, includes the use of the PSTN between the GSM network and the access-server on the Intranet, or the access-server at the Internet Service Provider. Due to modem handshaking, the time for connection is very long, up to 40 seconds. The unit converting the GSM digital signals to PSTN analogue signals is called the GSM Interworking Unit (GIWU). It can be integrated in the MSC, and is then called DTI, Digital Transmission Internetworking Unit. On the established circuit-switched connection PPP (Point-to-Point protocol) is used. The IP traffic is carried by PPP. The long time for establishing this connection is a great disadvantage for both users and network operators alike. High Speed Circuit Switched Data (HSCSD), which uses more time slots, is available to circuit switched GSM. The standard is ready and Ericsson are shipping products. The standard allows for speeds of up to 64 kbps and is intended to be used by, for example, video conferencing and telemedicine applications. 3
Ericsson also has an improved solution for GSM connections to the Intranet/Internet. By switching the modem link over PSTN to an ISDN PRA (Primary Rate Access) in the MSC, and a commercial router, analog communications is never required. In this case, the time for establishing connection drops to less than 10 seconds due to ISDN's fast connection times. ISDN with rate adaption is run between the router and the MSC according to the V.110 standard. This is a stopgap solution. 4
GPRS, General Packet Radio Services, is a standard for packet switched data in GSM. The gross bit-rate is 33-270 kbps, depending on the number of time slots used. The standardization is ongoing under the management of ETSI, with the Point-to-Point service standard fixed in spring of 1998. The biggest news is that you don't establish a connection, but only use the channels when data is transmitted. It's possible to use more than one time slot to increase the users bit-rate. The great advantage with GPRS compared to circuit switched computer communication is that the data communication will be cheaper for the user, and at the same time more profitable for the network operator. Today the GSM computer communication is debited for the time the connection is established. If several users share the channel, a lower price could be given to each user, based on the amount of data transmitted. The network operator can earn more money from the packet switched channel than from a circuit switched computer communication. The intention of the GPRS was that it should support all the possible network protocols like IP, X.25, etc. In phase one of the standardization only X.25 and IP are supported. For the support of GPRS, two new switching nodes are introduced: a "Serving Node", called SGSN, and a "Gateway Node", called GGSN. The terms SGSN and GGSN have already been explained in the lesson "GSM and GPRS". Even though the GPRS standardization procedure is not complete, Ericsson has started work on the implementation of these nodes. 5
Mobitex is a well established, widely deployed dedicated, packet switched system. It is a narrow-band network offering a gross bit-rate of 8 kbps on 12.5 khz channels. Note that several users may have to share this available bandwidth. Ericsson is the only supplier of network equipment for Mobitex, but radio modems, software and network add-ons are available from third party vendors. The specification for 8 kbps was ready in 1990. In Sweden and Nigeria the systems are run on an older specification from 1986, which only offers 1.2 kbps. Because of this low bitrate, you don't hear so much about Mobitex in Sweden. The biggest network, run by RAM Mobile Data in the US, consists of about 1200 base stations with 93 % coverage of the urban business population. The interface towards the net is specified in an open specification, developed by Telia, and nowadays administrated by the Mobitex Operators Association (MOA). The network layer is specially adapted to mobile computer communication, making it possible to develop applications for the vertical market. For the horizontal market, where you would rather use standard protocols and standard APIs, a specially adapted network layer is a disadvantage, but standard protocols can be run over the Mobitex network layer. These solutions are not specified in the interface specification. A number of products to solve the problem of standard protocol communication have emerged on the market. To make connections to the X.25 environment easier, a network integrated X.25 gateway has been developed, and an integrated IP solution is under development. 6
The network architecture is hierarchic. There are two levels of switches, main switch and area switch. For large networks a subdivision in subnets, inter-connected with a backbone, can be used. On the backbone, TCP/IP is used, leaving the Mobitex provider free to choose the backbone supplier. 7
Cellular Digital Packet Data is a packet switched computer communication network built on top of the AMPS-system. The system offers a gross bit-rate of 19.2 kbps over 30 khz channels. The standard was arrived at by a committee made up of several Bell-companies, IBM and other data communication players. The standard was ready in 1993, and the construction of CDPD is ongoing in the US, but the coverage is still inadequate. The interest in CDPD has become an important selling point for AMPS. Vendors use the existence of CDPD solutions to push AMPS networks. Ericsson has had a CDPD product since the end of 1996, and several other suppliers also offer CDPD solutions, the earliest of which were introduced in 1994. The CDPD standard defines both an IP and a ConnectionLess Network Protocol (CLNP) interface. (CLNP is the OSI equivalence of IP.) Ericsson has no CLNP service in the system, and has no intention to introduce it. The mobility solution is similar to that found in Mobile- IP. There is a base node that controls all it's subscribers' location, i.e. which Serving Node they are under. The Serving Node knows only which subscriber is under it for the moment. 8
Packet Personal Digital Cellular is a standard for packet switched data in PDC. The gross bit-rate is 14-42 kbps depending on the number of time slots available. A maximum of three can be allocated to a single user. The carrier width is 25 khz. The standardized interfaces are the radio interface and interface towards other nets. The radio interface standard was ready in the beginning of 1997, and the other standard (towards other networks) was also ready during 1997. 9
The Universal Mobile Telecommunications System, UMTS, will take the personal communications user into the Information Society of the 21st century. It will deliver advanced information directly to people and provide them with access to new and innovative services. The fundamental difference between GPRS and UMTS resides in the support of high bit rate bearer services with the notion of negotiated traffic and QoS characteristics. This shall allow UMTS to support single- and multi-media N-ISDN applications and single- and multimedia IP applications. In the final stage UMTS supports packet switched data service capabilities of at least 2 Mbps peak bit rate per user in urban areas and 384 kbps in wider areas along with Point-to-Multipoint communication. Moreover, the core network allows one mobile terminal to handle more than one bearer service simultaneously and to have bearer services of different connection modes. It is nevertheless expected that the terminal and network capabilities will put some limitations on the number of bearer services that can be handled simultaneously. Each connection has independent traffic and performance characteristics in the sense of throughput, delay and packet loss. The core network is IP based while the network between SGSN and RNC is ATM based. Standardization will have been finished in fall 1999 and the first releases are expected on the marked by summer 2000. Licensing is under progress and Japan might be the first country ready to introduce UMTS. 10
The first generation cellular telephone systems were introduced up to the mid-80's and they are analogue systems. These include NMT450 and 900, AMPS and TACS. The second generation of cellular systems came in the beginning of the 90's, they are digital systems and they behave almost identically. These systems include GSM and PDC. Future systems, also called the third generation, will have to cope with very high demands on voice traffic and data rates much higher than today's systems are able to handle. Furthermore, it must be easy to introduce new services, and the cellular phones must be cheap and consume very little energy. It is likely that they will also have to handle roaming between traditional ground-based cellular systems and satellite-based PCS (Personal Communication Systems). In a few years we could have this scenario: your car breaks down in a desolate area, you call your local garage and ask for help. You put on your Virtual Reality-helmet, and a repairman can, using real-time audio and video, guide you through the whole repair. The pictures may show what bolt to unscrew, what tools you need in different stages, etc. This puts high demands on the transmitting capacity of the cellular system and is only one example of what kind of situation the third generation of cellular systems would have to handle. 11
In this table you can compare the different packet based wireless WAN systems. The first commercially available Mobitex services offered a bitrate of 1.2 kbps. This was available in 1986 - a long way ahead of it's current competitors. A new Mobitex standard appeared in 1990 offering a gross bit-rate of 8 kbps. Of the packet data systems that build on mobile telephone infrastructures, CDPD was the first to be standardized in 1993. The gross bit-rate is 19.2 kbps and the net bitrate is approximately 10 kbps. Although the CDPD standard specifies both an IP and a CLNP interface, Ericsson's CDPD product offers only the IP version. The first CDPD networks went into operation in 1994. Ericsson's first CDPD network went into operation in 1997 in New Zealand. PPDC was standardized in 1997. It offers a net bit-rate of between 9 and 26 kbps. This flexibility derives from the ability to utilize more than a single time-slot. The PPDC standard specifies an IP interface. The first PPDC networks was in operation in 1996 - before the standard was officially complete! The GPRS standard will be the last to be completed with phase 1 standardized in spring of 1998 and phase 2 standardization ongoing. The net bit-rate offered is between 9 and 168 kbps, depending on the number of timeslots used. Phase 1 of the standard calls for both an IP and an X.25 interface, phase 2 may specify others. 12
Here we can compare the characteristics of some of the circuit switched systems. The GSM standard was completed in 1990 with the first networks in operation in 1991. The maximum user bit-rate for GSM is 9.6 kbps. As GSM is primarily a voice based digital system then no data interface is specified. Remember that because GSM is circuit switched a call setup is required before data is transmitted - this can take up to 40 seconds. The High-Speed Circuit Switched Data standard was completed in 1997 and Ericsson s product is on the market. HSCSD offers higher speeds than GSM by utilizing several timeslots simultaneously; using all 8 timeslots in a single frame means a net bitrate of about 64 kbps. It's envisaged that the market for HSCSD will be quite small. Essentially, HSCSD makes several GSM calls simultaneously which means that a single user can hog a large portion of the air interface. Add that to the fact that making multiple GSM telephone call simultaneously is not a cheap pastime and it is easy to see that HSCSD is likely to be a niche market. The Universal Mobile Telephone System is scheduled to be specified by 1999. The channel separation is undecided as of yet, but the channel access method will be Wideband-CDMA. The planned net bit-rate is between 144 kbps to 2 Mbps, depending on the users location. Remote areas with lower coverage should offer the minimum bit-rates, while urban areas with higher coverage should provide the higher bit-rates. The plan is to have UMTS in operation by the year 2002. To achieve this ambitious goal and to protect current network operators from capital dis-investment it is likely that UMTS will utilize the existing GSM infrastructure in some way. 13