Friday, July 29, 2011

Mobile Wireless Communications

Mobile Wireless Communications

Global System for Mobile Communication (GSM). Mobile Wireless Market: Technology Forecasts

Everything is converging. The wired world and the wireless world are converging. The Internet and mobile wireless is converging. The distinction between the wireless, wireline and the Internet service providers is beginning to blur. And the glue certainly is "mobile wireless".

Mobile wireless has exploded in popularity because of the fact that it simplifies and revolutionizes communication. The market for mobile wireless is increasing by leaps and bounds. The success of mobile communications lies in the ability to provide instant connectivity anytime and anywhere and the ability to provide high-speed data services to the mobile user. The quality and speeds available in the mobile environment must match the fixed networks if the convergence of the mobile wireless and fixed communication networks is to happen in the real sense. So, the challenges for the mobile networks lie in providing a very large footprint of mobile services (to make the movement from one network to another as transparent to the user as possible) and the availability of high speed reliable data services along with high quality voice. A range of successful mobile technologies exists today in various parts of the world and every technology must evolve to fulfill all these requirements. In the following sections I'll talk about the mobile technologies existing today, how these technologies compare, how these technologies are shaping up and what we can expect to see in the near future.

The mobile wireless market is predominantly voice-oriented with low speed data services. The popularity of mobile voice services has been the deciding factor for the development of mobile networks so far. Data, mainly in the form of SMS has basically been an extra service. However, SMS is fast becoming very popular and in many European countries subscribers are spending more on SMS than voice. Both the voice and data markets continue to grow and the 2nd Generation networks are evolving to keep up and, in fact, are generating demands for newer services. Although digital technologies have improved the quality of service provided by the mobile networks, the voice quality is still not the same as the toll quality. New speech coding techniques like EFR and adaptive multi-rate are bridging this gap.

Technologies And Services Existing Today

Many second-generation mobile technologies exist today each having influence in specific parts of the world. GSM, TDMA (IS 136), and CDMA (IS 95) are the main technologies in the second-generation mobile market. GSM by far has been the most successful standard in terms of it's coverage. All these systems have different features and capabilities. Although both GSM and TDMA based networks use time division multiplexing on the air interfaces, their channel sizes, structures and core networks are different. CDMA has an entirely different air interface.

In the following sections I will discuss the existing standards, the technologies, the situation today and also talk about some of the forecasts for these technologies. This should help you understand the situation today and also dispel some of the basic misconceptions about the viability of these technologies.

Global System for Mobile Communication (GSM)

GSM's air interface is based on narrowband TDMA technology, where available frequency bands are divided into time slots, with each user having access to one time slot at regular intervals. Narrow band TDMA allows eight simultaneous communications on a single 200Khz carrier and is designed to support 16 half-rate channels. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing and location updating functions exist and be standardized in GSM networks.

GSM offers a variety of data services. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adapter. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bi-directional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval.

The European version of GSM operates at the 900 MHz frequency (and now at the newer 1800 MHz frequency). Since the North American version of GSM operates at the 1900 MHz frequency, the phones are not interoperable, but the SIMs are. Dual-band 900 -1800 and 900 -1900 phones are already released and in production. Tri-band 900 -1800 -1900 GSM phone are expected to be manufactured in the next few years, which will allow interoperability between Europe and North America.

A GSM network consists of mobile stations talking to the base transceiver station, on the Um interface. Many BTS are connected to a BSC via the Abis interface and the BSC connect to the MSC (The core switching network) via the A interface.

GSM network

HLR and VLR provide customized subscriber services and allow seamless movement from one cell to another. The Authentication register and the equipment register provide security and authentication. An OMC and a cell broadcast center allow configuration of the network and provide the cell broadcast service in the GSM network (not shown in the diagram).The voice transmitted on the air interface can be encrypted. The speech is coded at 13kbps over the air interface. Using EFR (Enhanced Fullrate Coding) the voice quality approaches the land line quality. Recent developments like AMR (adaptive multi-rate coding) allow speech coding and channel coding to be dynamically adjusted giving acceptable performance even in case of bad radio conditions. The GSM network supports automatic handovers. Since the mobiles are not transmitting or receiving at all times battery consumption can be conserved. Further using DTX and DRX (Discontinuous transmission and reception, mobile transmits or receives only when there is a voice activity detection) batter power can be conserved even more - a highly desirable characteristic of any mobile system. Also since the mobile is not transmitting or receiving at all times, this allows the mobile to listen to control channels and to provide useful information about other channels back to the cell.

Recent developments and initiatives include:

GSM Association together with the Universal Wireless Communications Consortium (UWCC), which represents the interests of the TDMA community, are working towards inter-standard roaming between GSM and TDMA (ANSI-136) networks.

The majority of European GSM operators plan to implement general packet radio system (GPRS) technology as their network evolution path to third-generation.

MExE will allow operators to provide customized, user-friendly interfaces to a host of services from GSM, through GPRS and eventually UMTS. The first implementations of MExE are expected to support the wireless application protocol (WAP) and Java applications. MExE can extend the capabilities that currently exist within WAP by enabling a more flexible user- interface, more powerful features and security.

GSM cordless telephony system to provide a small home base station to work with a standard GSM mobile phone in similar mode to a cordless phone. The base station would be connected to the PSTN.

Number portability will allow customers to retain their mobile numbers when they change operators or service providers.

Location services to standardize the methods for determining a GSM subscriber's physical location.

Tandem free operation where the compressed speech is passed unchanged over the 64 kbps links between the transcoders, hence improving the voice quality.

Mobile Wireless Communications Tomorrow

The drive for 3G is the need for higher capacities and higher data rates. This article takes an in-depth look at 3G (and beyond) technologies such as GPRS and EDGE.

A Look At GPRS, HCSD, and EDGE

3rd Generation Wireless, or 3G, is the generic term used for the next generation of mobile communications systems. 3G systems aim to provide enhanced voice, text and data services to user. The main benefit of the 3G technologies will be substantially enhanced capacity, quality and data rates than are currently available. This will enable the provision of advanced services transparently to the end user (irrespective of the underlying network and technology, by means of seamless roaming between different networks) and will bridge the gap between the wireless world and the computing/Internet world, making inter-operation apparently seamless. The third generation networks should be in a position to support real-time video, high-speed multimedia and mobile Internet access. All this should be possible by means of highly evolved air interfaces, packet core networks, and increased availability of spectrum. Although ability to provide high-speed data is one of the key features of third generation networks, the real strength of these networks will be providing enhanced capacity for high quality voice services. The need for landline quality voice capacity is increasing more rapidly than the current 2nd generation networks will be able to support. High data capacities will open new revenue sources for the operators and bring the Internet more closer to the mobile customer. The use of all-ATM or all-IP based communications between the network elements will also bring down the operational costs of handling both voice and data, in addition to adding flexibility.

On The Way To 3G

As reflected in the introduction above, the drive for 3G is the need for higher capacities and higher data rates. Whereas higher capacities can basically be obtained by having a greater chunk of spectrum or by using new evolved air interfaces, the data requirements can be served to a certain extent by overlaying 2.5G technologies on the existing networks. In many cases it is possible to provide higher speed packet data by adding few network elements and a software upgrade.

A Look At GPRS, HCSD, and EDGE

Technologies like GPRS (General Packet Radio Service), High Speed Circuit Switched Data (HSCSD) and EDGE fulfill the requirements for packet data service and increased data rates in the existing GSM/TDMA networks. I'll talk about EDGE separately under the section "Migration To 3G". GPRS is actually an overlay over the existing GSM network, providing packet data sevices using the same air interface by the addition of two new network elements, the SGSN and GGSN, and a software upgrade. Although GPRS was basically designed for GSM networks, the IS-136 Time Division Multiple Access (TDMA) standard, popular in North and South America, will also support GPRS. This follows an agreement to follow the same evolution path towards third generation mobile phone networks concluded in early 1999 by the industry associations that support these two network types.

The General Packet Radio Service (GPRS)

The General Packet Radio Service (GPRS) is a wireless service that is designed to provide a foundation for a number of data services based on packet transmission. Customers will only be charged for the communication resources they use. The operator's most valuable resource, the radio spectrum, can be leveraged over multiple users simultaneously because it can support many more data users. Additionally more than one time slots can be used by a user to get higher data rates.

GPRS introduces two new major network nodes in the GSM PLMN:

  • Serving GPRS Support Node (SGSN)
    • The SGSN is the same hierarchical level as an MSC. The SGSN tracks packet capable mobile locations, performs security functions and access control. The SGSN is connected to the BSS via Frame Relay.
  • Gateway GPRS Support Node (GGSN)
    • The GGSN interfaces with external packet data networks (PDNs) to provide the routing destination for data to be delivered to the MS and to send mobile originated data to its intended destination. The GGSN is designed to provide inter-working with external packet switched networks, and is connected with SGSNs via an IP based GPRS backbone network.

A packet control unit is also required which may be placed at the BTS or at the BSC. A number of new interfaces have been defined between the existing network elements and the new elements and between the new network elements. Theoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today's fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. Actually we may not see speeds greater than 64 kbps however it would be much higher than the speeds possible in any 2G network. Also, another advantage is the fact that the user is always connected and is charged only for the amount of data transferred and not for the time he is connected to the network.

Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours.

Already many field trials and also some commercial GPRS implementations have taken place. GPRS is the evolution step that almost all GSM operators are considering. Also, coupled with other technologies like WAP, GPRS can act as a stepping stone towards convergence of cellular service providers and the internet service providers.

HSCSD (High speed circuit swiched data) is the evolution of circuit switched data within the GSM environment. HSCSD will enable the transmission of data over a GSM link at speeds of up to 57.6kbit/s. This is achieved by cocatenating, i.e. adding together, consecutive GSM timeslots, each of which is capable of supporting 14.4kbit/s. Up to four GSM timeslots are needed for the transmission of HSCSD. This allows theoretical speeds of up to 57.6 kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel. HSCSD is part of the planned evolution of the GSM specification and is included in the GSM Phase 2 development. In using HSCSD a permanent connection is established between the called and calling parties for the exchange of data. As it is circuit switched, HSCSD is more suited to applications such as video conferencing and multimedia than 'bursty' type applications such as email, which is more suited to packet switched data. In networks where High Speed Circuit Switched Data (HSCSD) is deployed, GPRS may only be assigned third priority, after voice as number one priority and HSCSD as number two. In theory, HSCSD can be preempted by voice calls- such that HSCSD calls can be reduced to one channel if voice calls are seeking to occupy these channels. HSCSD does not disrupt voice service availability, but it does affect GPRS. Even given preemption, it is difficult to see how HSCSD can be deployed in busy networks and still confer an agreeable user experience, i.e. continuously high data rate. HSCSD is therefore more likely to be deployed in start up networks or those with plenty of spare capacity since it is relatively inexpensive to deploy and can turn some spare channels into revenue streams.

An advantage for HSCSD could be the fact that while GPRS is complementary for communicating with other packet-based networks such as the Internet, HSCSD could be the best way of communicating with other circuit switched communications media such as the PSTN and ISDN. But one potential technical difficulty with High Speed Circuit Switched Data (HSCSD) arises because in a multi-timeslot environment, dynamic call transfer between different cells on a mobile network (called "handover") is complicated unless the same slots are available end-to-end throughout the duration of the Circuit Switched Data call.

Because of the way these technologies are evolving, the market need for high speed circuit switched data and the market response to GPRS, the mobile infrastructure vendors are not as committed to High Speed Circuit Switched Data (HSCSD) as they are to General Packet Radio Service (GPRS). So, we may only see HSCSD in isolated networks around the world. HSCSD may be used by operators with enough capacity to offer it at lower prices, such as Orange. [1] believes that every GSM operator in Europe will deploy GPRS, and by 2005 GPRS users will almost match the number of voice only users. Right now there are 300 million wireless phones in the world. By 2005 we expect one billion.

A quick look at the table below would help you appreciate and understand clearly the technology characterizations as 2nd generation, 2.5 generation and 3G. We have looked into 2G and some 2.5G technologies.

2G and 3G mobile communications

author: anonymous
Wireless Communications
Mobile Wireless Communications Today Report