Global System for Mobile communications (GSM)
GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.
The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.
In the 900 MHz band the uplink frequency band is 890–915 MHz, and the downlink frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.
The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
GSM was further enhanced in 1997[10] with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.
There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen meters; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.
The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).
The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.
In the 900 MHz band the uplink frequency band is 890–915 MHz, and the downlink frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.
The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
GSM was further enhanced in 1997[10] with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.
There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen meters; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.
The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).
Enhanced Data rates for GSM Evolution (EDGE)
EDGE Services, Inc. was founded in 1993 as an IT Consulting company, with a primary focus on the financial services industry. By 1999, EDGE was listed on the Inc. 500 roster of the country’s most successful and fastest-growing private companies.
EDGE’s mission is to continue as a premier IT services company, providing talented resources to meet the IT challenges our customers face. Our contributions will add value to any organization and will result in high quality, cost-effective solutions for every customer, every time.
EDGE attributes its incredible success to three things: our EDGE Associates, our financial services expertise, and our EDGE customers.In-depth skills - deep financial services knowledge - detailed business analysis - certified technicians - extraordinary partnerships. This is what EDGE and our Associates are all about. With an average level of 15+ years of financial IT services background, each EDGE Associate brings to the table a wealth of knowledge and experience – much more than most companies can afford to maintain in-house. Between our customers and our Associates a cycle of trust, collaboration, and growth has been established.
EDGE can supply the full scope of services, from providing a single resource with a particular skill set, to fielding a team delivering all aspects of a major project.
EDGE’s mission is to continue as a premier IT services company, providing talented resources to meet the IT challenges our customers face. Our contributions will add value to any organization and will result in high quality, cost-effective solutions for every customer, every time.
EDGE attributes its incredible success to three things: our EDGE Associates, our financial services expertise, and our EDGE customers.In-depth skills - deep financial services knowledge - detailed business analysis - certified technicians - extraordinary partnerships. This is what EDGE and our Associates are all about. With an average level of 15+ years of financial IT services background, each EDGE Associate brings to the table a wealth of knowledge and experience – much more than most companies can afford to maintain in-house. Between our customers and our Associates a cycle of trust, collaboration, and growth has been established.
EDGE can supply the full scope of services, from providing a single resource with a particular skill set, to fielding a team delivering all aspects of a major project.
General Packet Radio Service(GPRS)
GPRS is an overlay on the existing GSM radio interface to make it more 'packet data-friendly' it uses the existing GSM spectrum and cells. It allows the GSM network to provide some dedicated capacity to be used as a shared data pipe between several users, catering for IP traffic, such as web browsing, which has peaks and troughs and doesn't expect a single dedicated circuit.
GPRS is the first step away from a pure circuit switched architecture into the world of data packet technology, taking it much closer to the computing world by using Internet Protocol (IP). The service offers users the experience of having a constant IP connection, and it has radically changed how the industry approaches the development of new applications and services.
The ability of GPRS to support the IP-based infrastructure means that mobile users find a smooth and easy entry into on-line and internet access, almost identical to what they experience using a desktop PC.
To establish a network connection, the GPRS terminal only needs to conduct a set-up session once, which takes one to two seconds, with the terminal then staying on-line but without using any network resource. When a packet of data is sent or received, a delay of a few hundred milliseconds is experienced as the actual connection is made and the data transferred. This gives the user the impression that the terminal is 'always connected'.
Instead of maintaining a permanent link as with circuit switched technology, GPRS encapsulates the data into individual packets and transmits them over the network. Each data packet is tagged with an origin and destination address and may take a different path through the network on route to the receiver. This characteristic means that network capacity is only allocated when a packet is being sent - it is then released for other traffic. All the packets from the different users are interleaved so that the transmission resource is shared, with no pre-set timeslots being used. This statistical multiplexing technique enables the maximum amount of traffic to pass through the cell, using the radio spectrum more efficiently.
This packet-based technique is in contrast to static time division multiplexing where timeslots are allocated for the length of the call, as in the circuit switched HSCSD, regardless of the quantity of data traffic. The packet-based nature of GPRS means that if there are a number of users vying for capacity in a cell, the resource will be shared across all the users.
The ability of GPRS to transmit a single packet of data allows for remote control and monitoring of vending machines, mobile credit card verification, remote meter reading and simple surveillance services. These applications, normally termed "point to point", typically require very low average throughputs and are highly suited to GPRS.
GPRS is also capable of offering a set of point to multipoint services. This feature enables one data transmitter to send the same data simultaneously to a group of receivers. Known as groupcast or multicast, several applications can be created on top of these services such as news broadcasting, road traffic information and advertising.
GPRS is the first step away from a pure circuit switched architecture into the world of data packet technology, taking it much closer to the computing world by using Internet Protocol (IP). The service offers users the experience of having a constant IP connection, and it has radically changed how the industry approaches the development of new applications and services.
The ability of GPRS to support the IP-based infrastructure means that mobile users find a smooth and easy entry into on-line and internet access, almost identical to what they experience using a desktop PC.
To establish a network connection, the GPRS terminal only needs to conduct a set-up session once, which takes one to two seconds, with the terminal then staying on-line but without using any network resource. When a packet of data is sent or received, a delay of a few hundred milliseconds is experienced as the actual connection is made and the data transferred. This gives the user the impression that the terminal is 'always connected'.
Instead of maintaining a permanent link as with circuit switched technology, GPRS encapsulates the data into individual packets and transmits them over the network. Each data packet is tagged with an origin and destination address and may take a different path through the network on route to the receiver. This characteristic means that network capacity is only allocated when a packet is being sent - it is then released for other traffic. All the packets from the different users are interleaved so that the transmission resource is shared, with no pre-set timeslots being used. This statistical multiplexing technique enables the maximum amount of traffic to pass through the cell, using the radio spectrum more efficiently.
This packet-based technique is in contrast to static time division multiplexing where timeslots are allocated for the length of the call, as in the circuit switched HSCSD, regardless of the quantity of data traffic. The packet-based nature of GPRS means that if there are a number of users vying for capacity in a cell, the resource will be shared across all the users.
The ability of GPRS to transmit a single packet of data allows for remote control and monitoring of vending machines, mobile credit card verification, remote meter reading and simple surveillance services. These applications, normally termed "point to point", typically require very low average throughputs and are highly suited to GPRS.
GPRS is also capable of offering a set of point to multipoint services. This feature enables one data transmitter to send the same data simultaneously to a group of receivers. Known as groupcast or multicast, several applications can be created on top of these services such as news broadcasting, road traffic information and advertising.
0 comments:
Post a Comment