GSM is the shortened form of the proper name for a type of cellular signal. The proper name is Global System for Mobile Communications. GSM was named so because the signal is used more than any other second generation signal in the world. GSM is currently used in more than 210 countries, by more than 2 Billion people. GSM operates on 4 different frequencies 850, 900,1800, and 1900 Mhz. Most mobile devices make use of at least 2 of the signals at a time. The exact pairing of mobile frequencies depends on the country you are visiting. GSM’s standardization means that devices are able to work in multiple countries around the world just like they do in the users home country.
There are 3 main types of cellular sites use by a GSM network. The Macro site is the mother of all cell sites in the GSM world. Macro sites are a full base station with a tower, or building mounted antenna, and generally have the most coverage of any of the GSM cellular sites. Pico cell sites are smaller, in building, type sites. They are usually found inside a building where external cell sites are having difficulty penetrating the exterior of the building. The last type of cellular site is an Umbrella site. These Umbrella sites are larger than a Pico cell, and smaller than a Macro cell. They provide a filling the gap style coverage and are usually located outside. Most Umbrella sites are found mounted on a building or water tower, and do not provide extreme range. Umbrella sites are also usually very directionalized to cover an area missed by the Macro site.
GSM uses a Circuit-Switched technology to establish a connection with another device. Circuit-Switched technology allows for a bi-directional full-duplex connection between two devices. Circuit-Switched technology also allows for lower power consumption in the device, and highly reliable functionality from the network. The only down time that is typically experienced with a GSM system is when physical damage is involved. Lets take a closer look at what makes GSM so different from other second generation standards.
GMS uses a modified Hybrid of TDMA or Time Delay Multiple Access technology and FDMA or Frequency Division Multiple Access technology. TDMA uses coded time cycles to divide the signal into slots. Specific connections are assigned a slot on the rotation, and is only allowed to communicate at a specific timed interval. TDMA uses a system of 8 slots for this feat. Each slot is assigned a number 0 to 7. Each numbered slot is rotated in sequence and checked for information. The re-acurrance of a single slot defines one complete cycle. This is called a Slot Cycle. A Slot cycle index is used by all devices as a map of sorts to determine what devices are located at what slot. FDMA uses a division of frequency channels based on your physical properties and is then meshed with the time sort done by TDMA. FDMA divides the available frequency into 124 pairs of frequency 200 Khz wide starting at 890.2 Mhz.
TDMA is applied both forward (toward mobile device) and backward (away from mobile device). Because communications between the mobile and the network are full duplex (simultaneous send and receive) the send and receive slots are shifted by 3 slots to keep the system from trying to send and receive on the same slot at the same time.
Within these time slots there are segments named multi-frames. Multi-Frames split the time slots into smaller timed sections where certain parts of control are handled. There are two Multi-Frame sets one is 26 frames with a 120 ms interval, the other contains 51 frames with a 236 ms interval.
The 26 Multi-Frame subset controls traffic related functions. The Traffic Channel (TCH) and the Fast as well as the Slow Associated Control Channels all reside here. The traffic channel carries voice information spread between 24 120 ms frames. The information maintains 1 time slot in each frame with a typical bandwidth of about 13 kb/s for each of the TCH. There are eight Traffic channels.
The Slow Associated Control Channel (SACCH) is implemented on frame 12. One frame is reserved for each of the TCH. Frames are numbered from 0-26. The Fast Associated Control Channel (FACCH) is called upon by stealing slots from the TCH. The TCH are not active when the FACCH is implemented. The FACCH contains information for quick call set up, and call termination signaling. The SACCH controls various functions of link control and slower call signaling functions.
The 51 frame Multi-Frame Subset controls more complicated call functions like Stand Along Dedicated Control (SDCCH), Broadcast Control Channel (BCCH), Synchronization Channel (SCH), Frequency Control Channel (FCCH), and Common Control Channel (CCCH).
The SDCCH carries information for call signaling to and from the mobile phone. The SDCCH has a Slow Associated Control Channel that is released once the call finishes. BCCH controls the mobile direction to broadcast information. Sync parameters, mobile services, and Cell ID are all contained on this channel. The BCCH never stops transmitting. When no useful information is available, the system will transmit empty bursts. The mobile phone monitors the transmission signal to determine call hand-off functions to another broadcast cell. Other sync functions are handled between the SCH and FCCH. The SCH maintains the frame synchronization, while FCCH maintains carrier synchronization.
The CCCH maintains an open gateway for signaling information to and from the mobile. There are 3 resources that access this channel to perform the signaling functions. The resources share this channel. The paging channel handles functions for calling on the mobile device. A Random-Access channel is used to request a Direct Control Channel from the system during call set up. The Access Grant Channel is used to assign resource access to the mobile.
Technology will continue to advance just as GSM has advanced from the older analog systems. New ways of dividing the available spectrum to hold more calls at a higher quality will be the priority of the engineers designing these complicated systems. The basic principals of GSM are expected to continue to be used for many years to come.
