GSM Services and Features

Dec 6 • General • 5755 Views • No Comments on GSM Services and Features

GSM Mobile Services and Features:

GSM Services follows ISDN guidelines and are classified as either Teleservices or Date Services. Teleservices includes standards mobile telephony and mobile originated or base originated traffic. Data Services includes computer to computer communication and packet switched traffic. This post includes Wireless Communication Notes on GSM Services and Features including introduction of GSM Radio Interface. User may be divided into three major categories.

GSM

GSM Services

  • Telephonic Services
  • Bearer Services or Data Services
  • Supplementary ISDN Services

(1) Telephone Services: These services includes emergency calling and facsimile. GSM also support Videotext and Teletex, though they are not integral part of GSM standard.

(2) Bearer Services or Data Services: These services are limited to layer 1,2 and 3 of open system interconnection (OSI)reference model. Supported services include packet switched protocols and data rates from 300 bps to approx. 9.6 kbps.

  • Data may be transmitted using either a transparent mode (where GSM provides standard channel coding for the user data) or
  • The non-transparent mode where GSM offers special coding efficiencies based on the particular data interface.

(3) Supplementary ISDN Services: These are digital in nature and include call diversion, closed user groups and caller identification and are not available in analog mobile networks. Supplementary services also include the SMS (Short Messaging Services). This SMS service allows GSM subscribers and base station to transmit alphanumeric pages of limited length.

SMS also provide cell broadcast, which allow GSM base station to repetitively transmit ASCII message with as many as fifteen 93 character string in concatenated fashion.

SMS may be used for safety and advisory application, such as the broadcast of highway or weather information to all GSM subscribers with in reception range.

GSM Radio Subsystem

GSM originally used two 25 MHz cellular bands set aside for all members countries, but now it is used in many bands. The 890-915 MHz band was for subscribers to base transmissions and the 935-960 MHz base to subscribers transmission.

GSM uses FDD and a combination of TDMA and FHMA scheme to provide multiple accesses to mobile users. The availability of forward and reverse frequency  bands are divided into 200 KHz wide channels called ARFCNs ( Absolute Radio Frequency Channel Numbers).

The ARFCN denotes a forward and Reverse channel pair which is separated in frequency by 45 MHz and each channel is time shared between as many as eight subscribers using TDMA.

Radio Interface and Protocol

AS we know that the GSM works on radio frequency which can be describe by frequency frame. Frequency frame consist of bytes which is of 8 bits in which first bit is start bit,58 is encrypted bits. 26 is training bits, 58 is data/encrypted bits, 3 is stop bit, 8.25 is guard level. 26 bits on combining makes multi-frame and combination of 51 multi-frame is called super-frame. 2048 super-frame on combining makes it hyper-frame. The time frame of each bit is different like for frame it is 465.92 microsocs for multi frame it is 120 ms and for super-frame it is 6.12 secs.

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