Each carrier is further time divided into timeslots (TSL) and each timeslot is referred to as a physical channel. It is possible to share a physical channel amongst many processes or users. This sharing is referred to as logical channels.
Physical channel and TDMA frame
Time Division Multiple Access (TDMA), is a method of sharing a resource (in this case a radio frequency) between multiple users, by allocating a specific time (known as the time slot) for each user. This is in contrast to the analogue mobile systems where one radio frequency is used by a single user for the duration of the conversation. In Time Division Multiple Access (TDMA) systems each user either receives or transmits bursts of information only in the allocated time slot. These time slots are allocated for speech only when a user has set up the call. Some timeslots are, however, used to provide signalling and location updates etc. between calls. The main benefit of the time based system is higher capacity than in earlier systems.
In GSM, a TDMA frame is defined as a grouping of TSLs which are numbered 0 to 7 as shown above. It has duration of 4.615ms (8 x 577us).
TDMA frames are transmitted one after another. Every TDMA frame is allocated a frame number.
Um is the acronym for the GSM radio interface. It is an open interface, i.e. it is very accurately specified and thus vendor independent. A subscriber can use mobile phones from any manufacturer without bothering about the operator’s GSM infrastructure and supplier, as long as the network elements are compliant with the GSM specifications.
Tags: