b) TDMA – Time Division Multiple Access
2 PHYSICAL CHANNELS
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3
2.2 Physical Channel Offset
The physical channel is the channel in which an MS transmits and receives information. It consists of one timeslot on a downlink carrier, and the corresponding timeslot on the uplink carrier. This full-duplex method of operation is known as Frequency Division Duplex (FDD).
The example in Figure 3 shows timeslot 2 being used. The MS in this case will only transmit and receive in timeslot 2.
Note the three-timeslot offset in uplink and downlink. This is so the MS is not transmitting and receiving at the same time, allowing for simpler and cheaper equipment.
UPLINK DOWNLINK
Ch.nCh.n+1 Ch.N Ch.n Ch.N
Ch.n+2
2.3 Implementation of Channels
Each BTS may transmit and receive a number of carriers, each of which may carry up to eight simultaneous phone calls on its eight timeslots. However, not all timeslots are available for traffic as at least one of them (often several) is reserved exclusively for control channels. These control channels are normally transmitted on one of the cell’s carriers, known as the Broadcast Control Channel (BCCH). These channels provide facilities to the mobile phone which are necessary for it to establish telephone calls such as:
• informing the MS of an incoming call (paging)
• allowing the MS to contact the BTS requesting service
• providing information, such as which channels are available
• allowing the BTS and the MS to exchange information necessary to set up a call (e.g. phone number)
2.3.1 BCCH Allocation
Every GSM cell has one of its carriers designated as the BCCH carrier. Whereas all other carriers have eight timeslots available for telephone calls (although some may be allocated for GPRS use), the BCCH carrier has a reduced number (typically six) due to the presence of the control channels.
Figure 4 shows two scenarios:
a) shows a cell with three radio channels and the standard control and signalling structure;
b) shows a cell with only one radio channel where TS0 contains a combined control and signalling multiframe.
If more than three radio channels are used, it is possible to use extended control and signalling channels.
The dedicated signalling channels are used for signalling between the BTS and the mobile after the initial communication on the BCCH.
0 1 2 3 Combined Mult
iframe
4 5 6 7
Control + 4 x Dedicated Signalling Channels
Traffic Channels
8 x Dedicated Signalling Channels Traffic Channels
2.4 Allocation of Resources to GPRS
In GSM, timeslots are allocated on a user-by-user basis. That is, when users tell the network that they want to make a call, they will be allocated a single timeslot (physical channel) or, if HSCSD is supported, up to four timeslots. They will use this timeslot or timeslots for the duration of their circuit-switched connection, and when the connection is closed the timeslots will be released for reallocation. The user may not have been continually sending information during the time they were connected, in which case resources may have been wasted.
As a packet-switched bearer service, GPRS transmits user data in bursts. In other words, it seeks to optimize network usage by only granting resources to users when they have data to transmit. This means that physical channels are not allocated permanently, but on a ‘capacity on demand’ basis, and the number of channels allocated to GPRS will fluctuate according to need. The physical channel in GPRS is known as the Packet Data Channel (PDCH).
GSM and GPRS share network resources; PDCHs are allocated to GPRS from the common pool of channels available on a cell. If the demand for the GPRS bearer is high and resources are available, then further PDCHs may be allocated. They may then be reallocated to circuit-switched services if required. Information indicating if adjacent cells support GPRS is transmitted on the Broadcast Control Channel (BCCH).
The specifications do not require that PDCHs are permanently allocated. It remains an operator decision whether to grant resources permanently or temporarily.
However, any allocation of PDCHs will always reduce the number of circuit-switched channels available in a cell.
The number of timeslots allocated to a mobile will depend upon the following:
• how much data the user has to send
• the mobile’s multislot class
• QoS parameters
• resources available in the network
The term multislot refers to multiple traffic channels and associated control channels allocated to the mobile. Different mobiles have different multislot classes, and allocation will be dependent on this.
TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7
Timeslot Allocation in GPRS
2.5 Channel Functions
A cell supporting GSM and GPRS may share resources on one or several carriers.
Figure 6 shows a single carrier with shared resources for GPRS and GSM. It shows timeslot 0 allocated as BCCH, which can be read by both GSM and GPRS mobiles.
Timeslot 1 is allocated as a Dedicated Control Channel (DCCH) for GSM; timeslots 2–6 are allocated as Traffic Channels (TCH) and timeslot 7 is allocated as a PDCH for GPRS.
A cell supporting GPRS may have GPRS radio resources allocated at any time. If no resources are allocated, a mobile can request allocation of such resources and then proceed to use them. The network may dynamically increase or decrease the radio resources allocated to GPRS from GSM to a predefined maximum or minimum.
Information indicating if adjacent cells support GPRS is transmitted on the BCCH.
Broadcast Control Channel
Dedicated Common
Control Channel
Traffic Channels (TCH)
Packet Data Channel
(PDCH)