A random-access channel (RACH) is a shared channel used by wireless
terminals to access the mobile network (
TDMA/
FDMA
Frequency-division multiple access (FDMA) is a channel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing ...
, and
CDMA
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
based network) for call set-up and bursty data transmission. Whenever mobile wants to make an MO (Mobile Originating) call it schedules the RACH. RACH is
transport-layer
In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end c ...
channel; the corresponding
physical-layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
The ...
channel is PRACH.
RACH for GSM standard
Since RACH is shared, there is a probability that two or more mobiles transmit at the same time and their transmissions collide in the medium (air) and they will not be granted access to the network. This happens because limit for number of mobiles transmitting in one RACH time slot is not defined in the GSM standard. If collision happens then mobile waits for random period of time and transmits the RACH again. RACH uses GSM 51 frame multiframe structure in the uplink to transmit information. Mostly TS1 is used but in high capacity GSM cell areas, TS2, TS4 and TS6 is also used for RACH leaving TS0 (mainly used for broadcast frequency).
A random-access channel like that of mobile phone networks is also used in the OpenAirMesh network, between cluster heads and mesh routers.
[ H. Anouar, C. Bonnet, C., F. Kaltenberger, and R. Knopp (2008),
"OpenAirMesh—an experimental platform for cooperative mesh networks", Proceedings of the 1st COST2100 Workshop on MIMO and Cooperative Communications.]
A key feature of a random-access channel is that messages are not scheduled (compared to, for example, a "dedicated channel" in UMTS, which is assigned exclusively to one user at a time). There is no certainty that only a single device makes a connection attempt at one time, so collisions can result.
References
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External links
Cell Access
Channel access methods