Definition
Logical channels are channels composed of different types of information that are transmitted on physical channels. The MAC layer provides data transmission services on logical channels, and the logical channel type set is defined for different types of data transmission services provided by the MAC layer.
Classification
Logical channels can generally be divided into two categories: control channels and traffic channels. The control channel is used to transmit control plane information, and the traffic channel is used to transmit user plane information. The control channel (CCH) is used to transmit signaling or synchronization data, and the traffic channel (TCH) is used to transmit encoded and encrypted voice or data.
Control channels can be divided into three categories: broadcast channels, common control channels and dedicated control channels. Broadcast channel (BCH) includes frequency correction channel (FCCH), synchronization channel (SCH) and broadcast control channel (BCCH); common control channel (CCCH) includes paging channel (PCH), random access channel (RACH), and allows access Incoming Channel (AGCH) and Cell Broadcast Control Channel (CBCH); Dedicated Control Channel (DCCH) includes Independent Dedicated Control Channel (SDCCH), Slow Associated Control Channel (SACCH) and Fast Associated Control Channel (FACCH).
Function
Traffic channel (TCH) transmits encoded and encrypted voice or data, and control channel (CCH) is used to transmit signaling or synchronization data. The frequency correction channel (FCCH) transmits information to the user to correct the frequency of the mobile station (MS). The synchronization channel (SCH) transmits the MS frame synchronization (TDMA frame number) and the BTS identification code (BSIC) information. The broadcast control channel (BCCH) includes paging mobile station grouping, paging information multi-frame number and CCCH slot number, etc. The Paging Channel (PCH) is used to page (search) the MS. The MS applies for the allocation of an independent dedicated control channel (SDCCH) through the random access channel (RACH), which can be used as a response to paging or as an access during the MS caller registration. The Allowed Access Channel (AGCH) is used to allocate an SDCCH for the MS. In the downlink, the cell broadcast control channel (CBCH) is used to borrow time slots from the SDCCH to transmit short messages and broadcast information. Before the TCH is allocated, the SDCCH is used to transmit system information during the call establishment process, for example, registration and authentication are performed on this channel. The Slow Associated Control Channel (SACCH) is related to a traffic channel TCH or an independent dedicated control channel SDCCH. It is a continuous data channel that transmits connection information, such as the signal strength of the service and neighboring cells received by the mobile station. The test report is necessary to realize the handover function that the mobile station participates in. It is also used for power management and time adjustment of the MS. The Fast Associated Control Channel (FACCH) is related to a traffic channel TCH. During the voice transmission process, if the FACCH suddenly needs to transmit signaling information at a much higher speed than the slow Associated Control Channel (SACCH) can handle, Borrowing voice, just because the interruption is not noticed by the user.
GSM system
The logical channels of the GSM system are divided into two categories: common channels and dedicated channels.
The common channel mainly refers to the broadcast control channel used to transmit the broadcast message of the base station to the mobile station and the common control channel used to transmit the two-way signal required to establish a connection between the MSC and the MS.
Dedicated channels mainly refer to business channels used to transmit user voice or data, and also include some dedicated control channels for control.
Common Channel
1) The Broadcast Channel (BCH) is a one-way channel from the base station to the mobile station. It includes:
a. Frequency Correction Channel (FCCH): This channel is used to send users the information of correcting MS frequency. The mobile station receives frequency correction information on the channel and uses it to correct the mobile station user's own time base frequency.
b. Synchronization channel (SCH): The synchronization channel (SCH) is used to transmit frame synchronization (TDMA frame number) information and BTS identification code (BSIC) information to the MS.
c. Broadcast Control Channel (BCCH): Broadcast Control Channel (BCCH) is used to broadcast general information to each BTS. For example, broadcast the information of this cell and neighboring cells and synchronization information (frequency and time information) on this channel.
2) The Common Control Channel (CCCH) is a point-to-multipoint two-way channel between the base station and the mobile station. It includes:
a. Paging Channel (PCH) This channel is used to broadcast the paging message of the base station paging the mobile station. It is a downlink channel.
b. Random Access Channel (RACH) MS uses this channel to send information to the base station when it randomly accesses the network. The information sent includes: the response to the paging message of the base station; the access of the MS when it initiates a call. And the MS also applies to the base station to assign an independent dedicated control channel SDCCH on this channel. This is the upstream channel.
c. Allowed Access Channel (AGCH) AGCH is used by the base station to send the assigned independent dedicated control channel SDCCH, downlink channel to the mobile station with successful random access.
Dedicated Channel
1) Dedicated Control Channel (DCCH) is a point-to-point two-way channel between base station and mobile station. Including:
a. Independent dedicated control channel (SDCCH) Independent dedicated control channel (SDCCH) is used to transmit instructions and channel assignment information between base stations and mobile stations, such as authentication, registration signaling messages, etc. . This channel supports two-way data transmission during call establishment and supports the transmission of short message service information.
b. Associated Channel (ACCH) This channel can be shared with an independent dedicated control channel (SDCCH) or a traffic channel to transmit signaling messages on a physical channel. ACCH is divided into two types of channels:
a) Slow Associated Channel (SACCH) base station uses this channel to transmit power control information and frame adjustment information to the mobile station. On the other hand, the base station uses this channel to receive the signal strength report and link quality report received by the mobile station from the mobile station.
b) Fast Associated Channel (FACCH) This channel is mainly used to transmit the signaling message of the handover between the base station and the mobile station.
2) The traffic channel (TCH) is a channel used to transmit the user's voice and data services. According to the different service channels of the switching mode, it can be divided into circuit switching channels and data switching channels; depending on the transmission rate, it can be divided into full-rate channels and half-rate channels. The rate of the full rate channel of the GSM system is 13kbit/s; the rate of the half rate channel is 6.5kbit/s. In addition, the enhanced full-rate traffic channel means that its rate is the same as the rate of the full-rate channel at 13 kbit/s, but its compression coding scheme is superior to that of the full-rate channel, so it has better voice quality.