Logical Channels

Introduction

As you remember from the Introduction to TDMA tutorial. GSM divides up each ARFCN into 8 time slots.

These 8 timeslots are further broken up into logical channels.

Logical channels can be thought of as just different types of data that is transmitted only on certain frames in a certain timeslot.

Different time slots will carry different logical channels, depending on the structure the BSS uses.
There are two main categories of logical channels in GSM:

Signaling Channels
Traffic Channels (TCH)

Signaling Channels

These are the main types of signaling Channels:Broadcast Channels (BCH) - Transmitted by the BTS to the MS. This channel carries system parameters needed to identify the network, synchronize time and frequency with the network, and gain access to the network.

Common Control Channels (CCH) - Used for signaling between the BTS and the MS and to request and grant access to the network.

Standalone Dedicated Control Channels (SDCCH) - Used for call setup.

Associated Control Channels (ACCH) - Used for signaling associated with calls and call-setup. An ACCH is always allocated in conjunction with a TCH or a SDCCH.

*keep in mind, these are only categories of logical channels, they are not logical channels themselves.

The above categories can be divided into the following logical channels:

Broadcast Channels (BCH)
     Broadcast Control Channel (BCCH)
     Frequency Correction Channel (FCCH)
     Synchronization Channel (SCH)
     Cell Broadcast Channel (CBCH)

Common Control Channels (CCCH)
     Paging Channel (PCH)
     Random Access Channel (RACH)
     Access Grant Channel (AGCH)

Standalone Dedicated Control Channel (SDCCH)
     Associated Control Channel (ACCH)
     Fast Associated Control Channel (FACCH)
     Slow Associated Control Channel (SACCH)

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Let's examine each type of logical channel individually.

Broadcast Channels (BCH)

Broadcast Control Channel (BCCH) - DOWNLINK - This channel contains system parameters needed to identify the network and gain access. These paramters include the Location Area Code (LAC), the Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters.

Frequency Correction Channel (FCCH) - DOWNLINK - This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts.

Synchronization Channel (SCH) - DOWNLINK - This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know what TDMA frame they are on within the hyperframe.
* The BSIC was covered in the Introduction to GSM Tutorial. You can also read about the numbering schemes used in GSM.

Cell Broadcast Channel (CBCH) - DOWNLINK - This channel is not truly its own type of logical channel. The CBCH is for point-to-omnipoint messages. It is used to broadcast specific information to network subscribers; such as weather, traffic, sports, stocks, etc. Messages can be of any nature depending on what service is provided. Messages are normally public service type messages or announcements. The CBCH isnt allocated a slot for itself, it is assigned to an SDCCH. It only occurs on the downlink. The CBCH usually occupies the second subslot of the SDCCH. The mobile will not acknowledge any of the messages.

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Common Control Channels (CCCH)

Paging Channel (PCH) - DOWNLINK - This channel is used to inform the MS that it has incoming traffic. The traffic could be a voice call, SMS, or some other form of traffic.

Random Access Channel (RACH) - UPLINK This channel is used by a MS to request an initial dedicated channel from the BTS. This would be the first transmission made by a MS to access the network and request radio resources. The MS sends an Access Burst on this channel in order to request access.

Access Grant Channel (AGCH) - DOWNLINK - This channel is used by a BTS to notify the MS of the assignement of an initial SDCCH for initial signaling.

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Standalone Dedicated Control Channel (SDCCH) - UPLINK/DOWNLINK - This channel is used for signaling and call setup between the MS and the BTS.

Associated Control Channels (ACCH)

Fast Associated Control Channel (FACCH) - UPLINK/DOWNLINK - This channel is used for control requirements such as handoffs. There is no TS and frame allocation dedicated to a FAACH. The FAACH is a burst-stealing channel, it steals a Timeslot from a Traffic Channel (TCH).

Slow Associated Control Channel (SACCH) - UPLINK/DOWNLINK - This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.

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Signaling Channel Mapping

Normally the first two timeslots are allocated to signaling channels.

Remember that Control Channel (aka signaling channels) are composed of 51 TDMA frames. On a time slot Within the multiframe, the 51 TDMA frames are divided up and allocated to the various logical channels.

There are several channel combinations allowed in GSM. Some of the more common ones are:
FCCH + SCH + BCCH + CCCH
BCCH + CCCH
FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)
SDCCH/8(0 .7) + SACCH/C8(0 . 7)

FCCH + SCH + BCCH + CCCH

Downlink

Uplink

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BCCH + CCCH

Downlink

Uplink

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FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)

The SACCH that is associated with each SDCCH is only transmitted every other multiframe. Each SACCH only gets half of the transmit time as the SDCCH that it is associated with. So, in one multiframe, SACCH0 and SACCH1 would be transmitted, and in the next multiframe, SACCH2 and SACCH3 would be transmitted. The two sequential multiframes would look like this:

Downlink

Uplink

You will also notice that the downlink and uplink multiframes do not align with each other. This is done so that if the BTS sends an information request to the MS, it does not have to wait an entire multiframes to receive the needed information. The uplink is transmitted 15 TDMA frames behind the downlink. For example, the BTS might send an authentication request to the MS on SDCCH0 (downlink) which corresponds to TDMA frames 22-25. The MS then has enough time to process the request and reply on SDCCH0 (uplink) which immediately follows it on TDMA frames 37-40.

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SDCCH/8(0 .7) + SACCH/C8(0 . 7)

Once again, the SACCH that is associated with an SDCCH is only transmitted every other multiframe. Two consecutive multiframes would look like this:

Downlink

Uplink

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Traffic Channels (TCH)

Traffic Channels are used to carry two types of information to and from the user:

Encoded Speech
Data

There are two basic types of Encoded Speech channels:

Encoded Speech - Encoded speech is voice audio that is converted into digital form and compressed. See the Speech Encoding tutorial to see the process.
    Full Rate Speech TCH (TCH/FS) - 13 kb/s
    Half Rate Speech TCH (TCH/HS) - 5.6 kb/s

Data - Data refers to user data such as text messages, picture messages, internet browsing, etc. It includes pretty much everything except speech.

    Full rate Data TCH (TCH/F14.1) - 14.4 kb/s
    Full rate Data TCH (TCH/F9.6) - 9.6 kb/s
    Full rate Data TCH (TCH/F4.8) - 4.8 kb/s
    Half rate Data TCH (TCH/F4.8) - 4.8 kb/s
    Full rate Data TCH (TCH/F2.4) - ≤2.4 kb/s
    Half rate Data TCH (TCH/H2.4) - ≤2.4 kb/s

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Traffic Channel Mapping

Time slots 2 through 7 are normally used for Traffic Channels (TCH)

Traffic Channel Multiframes are composed of only 26 TDMA frames. On each multiframe, there are 24 frames for Traffic Channels, 1 frame for a SACCH, and the last frame is Idle. Remember that a MS (or other device) only gets one time slot per TDMA frame to transmit, so in the following diagrams we are looking at a single time slot.

Full Rate Traffic Channel (TCH/FS)

When using Half-Rate Speech Encoding (TCH/HS), the speech encoding bit rate is 5.6 kb/s, so one time slot can handle two half-rate channels. In this case, one channel will transmit every other TDMA frame, and the other channel would be transmitted on the other frames. The final frame (25), which is normally used as an Idle frame, is now used as a SACCH for the second half-rate channel.

Half Rate Traffic Channel (TCH/HS)

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ARFCN Mapping

This diagram shows a sample Multiframe with logical channels mapped to time slots and TDMA frames. This is just one possible configuration for an ARFCN.
*For illustrative purposes, half of the traffic channels are full-rate and the other half are half-rate

TS0

TS1

TS2

TS3

TS4

TS5

TS6

TS7

*Remember that CCH Multiframes have 51 frames and TCH Multiframes only have 26. Their sequences will synchronize every superframe.

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Offset

Even though GSM uses a full duplex radio channel, the MS and the BTS do not transmit at the exact same time. If a MS is assigned a given time slot, both the MS and the BTS will transmit during that given time slot, but their timing is offset. The uplink is exactly 3 time slots behind the downlink. For example, if the MS was allocated a TCH on TS3, the BTS would transmit when the downlink is on TS3 and the MS is set to receive on TS3. At this point, the uplink is only on TS0. Once the uplink reaches TS3, the MS would begin to transmit, and the BTS is set to receive on TS3. At this point, the downlink would be at TS6. When the MS is not transmitting or receiving, it switches frequencies to monitor the BCCH of adjacent cells.

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Speech Data Throughput

When looking at a Time slot allocated to a TCH, you will notice that TCH does not occur on every single frame within a time slot. There is one reserved for a SACCH and one that is Idle. So, in a TCH Multiframe, only 24 of the 26 frames are used for traffic (voice/data). This leaves us with a data throughput of 22.8 kb/s.

Here is the math:

1. Calculate bits per TCH Multiframe:
We know that there are 114 bits of data on a single burst, and we know that only 24 of the 26 frames in a TCH multiframe are used to send user data.
114 bits × 24 frames = 2736 bits per TCH multiframe

So, we know that on a single timeslot over the duration of one TCH multiframe, the data throughput is 2736 bits.

2. Calculate bits per millisecond (ms):
From step one above, we know that the throughput of a single TCH multiframe is 2736 bits. We also know that the duration of a TCH multiframe is 120ms.
2736 bits / 120 ms = 22.8 bits per millisecond

3. Convert milliseconds (ms) to seconds:
Now we need to put the value into terms of seconds. There are 1000 milliseconds in a second, so we simply multiply the value by 1000.
22.8 bits/millisecond × 1000 = 22,800 bits per second (22.8 kb/s)

4. Convert bits to kilobits:
Finally, we want to put it into terms of kilobits per second, wich is the most common term for referring to data throughput. We know a kilobit is 1000 bits, so we simply divide the term by 1000.
22,800 bits/s ÷ 1000 = 22.8 kb/s

So now we see why the data throughput of a single allocated timeslot is 22.8 kb/s.

There is an easier method to come to this number:

We know that only 24 of the 26 frames carry data, so we can say that the new throughput would be 24/26 of the original throughput. If we convert this to decimal form:
24÷26 = .9231

We know from the TDMA Tutorial that the data throughput of a single timeslot is 24.7 kb/s. Apply this 24/26 ratio to the 24.7 kb/s throughput:
24.7 × .9231 = 22.8 kb/s

You can see that we get the same answer as above.

A single BTS may have several Transceivers (TRX) assigned to it, each having its own ARFCN, each ARFCN having 8 time slots.

The logical channels that support signaling will normally only be on one ARFCN. All of the other ARFCNs assigned to a BTS will allocate all 8 time slots to Traffic Channels, to support multiple users.

The following diagram is an example of how a medium-sized cell might be set up with 4 TRX (ARFCNs).

Sample Medium-Size Cell

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Frequency Hopping

Each radio frequency Channel (ARFCN) is influenced differently by propagation conditions. What affects channel 23 may not affect channel 78 at all. Within a given cell, some frequencies will have good propagation in a certain area and some will have poor propagation in that area. In order to take advantage of the good propagation and to defeat the poor propagation, GSM utilizes frequency hopping. Frequency hopping means that a transceiver hops from one frequency to another in a predetermined sequence. If a transceiver hops through all of the avilable frequencies in a cell then it will average out the propagation. GSM uses Slow Frequency Hopping (SFH). It is considered slow becuase the system hops relatively slow, compared with other frequency hopping systems. In GSM, the operating frequency is changed every TDMA frame.

The main reason for using slow frequency hopping is because the MS must also change its frequency often in order to monitor adjacent cells. The device in a transceiver that generates the frequency is called a frequency synthesizer. On a MS, a synthesizer must be able to change its frequency within the time frame of one time slot, which is equal to 577 µs. GSM does not require the BTS to utilize frequency hopping. However, a MS must be capable of utilizing frequency hopping when told to do so.

The frequency hopping and timing sequence is known as the hopping algorithm. There are two types of hopping algorithms available to a MS.

Cyclic Hopping - The transceiver hops through a predefined list of frequencies in sequential order.
Random Hopping - The transceiver hops through the list of frequencies in a random manner. The sequence appears random but it is actually a set order.

There are a total of 63 different hopping algorithms available in GSM. When the MS is told to switch to frequency hopping mode, the BTS will assign it a list of channels and the Hopping Sequence Number (HSN), which corresponds to the particular hopping algorithm that will be used.

The base channel on the BTS does not frequency hop. This channel, located in time slot 0, holds the Broadcast Control Channels which the MS needs to monitor to determine strength measurements, determine access parameters, and synchronize with the system.

If a BTS uses multiple transceivers (TRX) then only one TRX will hold the the Broadcast Channels on time slot 0. All of the other TRXs may use time slot 0 for traffic or signaling and may take part in the frequency hopping.

There are two types of frequency hopping method available for the BTS: synthesizer hopping and baseband hopping.

Synthesizer Hopping - This requires the TRX itself to change frequencies according to the hopping sequence. So, one TRX would hop between multiple frequencies on the same sequence that the MS is required to.
Baseband Hopping - In this method there are several TRX and each one stays on a fixed frequency within the hopping frequency plan. Each TRX would be assigned a single time slot within a TDMA frame. For example, time slot 1 might be assigned to TRX 2 in one TDMA frame and in the next TDMA frame it would be assigned to TRX 3, and the next frame would be TRX 3. So, the data on each time slot would be sent on a different frequency each frame, but the TRXs on the BTS do not need to change frequency. The BTS simply routes the data to the appropriate TRX, and the MS knows which TRX to be on for any given TDMA frame.

Baseband Frequency Hopping

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