GSM Channels

Traffic channels (TCH), control channels (BCCH, CCCH, DCCH), and their functions.

Physical Reality vs. Logical Purpose: Introducing GSM Channels

In the GSM system, communication happens over the air using radio waves. We have previously learned that the available radio spectrum is divided into frequency channels (using FDMA) and that each of these frequency channels is further divided into eight time slots (using TDMA). The combination of a specific frequency and a specific time slot creates a . You can think of a physical channel as a physical, recurring "container" or a "pipe" that can carry a small packet of data, called a burst.

However, simply having a container is not enough. The network needs to know what to put inside it. Is it a fragment of a phone conversation? Is it a command to the phone to increase its power? Or is it a network-wide broadcast telling all phones about the cell's identity? The purpose or the type of information carried by a physical channel is defined by a . In essence, while the physical channel is the "pipe," the logical channel is the "content" flowing through that pipe. The same physical channel can carry different logical channels at different times, making the system incredibly flexible and efficient.

The Two Families of Logical Channels

The multitude of tasks a GSM network must perform, from carrying your voice to managing network resources, are handled by mapping them onto different logical channels. These channels are broadly categorized into two main families, distinguished by their fundamental purpose.

Traffic Channels (TCH): The sole purpose of these channels is to carry user-specific information. This is the "payload" of the network, containing either digitized voice from a phone call or circuit-switched data for services like fax or early mobile internet (CSD).
Control Channels (CCH): This diverse family of channels carries all the signaling and control information necessary for the network to function. They are the nervous system of GSM, handling everything from call setup and location updates to power control and synchronization. They do not carry user voice or data.

Detailed Exploration: Traffic Channels (TCH)

The Traffic Channels are the reason the network exists: to transport the user's conversation or data. GSM defines two main types of TCH to balance between voice quality and network capacity.

A. Full-Rate Traffic Channel (TCH/F)

The TCH/F is the standard channel for a single voice call or data connection. It uses one entire time slot in every TDMA frame to transmit its information. A full-rate channel provides a raw data rate of $22.8 \text{ kbit/s}$ . This raw stream includes not only the digitized voice but also a significant amount of error-correction coding to protect the signal from radio interference. After the error correction overhead is accounted for, the net data rate available for the actual voice information is $13 \text{ kbit/s}$ . This rate was determined to be sufficient to provide "toll-quality" voice, meaning a quality comparable to a traditional landline call. For data, the TCH/F can support rates up to $9.6 \text{ kbit/s}$ .

B. Half-Rate Traffic Channel (TCH/H)

As GSM networks became more popular, operators needed a way to increase the number of simultaneous calls a cell could handle. The solution was the half-rate channel. The TCH/H uses a clever trick: it allows two different users to share a single time slot. This is done by having each user transmit only in every second TDMA frame. For example, User A would use the slot in frames 1, 3, 5, etc., while User B would use the same slot in frames 2, 4, 6, etc. This effectively doubles the voice capacity of a cell. The trade-off is a lower data rate per user, about $11.4 \text{ kbit/s}$ raw, resulting in a net voice rate of around $5.6 \text{ kbit/s}$ . This requires more advanced voice compression algorithms, and while the voice quality is generally acceptable, it is slightly lower than that of a full-rate channel.

Detailed Exploration: Control Channels (CCH)

Control channels are the unsung heroes of the GSM network. They manage the complex dance of signaling required for the network to operate. They are further divided into three sub-categories based on their function and how they are used.

I. Broadcast Channels (BCH) - One-to-Many Communication

Broadcast channels are like a town's public announcement system. They are unidirectional, transmitted only from the Base Transceiver Station (BTS) downwards to all Mobile Stations (MS) in the cell (downlink). A phone in idle mode is constantly listening to these channels to stay informed about the network. The BCH family consists of three key channels:

Broadcast Control Channel (BCCH): The BCCH is the main bulletin board for the cell. It continuously transmits a stream of system information that a phone needs to operate correctly. This vital information includes:
- The network and cell identity codes (PLMN, LAI, CI).
- The list of frequencies used by neighboring cells, which is essential for preparing for handovers.
- The specific rules for accessing the network in this cell, such as which control channels are available and the parameters for the Random Access Channel (RACH).
Frequency Correction Channel (FCCH): The FCCH acts as a precise tuning fork for the mobile phone. It transmits a special Frequency Correction Burst, which consists of a long sequence of all zeros. This creates a pure, unmodulated sine wave at the exact carrier frequency of the cell. By listening to the FCCH, a phone can calibrate its internal frequency synthesizer, ensuring that it is perfectly tuned to the network's frequency and can reliably receive other channels.
Synchronization Channel (SCH): Right after a phone has corrected its frequency using the FCCH, it listens for the SCH. This channel transmits a Synchronization Burst containing a long, predefined training sequence. By successfully decoding this sequence, the phone can achieve exact timing synchronization with the network's TDMA frame structure. The SCH also carries the Base Station Identity Code (BSIC) and the current TDMA frame number, allowing the phone to know precisely "when" it is in the network's time cycle and which BTS it is communicating with.

II. Common Control Channels (CCCH) - Shared Public Pathways

The CCCH group includes channels that are used for the initial contact and call setup procedures. They are a shared resource, accessible by all mobile phones in the cell that are not currently in a call. They serve as the public lobby of the communication system.

Paging Channel (PCH) (Downlink): When you receive an incoming call or text message, the network needs to find your phone. Since your phone is in idle mode to save battery, the network uses the PCH to "page" it. It broadcasts a message containing your phone's temporary identity, alerting it to wake up and respond.
Random Access Channel (RACH) (Uplink): This is the first channel your phone uses to contact the network. When you decide to make a call or when you need to respond to a page, your phone sends a very short Access Burst on the RACH. It's called "random access" because there is no coordination; multiple phones might try to use the RACH at the same instant. The system is designed to handle the resulting collisions and allow phones to try again.
Access Grant Channel (AGCH) (Downlink): After the network successfully receives your request on the RACH, it needs to move you to a private line to continue the call setup. The network sends a message on the AGCH, assigning your phone a temporary, dedicated control channel (an SDCCH) and telling it which frequency and time slot to use.

III. Dedicated Control Channels (DCCH) - Private Communication Lines

Once the initial contact is made via the CCCH, the network assigns a DCCH to a single mobile phone for a specific task. These are dedicated, point-to-point channels not shared with other users.

Standalone Dedicated Control Channel (SDCCH): The SDCCH is a private line assigned to a phone for the duration of the call setup process, after the initial AGCH message. All subsequent signaling, such as sending the dialed phone number, user authentication, and ciphering key setup, occurs on this channel. It is also used for services that don't require a full traffic channel, most notably sending and receiving SMS messages.
Slow Associated Control Channel (SACCH): This channel is always "associated" with a TCH or an SDCCH. It is a slow, continuous stream of control information that is exchanged during a call or an SDCCH session. The SACCH is used to transmit essential maintenance information, such as commands for power control and reports on the signal strength of neighboring cells, which are crucial for handover decisions. It is called "slow" because it transmits its data much less frequently than the TCH it is paired with.
Fast Associated Control Channel (FACCH): The FACCH is an "emergency" channel used for urgent signaling during an active call. The SACCH is too slow for time-critical commands, like the final instruction for a phone to switch to a new cell during a handover. To send such a command, the network temporarily "steals" a Traffic Channel burst that would normally be used for voice. It sets the "stealing flags" within the Normal Burst to indicate to the receiving phone that the data payload contains not voice, but a high-priority FACCH message.