Channel and Bands Concept in GSM

Channel and Bands Concept in GSM : Today, in this article, we will discuss what is the concept of channel and bands in GSM. Many people working in the GSM field know how many frequency bands are used for BTS, but some people are not aware of radio frequency carriers. With the help of this article, I will try to tell you about the channel and the bands in a very easy way.

In simple language, we can say that the channel serves to carry information, GSM uses a lot of channel in which data or information is carried. GSM channel is divided into two parts according to the quality or function of the channel, which we will discuss further. Now I am going to discuss about the frequency bands in GSM.

Complete Channel concept in GSM

Channel and Bands Concept in GSM

Frequency bands in GSM

GSM works on the below given frequency bands, with frequencies of up-link and down-link. I hope that there is no need to write here about Up-link and down-link frequency. 

Frequency bands in GSM with Up-link and Down-link

GSM Frequency bands

Carrier separation is considered 200 kHz, which provides different pairs of carriers. Now I am going to mentioned about that different pairs of carriers.

➤124 pairs of carriers in the GSM 900 band
➤374 pairs of carriers in the GSM 1800 band
➤299 pairs of carriers in the GSM 1900 band.

In the Time Division Multiple Access (TDMA) each  carriers are divided into eight Time Slots (TS). Single Time Slot on a TDMA frame is called a physical channel, i.e. there are eight physical channels on each duplex pair of carriers. 

Lots of  information is transmitted between the BTS and the MS. Logical channels are determined by the data present in the physical channel. It means these all information is grouped into different logical channels.

Here I would like to mentioned that each logical channel is used for a specific purpose like as paging, call set-up and speech. In this way, Channel divided into two parts which we will discuss further.

GSM Logical Channel

The logical channel can be divided into two parts. Traffic channels and signaling channels. Traffic channels (TCHs) further divided into two parts-

1. Full rate TCH (TCH/F) : Full rate channel (TCH/F) carries information at the rate of approx 22.8 kbit/s or we can say that TCH / F is called full rate traffic channel. It is used to carry full rate speech. TCH / F carries a 22.8 kbps data rate.

2. Half rate TCH (TCH/H) : Half rate channel (TCH/H) carries information at rate of approx 11.4 kbit/s or we can say in other words TCH / H is called half rate speech channel. It is used to carry half rate speech. The main purpose of TCH / H is to support two calls in only one GSM channel. TCH / H supports two calls in one channel due to which the capacity of the system is doubled. It carries an 11.4 kbps data rate.

Traffic channels (TCH) are used to carry encoded speech and user data. Traffic channels are defined by a group of 26 TDMA frames.

GSM Signaling channel

Signalling channels are used for set-up the calls, paging, maintaining calls and synchronization etc. Signaling channel divided into three parts-

1. Broadcast Channel (BCH)
2. Common Control Channel (CCCH)
3. Dedicated Control Channel (DCCH)

1. Broadcast Channel (BCH) : Broadcast channel carry only down-link information and their main function is to synchronize and correct the frequency. Broadcast channel also divided into three parts-

(A) Broadcast Control Channel (BCCH)
(B) Frequency Correction Channel (FCCH)
(C) Synchronization CHannel (SCH)

(A) Broadcast Control Channel (BCCH) : BCCH is used to send the indentity of the network through the base station in GSM network. This identity is used by mobile station (MS) to access the network.

This identity contains information such as mobile network code (MNC), local area code (LAC), access parameters etc.

MS receive information concerning the cell in order to start roaming, waiting for calls to arrive or making calls. This information is broadcast on the Broadcast Control Channel (BCCH) and information includes the Location Area Identity (LAI). You have to memorize that BCCH is transmitted on the down-link only.

(B) Frequency Correction Channel (FCCH) : FCCH is used to correct the frequency of mobile station (MS) and for synchronization. BCCH used for two purposes. First to conform that this is the BCCH carrier, and second for the MS frequency synchronization. FCCH is transmitted downlink only.

(C) Synchronization CHannel (SCH) : SCH provides information to the mobile station (MS). With the help of this information, mobile station (MS) searches the base station. 

Within particular cell the MS needs to synchronize to the time-structure, and also ensure that the chosen BTS is a GSM base station. MS receives information about the frame number in the cell and about BSIC of the chosen BTS. BSIC can be decoded only when the base station is of GSM network. SCH is transmitted down-link only.

2. Common Control Channel (CCCH) : CCCH is a group of uplink and downlink channels between MS and BTS. These channels are used to carry information from the network to the MS and to provide access to the network. Common Control Channel also divided into three parts-

(A) Paging Channel (PCH)
(B) Random Access Channel (RACH)
(C) Access Grant Channel (AGCH)

(A) Paging Channel (PCH) : PCH informs incoming calls to mobile station (MS). That is, whenever someone calls a mobile phone, PCH alerts by sending information to the mobile phone.

MS listens to PCH to check at certain time intervals the network wants to make contact with MS. The reason why the network wants to contact MS can be one incoming call or an incoming short message.

PCH information is a message that contains an MS Identification Number (IMSI) or a temporary number (TMSI). PCH is transmitted down-link only.

(B) Random Access Channel (RACH) : This is the reverse link channel. It is used by the subscriber to acknowledge PCH. And RACH is used by mobiles to organize calls.

In Random Access Channel MS listens to PCH to determine when it is being paged out. When MS is paged, it requests to RACH for Signaling channel. RACH can also be used if MS wants contact the network. For example, when setting up mobile origin of the call. Only up-link is transmitted to the RACH.

(C) Access Grant Channel (AGCH) : The network provides a signaling channel (stand-alone) Dedicated Control Channel (SDCCH) to MS. This assignment is done at AGCH. AGCH sent Down-link only. In other words we can say that AGCH is used to provide forward link communication to the mobile phone through the base station and to carry the data through a particular dedicated control channel.

3. Dedicated Control Channel (DCCH) : DCCH is responsible for roaming, handovers, encryption etc. Dedicated Control Channel also divided into following categories-

(A) Stand alone Dedicated Control Channel (SDCCH)
(B) Slow Associated Control Channel (SACCH)
(C) Fast Associated Control Channel (FACCH)
(D) Cell Broadcast Channel (CBCH)

(A) Stand alone Dedicated Control Channel (SDCCH) : SDCCH is used to setup the call. The MS and the BTS switches over to the particular SDCCH.  On the SDCCH the call set-up procedure is performed, as well as the textual message transmission also in the in idle mode.When the call is setup, MS instructs for the switch to TCH.

(B) Slow Associated Control Channel (SACCH) : SACCH is used to control signals connected to traffic channels. in the uplink, The averaged measurements send by MS to its own BTS  and neighboring BTSs . in the downlink, information received by MS from the concerning the transmitting power to use and instructions on the timing advance. SACCH is transmitted both uplink and downlink.

(C) Fast Associated Control Channel (FACCH) : FACCH is used for control requirements such as: - handoff and handovers. For the handover required the FACCH. 

FACCH function in stealing mode it means one 20 ms segment of speech is exchanged for signaling information necessary for the handover. Under normal circumstances the client does not have speech interruptions as the speech coder repeats the previous speech block.

(D) Cell Broadcast Channel (CBCH) : CBCH is used in downlink to carry  only short message service Cell Broadcast (SMSCB) and uses the same physical channel as SDCCH.

Channel Combinations

Only a few combinations of logical channels are allowed as per GSM recommendations. I would like me mentioned below which logical channels can be combined on to Basic Physical Channels (BPC).

(i) TCH/F + FACCH/F + SACCH/TF
(ii) TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)
(iii) TCH/H(0) + FACCH/H(0) + SACCH/TH(0) + TCH/H(1)
(iv) FCCH + SCH + BCCH + CCCH
(v) FCCH + SCH + BCCH + CCCH + SDCCH/4(0...3) + SACCH/C4(0...3)
(vi) BCCH + CCCH
(vii) SDCCH/8(0...7) + SACCH/C8(0...7)
(viii) TCH/F + FACCH/F + SACCH/M
(ix) TCH/F + SACCH/M
(x) TCH/FD + SACCH/MD
Where CCCH = PCH + AGCH + RACH

SACCH / T means that SACCH is associated with TCH SACCH / C is connected with a control channel.

Last Word

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