The Base Transceiver Station BTS

The BTS (see Figure 21-12) houses the radio transceivers that define a cell and handles the radio link protocols with the MS. In a large urban area, a large number of BTSs may be deployed. The requirements for a BTS are

• Ruggedness

• Reliability

• Portability

Figure 21-12: The BTS

The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1 and 16 transceivers, depending on the density of users in the cell. Each BTS serves a single cell. It also includes the following functions:

Figure 21-12: The BTS

The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1 and 16 transceivers, depending on the density of users in the cell. Each BTS serves a single cell. It also includes the following functions:

• Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna

• Transcoding and rate adaptation

• Time and frequency synchronizing

• Voice through full- or half-rate services

• Decoding, decrypting, and equalizing received signals

• Random access detection

• Timing advances

• Uplink channel measurements

The Base Station Controller (BSC)

The BSC manages the radio resources for one or more BTSs. It handles radio channel setup, frequency hopping, and handovers. The BSC is the connection between the mobile and the MSC. The BSC also translates the 13 Kbps voice channel used over the radio link to the standard 64 Kbps channel used by the Public Switched Telephone Network (PSDN) or ISDN. The BSC is between the BTS and the MSC and provides radio resource management for the cells under its control. It assigns and releases frequencies and time slots for the MS. The BSC also handles intercell handover. It controls the power transmission of the BSS and MS in its area. The function of the BSC is to allocate the necessary time slots between the BTS and the MSC. It is a switching device that handles the radio resources. Additional functions include

• Control of frequency hopping

• Performing traffic concentration to reduce the number of lines from the MSC

• Providing an interface to the Operations and Maintenance Center for the BSS

• Reallocation of frequencies among BTSs

• Time and frequency synchronization

• Power management

• Time-delay measurements of received signals from the MS

The BSS is composed of two parts: the BTS and the BSC. These communicate across the specified Abis interface, enabling (as in the rest of the system) operations between components that are made by different suppliers. The radio components of a BSS may consist of four to seven or nine cells. A BSS may have one or more base stations. The BSS uses the Abis interface between the BTS and the BSC. A separate high-speed line (T1 or E1) is then connected from the BSS to the Mobile central office (CO), as shown in the architecture in Figure 21-13.

o Combinalion of BTS plus BSC

° Uses Abis Interface

Figure 21-13: The BSS

o Combinalion of BTS plus BSC

° Uses Abis Interface

Figure 21-13: The BSS

The TRAU

Depending on the costs of transmission facilities from a cellular operator, it may be cost efficient to have the transcoder either at the BTS, BSC, or MSC. If the transcoder is located at the MSC, it is functionally still a part of the BSS. This creates maximum flexibility of the overall network operation. The transcoder takes the 13 Kbps speech or data (at 300, 600, and 1,200 bps) multiplexes 4 of them, and places them on a standard 64 Kbps digital PCM channel. First, the 13 Kbps voice is brought up to a 16 Kbps level by inserting additional synchronizing data to make up the difference of the lower data rate. Then, four 16 Kbps channels are multiplexed onto a DS0 (64 Kbps) channel.

Locating the TRAU

If the transcoder/rate adapter is outside the BTS, the Abis interface can only operate at 16 Kbps within the BSS. The TRAU output data rate is 64 Kbps standard digital channel capacity. Next, 30 of the 64 Kbps channels are multiplexed onto a 2.048 Mbps E1 service if the CEPT standards are used. The E1 can carry up to 120 traffic and control signals. The locations can be between the BTS and the BSC, whereby a 16 Kbps subchannel is used between the BTS and the TRAU, and 64 Kbps channels between the TRAU and the BSC. Alternatively, the TRAU can be located between the BSC and the MSC, as shown in Figure 21-14, using 16 Kbps between the BTS and the BSC and 64 Kbps between the BSC and the TRAU.

13 Kbps

Figure 21-14: The TRAU

13 Kbps

Figure 21-14: The TRAU

The central component of the Network Subsystem is the MSC, which is shown in Figure 21-15. It acts like a normal Class 5 CO in the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. The primary functions of the MSC include

• Coordination of call setup for all MSs in its operating area

• Dynamic allocation of resources

• Location registration

• Interworking functions

• Handover management

• Reallocation of frequencies to BTSs

• Encryption

• Echo cancellation

• Signaling exchange

• Synchronizing the BSS

El or Better

Figure 21-15: The MSC

As a CO function, it uses the digital trunks in the form of E1 (or larger) to the other network interfaces such as

• Public Land Mobile Network (PLMN)

These services are provided in conjunction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the public-fixed network (PSTN or ISDN), and signaling between functional entities uses Signaling System Number 7 (SS7), which is used in ISDN and is widely used in current public networks.

The Gateway Mobile Services Switching Center (GMSC) is used in the PLMN. A gateway is a node interconnecting two networks. The GMSC is the interface between the mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC. A PLMN may have many MSCs, but it has only one gateway access to the wireline network to accommodate the network operator. The gateway then is the high-speed trunking machine connected via E1 or Synchronous Digital Hierarchy (SDH) to the outside world.

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Responses

  • florencia
    How and from where bsc allocates the time slots between bts and msc?
    7 months ago

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