Slide 8 – Global System for Mobile Communication (GSM)

GSM Architecture

Base Station Subsystem(BSS) BSS terdiri dari dua buah perangkat : Base Transceiver Station (BTS) BTS merupakan tranceiver yang mendefinisikan sebuah sel dan menangani hubungan link radio dengan MS. BTS terdiri dari perangkat pemancar dan penerima, seperti antenna dan pemroses sinyal untuk sebuah interface. Base Station Controller (BSC) BSC mengatur sumber radio untuk sebuah BTS atau lebih. BSC menangani radio-channel setup, frequency hopping, and handover intern BSC.

Network Sub-system (NSS) NSS terdiri dari : Mobile Switching Center (MSC). Home Location Register (HLR). Visitor Location Register (VLR). Authentication Center (AuC). Equipment Identity Register (EIR).

Mobile Switching Center (MSC) Melakukan fungsi switching dasar Sebagai penghubung antara satu jaringan GSM dengan jaringan lainnya melalui Internetworking Function (IWF)

Authentication Center (AuC) Berisi parameter authentikasi pelanggan untuk mengakses jaringan GSM. AuC mekanisme authentication.

Home Location Register (HLR) HLR berisi rekaman database permanen dari pelanggandan merupakan database user yang utama. HLR juga berisi rekaman lengkap lokasi terkini dari user.

Visitor Location Register (VLR) VLR berisi database smentara dari pelanggan, digunakan untuk pelanggan lokal dan yang sedang melakukan roaming. VLR memiliki pertukaran data yang luas daripada HLR dan diakses oleh MSC untuk setiap panggilan, dan MSC dihubungkan dengan VLR. Setiap MSC terhubung dengan sebuah VLR, tetapi satu VLR dapat terhubung dengan beberapa MSC.

Equipment Identity Register (EIR) EIR merupakan register penyimpan data seluruh mobile stations EIR berisi IMEIs (international Mobile Equipment Identities), yang merupakan nomor seri perangkat + tipe code tertentu.

Operation Sub-system (OSS) Operation and Maintenance Jaringan. Pengaturan pelanggan dan tagihan. Pengaturan Mobile Equipment.

Kanal Pada GSM (1) Struktur kanal pada GSM, terbagi menjadi dua, yaitu kanal fisik dan kanal logik. Kanal fisik berhubungan dengan kanal frekuensi radio dan time slot, sedangkan kanal logik berhubungan dengan informasi dan kontrol data pensinyalan. Seperti yang telah dibahas pada pertemuan sebelumnya, bahwa teknik multiplexing GSM menggunakan TDMA & FDMA.

Kanal Pada GSM (2) FDMA membagi kanal menjadi 125 kanal, dimana 124 kanal merupakan kanal komunikasi, dan 1 kanal untuk keperluan pensinyalan. Tiap kanal pada GSM memiliki lebar 200kHz, dimana setiap kanal dibagi lagi menjadi 8 time slot, dengan setiap slot memiliki durasi 576,9 µs. Sehingga 8 slot memiliki durasi 4,615 ms.

MS (Mobile Station) States (1) The mobile station does not answer paging messages, therefore there is no contact between the MS and the network. The network does not get any information on where the MS is. This state is considered as MS detached. When the MS is turned on – or MS attached – it can be in two states : MS Idle & MS Active.

MS (Mobile Station) States (2) ~ MS Idle While moving around - roaming - the mobile station listens to the ”best” cell for paging messages. The MS decides by itself which cell is the best one. This is done by comparing own cell to neighboring cells. If a neighboring cell is considered better, the MS will change cells and, if necessary, inform the system about the new Location Area (LA). This is called location updating. In idle state the MS can receive short messages or cell broadcast messages.

MS (Mobile Station) States (3) ~ MS Active MS is considered active (or busy) when there is a call (speech, fax or data), or a call set up procedure, going on. In this state the MS does not decide by itself if it is necessary to change cell. Based on measurements provided by the MS and the BTS, the BSC makes the decision on change of cell. The decision making is considered as locating, while the actual change of cells is considered as handover. In active state the MS can at the same time receive short messages, but not cell broadcast messages.

Handover Between Cell, Controlled by The Same BSC 1. BSC orders to the new BTS to activate a TCH (Traffic Channel). 2. BSC sends a message to the MS via the old BTS containing information about the frequency and time slot to change to, and also what output power to use. This information is sent over FACCH (Fast Associated Control Channel). 3. MS tunes to the new frequency, and transmits Handover (HO) access bursts in the correct time slot. 4. When the new BTS detects the HO bursts it sends information about timing advance. This is sent on FACCH. 5. MS sends a Handover Complete message to BSC via new BTS. 6. BSC tells the old BTS to release the old TCH.

Handover Between Cell : The Different BSC, Same MSC/VLR (1) 1. The serving (old) BSC sends a Handover required message to the MSC together with the identity of the target cell. 2. MSC knows which BSC that controls this BTS and sends a Handover request to this BSC. 3. New BSC orders target BTS to activate a TCH. 4. New BSC sends a message to the MS via MSC, old BSC and old BTS containing information about the frequency and time slot to change to, and what output power to use. This information is sent over FACCH (Fast Associated Control Channel). 5. MS tunes to the new frequency, and transmits Handover (HO) access bursts in the correct time slot. 6. When the new BTS detects the HO bursts it sends information about timing advance. This is sent on FACCH. 7. MS sends Handover Complete message to MSC via new BSC. 8. The MSC sends an order to old BSC previously to release the old TCH. 9. The old BSC tells the old BTS to release the previously used TCH.

Handover Between Cell : The Different MSC/VLR (1) 1. The serving (old) BSC sends a Handover required message to the serving MSC, MSC–A, together with the identity of the target cell. 2. MSC–A realizes that this cell belongs to another MSC, MSC–B, and asks for help. 3. MSC–B allocates a handover number in order to reroute the call. A Handover request is then sent to the new BSC. 4. New BSC orders target BTS to activate a TCH. 5. MSC–B receives the information, and passes it on to MSC–A together with the handover number. 6. A link is set up to MSC–B, possibly via PSTN. 7. MSC–A sends a HO command to MS, via old BSC containing information on which frequency and time slot to use, and what output power to use. This information is sent over FACCH (Fast Associated Control Channel). 8. MS tunes to the new frequency, and transmits Handover (HO) access bursts in the correct time slot. The HO bursts are transmitted on FACCH. 9. When the new BTS detects the HO bursts it sends information about timing advance. This is sent on FACCH. 10. MS sends Handover Complete message to old MSC via new BSC and new MSC/VLR. 11. A new path in the group switch in MSC–A is established, and the call is switched through.