NSS, MSC, dan MSS

Network Switching Subsystem

Network Switching Subsystem, or NSS, is the component of a GSM system that carries out switching functions and manages the communications between mobile phones and the Public Switched Telephone Network. It is owned and deployed by mobile phone operators and allows mobile phones to communicate with each other and telephones in the wider telecommunications network. The architecture closely resembles a telephone exchange, but there are additional functions which are needed because the phones are not fixed in one location. Each of these functions handle different aspects of mobility management and are described in more detail below.

The Network Switching Subsystem, also referred to as the GSM core network, usually refers to the circuit-switched core network, used for traditional GSM services such as voice calls, SMS, and Circuit Switched Data calls.

There is also an overlay architecture on the GSM core network to provide packet-switched data services and is known as the GPRS core network. This allows mobile phones to have access to services such as WAP, MMS, and Internet access.

All mobile phones manufactured today have both circuit and packet based services, so most operators have a GPRS network in addition to the standard GSM core network.

Mobile Switching Center (MSC)

The Mobile Switching Center or MSC is the primary service delivery node for GSM, responsible for handling voice calls and SMS as well as other services (such as conference calls, FAX and circuit switched data). The MSC sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring.

In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s timeslot, known as a DS0 in America).

There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.

A Gateway MSC is the MSC that determines which visited MSC the subscriber who is being called is currently located. It also interfaces with the Public Switched Telephone Network. All mobile to mobile calls and PSTN to mobile calls are routed through a GMSC. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufacturers design dedicated high capacity MSCs which do not have any BSSes connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.

The Visited MSC is the MSC where a customer is currently located. The VLR associated with this MSC will have the subscriber's data in it.

The Anchor MSC is the MSC from which a handover has been initiated. The Target MSC is the MSC toward which a Handover should take place. An MSC Server is a part of the redesigned MSC concept starting from 3GPP Release 5.

Mobile Switching Centre Server (MSS)

The Mobile Switching Centre Server or MSC Server is a soft switch variant of Mobile Switching Centre, which provides circuit-switched calling, mobility management, and GSM services to the mobile phones roaming within the area that it serves. MSC Server functionality enables split between control (signalling) and user plane (bearer in network element called as Media Gateway), which guarantees more optimal placement of network elements within the network.

MSC Server and MGW Media Gateway makes it possible to cross-connect circuit switched calls switched by using IP, ATM AAL2 as well as TDM.

More information is available in 3GPP TS 23.205.

Sumber : http://en.wikipedia.org/wiki/Network_Switching_Subsystem

BSS, BTS, dan BSC

Base Station Subsystem (BSS)

The Base Station Subsystem (BSS) is the section of a traditional cellular telephone network which is responsible for handling traffic and signaling between a mobile phone and the Network Switching Subsystem. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging, quality management of transmission and reception over the Air interface and many other tasks related to the radio network.

Base Transceiver Station

The Base Transceiver Station, or BTS, contains the equipment for transmitting and receiving of radio signals (transceivers), antennas, and equipment for encrypting and decrypting communications with the Base Station Controller (BSC). Typically a BTS for anything other than a picocell will have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations). A BTS is controlled by a parent BSC via the Base Station Control Function (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an Operations and Maintenance (O&M) connection to the Network Management System (NMS), and manages operational states of each TRX, as well as software handling and alarm collection.

The functions of a BTS vary depending on the cellular technology used and the cellular telephone provider. There are vendors in which the BTS is a plain transceiver which receives information from the MS (Mobile Station) through the Um (Air Interface) and then converts it to a TDM ("PCM") based interface, the Abis, and sends it towards the BSC. There are vendors which build their BTSs so the information is preprocessed, target cell lists are generated and even intracell handover (HO) can be fully handled. The advantage in this case is less load on the expensive Abis interface.

The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE-enabled networks it is GMSK and 8-PSK.

Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found in micro and pico cells only.

Frequency hopping is often used to increase overall BTS performance; this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, and the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets.

A TRX transmits and receives according to the GSM standards, which specify eight TDMA timeslots per radio frequency. A TRX may lose some of this capacity as some information is required to be broadcast to handsets in the area that the BTS serves. This information allows the handsets to identify the network and gain access to it. This signalling makes use of a channel known as the BCCH (Broadcast Control Channel).

Sectorisation

By using directional antennas on a base station, each pointing in different directions, it is possible to sectorise the base station so that several different cells are served from the same location. Typically these directional antennas have a beamwidth of 65 to 85 degrees. This increases the traffic capacity of the base station (each frequency can carry eight voice channels) whilst not greatly increasing the interference caused to neighboring cells (in any given direction, only a small number of frequencies are being broadcast). Typically two antennas are used per sector, at spacing of ten or more wavelengths apart. This allows the operator to overcome the effects of fading due to physical phenomena such as multipath reception. Some amplification of the received signal as it leaves the antenna is often used to preserve the balance between uplink and downlink signal.

Base Station Controller

The Base Station Controller (BSC) provides, classically, the intelligence behind the BTSs. Typically a BSC has 10s or even 100s of BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, controls handovers from BTS to BTS (except in the case of an inter-BSC handover in which case control is in part the responsibility of the Anchor MSC). A key function of the BSC is to act as a concentrator where many different low capacity connections to BTSs (with relatively low utilisation) become reduced to a smaller number of connections towards the Mobile Switching Center (MSC) (with a high level of utilisation). Overall, this means that networks are often structured to have many BSCs distributed into regions near their BTSs which are then connected to large centralised MSC sites.

The BSC is undoubtedly the most robust element in the

BSS as it is not only a BTS controller but, f

or some vendors, a full switching center, as well as an SS7 node with connections to the MSC and SGSN (when using GPRS). It also provides all the required data to the Operation Support Subsystem (OSS) as well as to the performance measuring centers.

A BSC is often based on a distributed computing architecture, with redundancy applied to critical functional units to ensure availability in the event of fault conditions. Redundancy often extends beyond the BSC equipment itself and is commonly used in the power supplies and in the transmission equipment providing th

e A-ter interface to PCU.

The databases for all the sites, including information such

as carrier frequencies, frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are stored in the BSC. This data is obtained directly from radio planning engineering which involves modellin

g of the signal propagation as well as traffic projections.

Transcoder

Although the Transcoding (compressing/decompressing) function is as standard defined as a BSC function, there are several ven dors which have implemented the solution in a stand-alone rack using a proprietary interface. This subsystem is also referred to as the TRAU (Transcoder and Rate Adaptation Unit). The transcoding function converts the voice channel coding between the GSM (Regular Pulse Excited-Long Term Prediction, also known as RPE-L PC) coder and the CCITT standard PCM (G.711 A-law or u-law). Since the PCM coding is 64 kbit/s and the GSM coding is 13 kbit/s, this also involves a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks, to compress voice channels from the 64 kbit/s PCM standard to the 13 kbit/s rate used on the air interface. Som e networks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its Rate Adaptation Unit function to give compatibility between the BSS data rates and the MSC capability.

However, at least in Siemens' and Nokia's architecture, the Transcoder is an identifiable separate sub-system which will normally be co-located with the M

SC. In some of Ericsson's systems it is

integrated to the MSC rather than the BSC. The reason for these designs is that if the compression of voice channels is done at the site of the MSC, fixed transmission link costs can be reduced.

Packet Control Unit

The Packet Control Unit (PCU) is a l ate addition to the GSM standard. It performs some of the processing tasks of the BSC, but for packet data. The allocation of channels between voice and data is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes full control over that channel.

The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site.

BSS interfaces

• Um – The air interface between the MS (Mobile Station) and the BTS. This interface uses LAPDm protocol for signaling, to cond uct call control, measurement reporting, Handover, Power control, Authentication, Authorization, Locati on Update and so on. Traffic and Signaling are sent in bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.

  

• Abis – The interface between the Base Transceiver Station and Base Station Controller. Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM subchannels for traffic (TCH), LAPD protocol for BTS supervision and telecom signaling, and carries synchronization from the BSC to the BTS and MS.
• A – The interface between the BSC and Mobile Switching Center. It is used for carrying Traffic channels and the BSSAP user part of the SS7 stack. Although there are usually transcoding units between BSC and MSC, the signaling communication takes place between these two ending points and the transcoder unit doesn't touch the SS7 information, only the voice or CS data are transcoded or rate adapted.
• Ater – The interface between the Base Station Controller and Transcoder. It is a proprietary interface whose name depends on the vendor (for example Ater by Nokia), it carries the A interface information from the BSC leaving it untouched.
• Gb – Connects the BSS to the Serving GPRS Support Node (SGSN) in the GPRS Core Network.

Sumber : http://en.wikipedia.org/wiki/Base_Station_Subsystem

MENGENAL GSM

Komunikasi bergerak (mobile communication) mulai dirasakan perlu sejak orang semakin sibuk pergi ke sana kemari dan memerlukan alat telekomunikasi yang siap dipakai sewaktu-waktu di mana saja ia berada. Kebutuhan ini ternyata tidak dibiarkan begitu saja oleh para engineer telekomunikasi. Mereka telah memikirkan standardisasi untuk komunikasi bergerak ini, salah satunya adalah GSM (Global System for Mobile communications)

Alokasi spektrum frekuensi untuk GSM awalnya dilakukan pada tahun 1979. Spektrum ini terdiri atas dua buah sub-band masing-masing sebesar 25MHz, antara 890MHz - 915MHz dan 935MHz - 960MHz. Sebuah sub-band dialokasikan untuk frekuensi uplink dan sub-band yang lain sebagai frekuensi downlink.

Karena konsekuensi logis dari kenaikan redaman atas kenaikan frekuensi, biasanya sub-band terendah dipakai untuk uplink, agar daya yang ditransmisikan oleh MS (mobile system atau lebih dikenal handphone) ke BTS (Base Transmitter Station yaitu seperti sentral telepon di PSTN/POTS, namun memiliki fungsi lebih) tidak perlu besar. Kalau digunakan sub-band yang satu lagi, mungkin anda perlu melakukan recharge batere handphone berulang kali untuk mendapatkan kualitas sama dengan saat ini.

Kemudian kedua sub-band tersebut dibagi lagi menjadi kanal-kanal, sebuah kanal pada satu sub-band memiliki pasangan dengan sebuah kanal pada sub-band yang lain. Tiap sub-band dibagi menjadi 124 kanal, yang kemudian masing-masing diberi nomor yang dikenal sebagai ARFCN (Absolute Radio Frequency Channel Number). Jadi sebuah MS yang dialokasikan pada sebuah ARFCN akan beroperasi pada satu frekuensi untuk mengirim dan satu frekuensi untuk menerima sinyal.

Untuk GSM, jarak antar pasangan dengan ARFCN sama selalu 45MHz, dan bandwidth tiap kanal sebesar 200kHz. Kanal pada tiap awal sub-band digunakan sebagai guard band. Silakan anda hitung, maka spektrum GSM akan menghasilkan 124 ARFCN, masing-masing diberi nomor 1 sampai 124. Kanal sebanyak 124 inilah yang nantinya dibagi-bagi buat operator-operator GSM yang ada di suatu negara.

Untuk mengantisipai perkembangan jaringan di masa mendatang, telah dilokasikan tambahan 10MHz frekuensi pada masing-masing awal sub-band. Ini dikenal sebagai EGSM (Extended GSM). Jadi spektrum EGSM ini 880MHz - 915MHz buat uplink dan 925MHz - 960MHz buat downlink. Hal tersebut memberi tambahan 50 ARFCN menjadi 174. Tambahan ARFCN ini diberi nomor 975 - 1023.

DCS 1800

Seiring dengan evolusi GSM, diputuskan untuk menerapkan teknologi ini pada PCN (Personal Communication Networks). Hal ini membutuhkan perubahan pada interafce udara untuk memodifikasi frekuensi operasinya. Frekuensi modifikasinya antara 1710MHz - 1785MHz untuk uplink dan 1805MHz - 1880MHz untuk downlink. Teknik ini menyediakan 374 ARFCN dengan pemisahan frekuensi sebesar 95MHz antara uplink dan downlink.

Teknik PCN ini dikembangkan di Eropa, khususnya di Inggris. Di Inggris (Raya) ARFCN ini telah dibagi-bagi antara keempat operator jaringan yang ada di sana. Dua di antaranya, Orange dan One to One, beroperasi pada daerah GSM 1800, sementara dua yang lainnya, Vodafone dan Cellnet, telah dialokasikan kanal GSM 1800 pada puncak jaringan GSM 900 mereka. ARFCN ini diberi nomor 512 - 885. Porsi pada puncak band digunakan oleh DECTs (Digital Enhanced Cordless Telephony).

PCS 1900

PCS 1900 merupakan adaptasi GSM yang lain ke dalam band 1900MHz. Teknik ini digunakan di Amerika Serikat di mana FCC (Federal Communication Commission) telah membaginya menjadi 300 ARFCN dan mengumumkan lisensi pada berbagai macam operator untuk mengimplementasikan jaringan GSM. Pemisahan frekuensinya sebesar 80MHz, dan pembagian frekuensinya adalah 1850MHz - 1910MHz untuk uplink dan 1930MHz - 1990MHz untuk downlink.

Teknik Modulasi dan Bandwidth

Teknik modulasi yang digunakan pada GSM adalah GMSK (Gaussian Minimum Shift Keying). Teknik ini bekerja dengan melewatkan data yang akan dimodulasikan melalui Filter Gaussian. Filter ini menghilangkan sinyal-sinyal harmonik dari gelombang pulsa data dan menghasilkan bentuk yang lebih bulat pada ujung-ujungnya. Jika hasil ini diaplikasikan pada modulator fasa, hasil yang didapat adalah bentuk envelope yang termodifikasi (ada sinyal pembawa). Bandwidth envelope ini lebih sempit dibandingkan dengan data yang tidak dilewatkan pada filter gaussian.

Bandwidth yang dialokasikan untuk tiap frekuensi pembawa pada GSM adalah sebesar 200kHz. Pada kenyataannya, bandwidth sinyal tersebut lebih besar dari 200kHz, bahkan setelah dilakukan pemfilteran gaussian pun hal itu tetap terjadi. Akibatnya sinyal akan memasuki kanal-kanal di sebelahnya. Jika pada satu sel (akan dijelaskan kemudian) terdapat BTS dengan frekuensi pembawa yang sama atau bersebelahan kanal, maka akan terjadi interferensi akibat overlapping tersebut. Begitu juga jika sel-sel yang bersebelahan memiliki frekuensi pembawa sama atau berdekatan. Alasan inilah yang menyebabkan mengapa dalam satu sel atau antara sel-sel yang berdekatan tidak boleh menggunakan kanal yang sama atau berdekatan.

Pembagian Sel

Pembagian area dalam kumpulan sel-sel merupakan prinsip penting GSM sebagai sistem telekomunikasi selular. Sel-sel tersebut dimodelkan sebagai bentuk heksagonal seperti pada gambar berikut. Tiap sel mengacu pada satu frekuensi pembawa / kanal / ARFCN tertentu. Pada kenyataannya jumlah kanal yang dialokasikan terbatas, sementara jumlah sel bisa saja berjumlah sangat banyak. Untuk memenuhi hal ini, dilakukan teknik pengulangan frekuensi (frequency re-use). Pada gambar terlihat contoh frequency re-use dengan jumlah kanal 7 buah. Antara sel-sel yang berdekatan frekuensi yang digunakan tidak boleh bersebelahan kanal atau bahkan sama.

Jelas bahwa semakin besar jumlah himpunan kanal, semakin sedikit jumlah kanal tersedia per sel dan oleh karenanya kapasitas sistem menurun. Namun, peningkatan jumlah himpunan kanal menyebabkan jarak antara sel yang berdekatan kanal semakin jauh, dan ini mengurangi resiko terjadi interferensi. Sekali lagi, desain sistem GSM memerlukan kompromi antara kualitas dan kapasitas.

Pada kenyataannya, model satu sel dengan satu kanal transceiver (TRx, tentunya menggunakan antena omni-directional) jarang digunakan. Untuk lebih meningkatkan kapasitas dan kualitas, desainer melakukan teknik sektorisasi. Prinsip dasar sektorisasi ini adalah membagi sel menjadi beberapa bagian (biasanya 3 atau 6 bagian; dikenal dengan sektorisasi 120^o atau 30^o). Tiap bagian ini kemudian menjadi sebuah BTS (Base Transceiver Station). Kebanyakan vendor memperbolehkan sampai dengan 4 TRx per BTS untuk sektorisasi 120^o. Jika digunakan TDMA pada TRx, menghasilkan 8 kanal TDMA tiap TRx, Anda bisa menghitung bahwa dalam satu sel dapat menampung trafik yang setara dengan 3 X 4 X 8 = 96 kanal TDMA atau sebesar 82,42 erlang dengan GoS 2%. (Erlang merupakan satuan trafik dan GoS(Grade of Service) menyatakan derajat keandalan layanan, berapa jumlah blocking yang terjadi terhadap panggilan total)

Pada prakteknya tidak semua kanal TDMA tersebut bisa digunakan untuk kanal pembicaraan (TCH = Traffic Channel). Dalam sebuah BTS juga diperlukan SDCCH (Stand-alone Dedicated Control Channel) yang digunakan untuk call setup dan location updating serta BCCH (Broadcast Control Channel) yang merupakan kanal downlink yang memberikan informasi dari BTS ke MS mengenai jaringan, sel yang kedatangan panggilan, dan sel-sel di sekitarnya.

Struktur Sistem Selular

Bagian paling rendah dari sistem GSM adalah MS (Mobile Station). Bagian ini berada pada tingkat pelanggan dan portable. Pada tiap sel terdapat BTS (Base Transceiver Station). BTS ini fungsinya sebagai stasiun penghubung dengan MS. Jadi, merupakan sistem yang langsung berhubungan dengan handphone Anda.

BTS pada dasarnya hanya merupakan "pesuruh" saja. Otak yang mengatur lalu-lintas trafik di BTS adalah BSC (Base Station Controller). Location Updating, penentuan BTS dan proses handover pada percakapan ditentukan oleh BSC ini. Beberapa BTS pada satu region diatur oleh sebuah BSC.

BSC-BSC ini dihubungkan dengan MSC (Mobile Switching Center). MSC merupakan pusat penyambungan yang mengatur jalur hubungan antar BSC maupun antara BSC dan jenis layanan telekomunikasi lain (PSTN, operator GSM lain, AMPS, dll).Saat ini teknik switching terus berkembang, dan begitu pula pada layanan GSM. Beberapa operator GSM di Indonesia telah menerapkan Intelegent Network lanjutan dalam teknik switchingnya.

Frequency Hopping

Frequency hopping merupakan fitur yang diterapkan pada interface udara, yakni lintasan radio ke MS. Teknik ini dapat mengurangi redaman akibat efek multipath fading. GSM hanya merekomendasikan satu jenis frequency hopping, yakni baseband hopping. Namun beberapa vendor, seperti Motorola, menyediakan tipe frequency hopping yang lain, yang disebut Synthesizer Hopping.

Baseband Hopping digunakan jika base station memiliki beberapa DRCU/TCU tersedia. Aliran data secara sederhana dilalukan pada frekuensi dasar ke berbagai macam DRCU/TCU. Setiap data beroperasi pada frekuensi yang tetap, mengacu pada urutan hopping yang ditentukan. DRCU/TCU yang berbeda akan menerima sebuah timeslot yang spesifik pada setiap frame TDMA, berisi informasi yang ditujukan kepada MS-MS yang berbeda.

Synthesizer Hopping menggunakan kelincahan ferkuensi dari DRCU/TCU untuk mengubah frekuensi-frekuensi pada sebuah basis timeslot untuk transmisi maupun menerima. SCB pada DRCU serta sistem kontrol dan pemrosesan digital pada TCU akan menghitung dan menentukan frekuensi selanjutnya, dan memprogram sebuah pasangan synthesizer Tx dan Rx untuk menuju ke frekuensi yang telah dihitung.

Teknik synthesizer hopping ini sangat baik untuk diterapkan pada sel-sel dengan jumlah carrier yang sedikit. Untuk sel-sel dengan jumlah carrier yang banyak, teknik baseband hopping merupakan teknik yang paling baik. Dan kedua teknik ini tidak bisa diterapkan sekaligus pada sebuah site BTS.

Sumber : http://cahbuton.tripod.com/id16.html