SIGNALING SISTEM

CHAPTER 6Message Transfer Part - MTP

GENERALAplikasi pertama dari Signaling System No 7 yaitu mengatur panggilan dalam jaringan telepon biasa, PSTN.SS No 7 melakukan fungsi sinyal yang sama dengan sinyal sistem konvensional tapi dengan teknik yang lebih cocok untuk sistem digital dan pertukaran SPC.Beberapa tahun setelah versi pertama dari sinyalsistem disajikan oleh CCITT pada tahun 1980, beberapa negara mulai menggunakan sistem dalam jaringan telepon nasional.Saat ini, banyak negara menggunakan SS 7, tidak hanya di jaringan telepon tetapi juga dalam ISDN, Intelligent Networks, JaringanPonsel ,dll.GENERALUntuk sinyal dalam jaringan telepon, PSTN, dua bagian dari sistem sinyal yang diperlukan.The Message Transfer Part (MTP), yang merupakan sistem transportasi umum untuk semua jenis aplikasi telekomunikasi, diperlukan untuk mentransfer pesan antar telepon (Signalling Poin).The Telephone User Part (TUP), bagian dari sistem sinyal,yang menciptakan sinyal telepon di yang berasal dari penerima dan sinyalMESSAGE TRANSFER PART - MTP

MESSAGE TRANSFER PART - MTPPesan Transfer Control System dibagi menjadi dua bagian,Signalling link Fungsi (Level 2) dan Jaringan Signalling Fungsi (Level 3)

Signalling Link FunctionsFungsi link mengawasi Signalling Signalling Data Link untuk kerusakan pesan dan kontrol bahwa pesan yang dikirim dan diterima dengan benar berurutan dan tanpa kehilangan atau duplikasiSignalling Network FunctionsFungsi Jaringan Signalling berfungsi untuk menanganani Pesan (penanganan lalu lintas) dan untuk Manajemen Jaringan SignallingSignalling Message HandlingPenanganan Pesan berisi fungsi untuk routing pesan sesuai link dan untuk mendistribusikan pesan yang diterima dalam pengiriman untuk pengguna yang benar.

Signalling Network ManagementDalam hal perubahan status dalam jaringan sinyal, Misalnya jika link sinyal atau titik sinyal tidak tersedia, Manajemen Jaringan Signalling berfungsi untuk mengaontrol ulang konfigurasi dan tindakan lain untuk mempertahankan atau mengembalikan transfer pesan yang normal kemampuanFungsi link Signalling terdiri dari:-Sinyal Unit pembatasan dan keselarasan -Error Detection-Error Correction-awal keselarasan-prosesor outage-Tingkat 2 aliran kontrol-Kemacetan indikasi ke Level 3 -Signalling Link kesalahan pemantauanTerimaksihSignalling Data Link (Level 1)A Signalling Data Link is a bi-directional transmission path for signalling, comprising two data channels operating together in opposite directions and at the same transmission rate.

A Signalling Data Link can be either digital or analogue. A digital Signalling Data Link is mode up of digital transmission channels (64 kb /s) and digital switches. An analogue Signalling Data Link is made up of voice-frequency analogue transmission channels (4 kHz) and modems.The Level 1 protocol defines the physical, electrical and functional characteristics of a Signalling Data Link. These characteristics are described in more detail in the CCITT Recommendations G.703, G.732 and G.734.The purpose of the Signalling Link Function is to ensure that messages are delivered to the far end correctly, in correct sequence and without loss or duplication. The purpose is also to perform initial alignment of the look and to supervise the performance of the link.

Signalling Link (Level 2)The Signalling Link Functions, together with the Signalling Data Link as transmission medium and with the Signalling Terminal as transmission/reception controller, provide a Signalling Link for reliable transfer of signalling messages between two directly connected Signalling Points.

Signalling Link Control Functions GeneralThe signalling information is assembled into a Message Signal Unit (MSU), which may be of varying length depending on the amount of information to be transferred. The MSU comprises a number of control fields in addition to the signalling information (SIF).

The Length Indicator (LI) is used to distinguish between MSU, LSSU (Link Status Signal Unit) and FISU (Fill In Signal Unit).

Signal Unit DelimitationThe beginning and the end of a signal unit are indicated by a unique 8-bit pattern.

To ensure that the pattern cannot be imitated elsewhere in the signal unit, bit-stuffing is used.

Bit-stuffing means to add an extra zero after five consecutive ones in the message. These extra zeros and deleted in the signalling terminal at the receiving end.The closing flag of a signal unit is normally also the opening flag of the subsequent signal unit.Signal Unit AlignmentSignal unit alignment is performed by the delimitation procedure described above.Loss of alignment occurs when a bit pattern not allowed by the delimitation procedure is received, or when a certain maximum length of a signal unit is exceeded.Note that the Length Indicator (LI) is not used to determine the length of a signal unit.Error DetectionThe error detection function is performed by means of 16 check bits (CK) provided at the end of each signal unit. The check bits (check sum) are generated by the transmitting signalling terminal by operating on the preceding bits of the signal unit following a specified algorithm.At the receiving signalling terminal the same algorithm is used to calculate a check sum. This check sum is then compared with the received check sum.If the two check sums are not equal, then the presence of errors is indicated and the signal unit is discarded.Error CorrectionThe Error Correction field contains 16 bits and consists of the Forward and Backward Sequence Numbers as well as the Forward and Backward Indicator Bits

Basic Error Correction MethodIn this method, a signal unit which has been transmitted is retained in the Retransmission Buffer until a positive acknowledgment is received from the receiving end.

If the message is received correctly, the receiving signalling terminal sends a POSITIVE ACKNOWLEDGMENT by inserting the sequence number of the received message (FSN) as a Backward Sequence Number (BSN), either in an ordinary Message Signal Unit ( MSU), a Fill -In Signal Unit (FISU) or a Link Status Signal Unit (LSSU).The Backward Indicator Bit (BIB) is set equal to the Forward Indicator Bit (FIB) in the received message.On receiving the positive acknowledgment, the originating signalling terminal removes the message from the retransmission buffer.If the received message is incorrect, the receiving signalling terminal will send a NEGATIVE ACKNOWLEDGMENT by inverting the Backward Indicator Bit.Basic Error Correction with RepetitionThis method has a few additions to the basic error correction method:Each MSU is transmitted twice and in consecutive order.Each MSU has its own opening and closing flags to ensure that the repeated MSU is not lost due to corruption of a single flag.Preventive Cyclic Retransmission MethodA signal unit, which has been transmitted, it retained in the retransmission buffer until a positive acknowledgment for that signal unit is received.During a period, when there are no new signal units to transmit, all the signal units which have not yet been positively acknowledged are retransmitted cyclically.The "forced retransmission procedure "is initiated when a predetermined number of retained, unacknowledged signal units exist. The transmission of new signal units is interrupted and the retained signal units are retransmitted cyclically until the number of unacknowledged signal units is reducedInitial AlignmentThe Initial Alignment procedure is appropriate to both first time initialization (for example after "switch on") and alignment in association with restoration after a link failure.The procedure is based on the compelled interchange of Link Status Signal Units (LSSU) between the two signalling points concerned and the provision of a proving period.In the Status Field (SF), the three least significant bits are used to mark the status of the signalling link.The approximate proving time for the two alignment procedures ("Normal" and "Emergency") are:Pn = 216 octets -->8.2s(64 kb/s)Pe = 212 octets -->0.5s(64 kb/s)Processor OutageProcessor Usage refers to a situation when signalling messages cannot be transferred to function levels 3 and/or 4. This may be because of, for example, a central processor failure. It may also be due to inhibiting of an individual signalling link.When Signalling Link Control identifies a local Processor Outage condition it begins to continuously transmit LSSUs with Status Indication Processor Outage (SIPO) and discards received MSUs.

Flow Control Level 2Flow control is initiated when congestion is detected at the receiving end of the signalling link. The congested receiving end of the link notifies the remote transmitting end of the condition by means of an LSSU; Status Indication Busy (SIB), and it withholds acknowledgments of all incoming Message Signal Units.When congestion decreases, acknowledgments of all incoming MSUs is resumed. While congestion exists, the remote transmitting end is periodically notified of this condition. The remote transmitting end will indicate the link as failed if the congestion continues too long.

Congestion Indications to Level 3The congestion levels in the Transmission Buffer and the Retransmission Buffer are monitored by Signalling Link Control to provide congestion indications to level 3.Signalling Link Error MonitoringTo make sure that the performance of a signalling link is adequate to meet the signalling service requirements, for example that the proportion of signal units received incorrectly is acceptable, the operation of each link is monitored by means of two monitors.Signal Unit Error Rate Monitor (SUERM)