UK Trunking Network Primer. by hybrid Written for F41th and 9x April 2001 ----------------------------------- The Local Distribution Network (Local Loop) =========================================== At the lowest level in the UK PSTN hierarchy, customers Terminal Equipment (TE) is connected to the Local Loop, which physicaly comprises of cables and copper. The Local Distribution Network provides the TE with access to the Local Exchange either via a standrard copper connection, where the connection to the exchange is analogue, and then conversion from the exchange to the PSTN is digital, or via a direct digital link (FAS Flexible Access Systems) from the TE to the Local Exchange. A Logical Local Distribution Network ==================================== P <--> +----------+ +-------+ Exch Side +-------+ S <--> | Local | +------>| PCP |<---------->| SCP | T <--> | Exchange | | ###| |############+-o-o-o-+ N <--> | MDF | | # +-o-o-o-+ Dist Side | | | +-------+--+ | # | | | | | | CC | | # | | | | | |<------+ # | | | +-------+ # | | +---------+ ############################## | +---------+ | +--------------+ | | | | | | | | +------R+D------+ +------F+T-+ +-O+D------+ | | | | | | | | | | | | | | +--+-+ +--+-+ +--+-+ +--+-+ +--+-+ +--+-+ +--+-+ | TE | | TE | | TE | | TE | | TE | | TE | | TE | +----+ +----+ +----+ +----+ +----+ +----+ +----+ RD = Radial Distribution (Underground Cables) FT = Frontage Tee Distribution OD = Overhead Distribution CC = Cable Chamber PCP = Primary Cross - Connect Point SCP = Secondary Cross - Connect Point MDF = Main Distribution Frame ### = Exchange Side/Distribution Side Barrier UK PSTN Node Hierarchy ====================== The UK PSTN is a complex array of network nodes that operate on a partent-child hirarchy, operating right from the top of the chain (International Exchange Boundarys) down to RCUs and RSSs to customers TE. BT implement a particular system known as System X, which will be covered later in this file. A basic layout of the UK PSN is as follows: +---------------+ | International | | Gateway | +---+---+-------+ DDSSC = Digital Derived | | Services Switching | +-------------------+ Centre | | +---+---+ +---+---+ +---------+ --> D | DMSU O<------------->O DMSU O<-------->O| DDSSC | --> D | | | | | | --> S +-+---+-+ +-+---+-+ +---------+ --> N | | | | | | +---------+ | DMSU = Digital Main | | | | Swithing Unit | | | | (System X) | +-------------------+ | DLE = Digital Local | | | | Exchange (System X | | | | or AXE10) | +---------+ | | RCU = Remote Concentrator | | | | Unit (System X) +-+---+-+ +-+---+-+ RSS = Remote Subscriber | DLE | | DLE | Switch (AXE10) | O<--------+ +-->O | +---+-+-+ | | +---+-+-+ | | | | | | | | | | | +-------------+ | | | | +------------+ | | | +-o-o-----------+ | | | | | Operator | | | | | | Service Centre| | | | | +---------------+ | | | +---------+ +------------+ | | | | +--+ +---+---+ | +---+---+ | | RSS | | | RCU | | | | | | | | +---+---+ | +---+---+ | | | | | | | | | \ TEs / \ TEs / The International Gateway ========================= The International Gateway, or ISC (International Switching Centre) is a switching centre that will handle international traffic to and from other countries. In the UK There are 5 ISCs that implement AXE10, 5ESS and DMS100 switches. As shown in the diagram above, the ISCs are connected to the inland PSN via System X DMSUs. The UK Trunk Network ==================== ISCs are connected to DMSUs (Digital Main Switching Units) which form the backbone of the UK trunking network. There are approximatly 59+ DMSUs in the UK PSTN which are all fully interconnected and switch/route traffic to and from System X catchment areas, along with around 4 Digital Switching Units (DSUs) which route traffic from London to ISCs via partially interconnected links. The DMSUs are interconnected to the "smaller" nodes in the Local network, forming the connection from the national switched PSN to the Local Level. Aswell as being interconnected to the ISCs, the DMSUs are also connected with the DDSN (Digital Derived Services Network) which provides BT LinkLine services, such as 0800s. The UK Local Network ==================== At the top part of the Local Network are the DLSUs (Digital Local Switching Units) which from the local level in the System X network. A zone will normaly contain more than 2 DLSUs (System X) and/or AXE10 Exchanges. From the 2+ Exchanges in the zone, 1 of them is used as a DCCE (Digital Cell Centre Exchange) or a DLE (Digital Local Exchange). So a DLSU will either be a DLE or a DCCE coupled with a non System X (AXE10 Exchange), Otherwise known as System Y. The DCCE has access to its parent (the DMSU) via its DDF (Digital Distribution Frame). Any other Exchange on this network level, such as AXE10 must also implement the DCCEs Digital Distribution Frame as a means of access to and from the parent DMSU. The DCCE performs variopus functions at a local level, including call charging and Tandem Traffic Switching between other DLSUs on the local network level. The DLSU acts as a parent node for the next type of local exchange, the RCUs (Remote Concentrator Units). In cases where older ALEs (Analouge Local Exchanges) have been replaced with RCUs a DLE will replace the DCCE and and act as a parent for the collection of replaced ALEs forming an RCC (Remote Concentrator Centre). When there are more than one Concentrators in an RCC, this will form an RMCC (Remote Multi Concentrator Unit) Because older ALEs such as TXE4 have no direct interconnection with the main DMSUs, the DCCE will provide them with the function to communicate with the DMSU. It is possible to determine your own Local Exchange type from your TE (Terminal Equipment(Phone)) via Testing the Exchange for Offered Services: System X: A System X type is the only Digital Exchange to offer "Charge Advise Per Call" *40*[DN]#. Also "Regular Reminder Call" *56*[hh/mm]*9# so if your Excahnge excepts these 2 services, it is most likely System X. System Y: "Charge Advice Per Call" this time, *40#[DN] will determine the System Y Exchange Type. If neither of these work, there is a possiblilty that your exchange type is UXD5(Non C7 or Enhanced). In some high call density zones in London, a DJSU (Digital Junction Switching Unit) to handle the high concentration. The TE Customers are connected to a RCU/RSS/ALE via the Local Loop, and the RCU is connected to the DLSU via digital links. The opposite of this is Low density rural areas whereas UXD5 is used. ######TRUNKING#LEVEL################################# ... <------>ISCs<------><----------->DDSSCs-----> ... ... <------>DMSU<------->DMSU<------>DMSU<------> ... | | | | | | ######LOCAL#LEVEL#################################### | | | DLSU DLSU DLSU<---> Other DMSU DCCE DLE DCCE | | | RCU RCU RCU (or RSS AXE10) ALE RCU ALE PCM === Pulse Code Modulation forms the backbone of the UK digital network, including direct links to and from digital Exchanges. One of the main functions of PCM is to convert reproduced analouge voice signals into a digitaly pulsed form, which can then be transmitted over the UK digital Trunking Network. The first part of the PCM proccess is to sample the origional transmission into PAM (Pulse Amplitude Modulation) Signals. This is done by sampling the voice pattern at fast intervals (8kHz on the UK network) into real-time PAM signals which are then encoded into the PCM signal. In order for the PAM pulses to be encoded, they must be treated as strings/samples, and then injected with a standard 8 bit binary string. When the 8 bit binary strings arive at the other side, ie: the destination of the decoder on the network, the binary samples can be decoded back into origional form, where the reconstruction of the PAM signals takes place. Now, because of the time intervals in the sampling rate of conversion, this will leave Spaces for other signals to be injected into the transmission, thus multiplexing occurs. This is more commonly known as TDM (Time Division Multiplexing), where each sample will take a Timeslot. In PCM the samples are converted at a rate of 8kHz, thus creating 32 Timeslots. BT implements a 30 CHANNEL Timeslot system, in which the remnaining 2 CHANNELS are used for Network Syncronisation (TIMESLOT 0) and Network Signalling (TIMESLOT 16), ((Loop Disconnect (LD) type signalling)). During each frame two sets of four signalling bits, (a,b,c,d) are allocated to carry signalling information for 2 traffic channels. Frame Signalling Codes: ======================= a b c d -------------------------------------------------------------------- 1 1 1 1 Circuit Idle Circuit Busy 0 0 1 1 Circuit Seized Answer 1 0 1 1 Dial Break Not Used 0 1 1 1 Not Used Circuit Free 0 0 0 1 Operator Override Not Used (The Operator Override code is a forward signal used to allow an operator to monitor a busy line) 1 0 0 1 Not Used Not Used 1 1 0 1 Not Used Not Used 0 1 0 1 Not Used Not Used 0 0 0 0 Not Used Not Used -------------------------------------------------------------------- +-------+ |TS 0 |< Network | |< Synchronisation +-------+<-----+ |TS 1 | | +-------+ | |TS 2 | | +-------+ | |TS 3 | +- Speech Channels | . | | 1 to 15 | . | | |TS 14 | | +-------+ | |TS 15 | | +-------+<-----+ |TS 16 |< Network | |< Signalling + Allignment Signals +-------+<-----+ |TS 17 | | +-------+ | |TS 18 | | | . | | | . | +- Speech Channels |TS 29 | | 16 to 30 +-------+ | |TS 30 | | CAS (Channel Associated Signalling) +-------+ | =================================== |TS 31 | | +-------+<-----+ Timeslot 0 is injected with allignment signals in the transmission path of the PCM signal, the PCM decoder will then be able to know if the synchronisation rate is correct. The following is a diagram of a PCM encoder/decoder circuit which is implemented in this proccess: PCM CIRCUIT =========== +<-- Ch (4) +---+ | | |-------------->+|<-- Ch (3) +---+ || Low Pass Filters || +---+ || | |------------->+||<-- Ch (2) Receive End ---------> +---+ ||| +--> Ch (16) Ch (1) ||| | +---+ || ||| +-----------------------+ +|--> No Channel #-#-->| |-||--->+||+->O | || Connected AUDIO +---+ || ||+-->O | +||--> Ch (15) |+--->O | ||| ------------------+---->O<----------+<-TS (0) O<----+|| Reconstructed +------->O | TS (16)->O<-----+| PAM Samples |+------>O | O<------+ || +-----------|-----------+ ---------------+| Ch (1)----->| Recognised Signals No Channel | Ch (3)----->|<---- Ch (2) Connected | |<---- Ch (4) | |<---------------------+ ----------------+ +-----------|-----------+ | C Ch (30) | | | o | DECODER | | m | | | m +-------------|---------+ | o . .+........................... . +---+ B P | n . Ch | 1 | i A | . +---+ n M | P . Ch | 2 | a | a Line +---+ r S | t System Ch | 3 | y i | h . +---+ g | . Ch | 4 | n <----------+ F . +---+ a | o . | | l | r . .+........................... . +---+ s | +-------------|---------+ | A | | | l | ENCODER | | l | | | +-----------|-----------+ | C Ch (1)----->| PAM Samples | h Ch (3)----->|<---- Ch (2) | a +<-- No Channel |<---- Ch (4) | n | Connected |<---------------------+ n | +-----------|-----------+ e | Ch (30) ---------->O | | l +---------------------->O | | s +--------------->O<----------+<-- TS (0) | | +--->O O<------+ | |+-->O Channel Sampling O<-----+|<-- TS (16) =A====== ||+->O Switch O<----+|| ==U===== ||| +-----------------------+ ||| ===D==== ||| ||| ====I=== ||| Transmit ||| =====O== ||| End ||| ======|= ||+-> Ch (4) ||| | |+--> Ch (3) Ch (16) <----||+ | +---> Ch (2) No Ch <----|+ +---------------> Ch (1) Ch (15) <----+ Timeslot 16 and CCS (Common Channel Signalling) =============================================== As with CAS, in CCS Timeslot 16 is used for network signalling information such as binary sieze codes etc. In CCS, Timeslot 16 is a 64/kbit channel, (8 bits x 8 kHz), as are the other channels. Digital Exchanges implement CCS for signalling between various other Exchanges, and is based on CCITT7. In System X, the 30 Channel PCM system forms the link between the customer and the Exchange switch (DS3), and from the Excahnge switch to Junction Access. At the Concentrator Unit in the RCC, subscribers lines are concentrated into PCM Channels, upto 2048 lines consentrated to the 8 x 30 PCM channels in each unit. DSSS Subscriber Line Units at the Concentrator: Sub 0 +-------+ Speech +-------+ ------->O O<------------->O L C | | SLU 0 | | i o | ------->O O<------------->O n n | Sun 7 +-------+ Signals | e t | +---------------+ | r | 32TS's| Concentrator | | o O<----->O Switch | | l | | | | l | | | Sub 24 +-------+ Speech | e | +---------------+ ------->O O<------------->O r O<--+ | SLU 3 | | | | ------->O O<------------->O | | Sub 31 +-------+ Signals +-------+ +--> Concentrator Control If the Remote Concentrator becomes Isolated from the Exchange, ie: unable to communicate with the Exchange, Subscribers are still able to set up calls, although calls made in this period of isolation will not be charged due to the charging equipment in the Exchange dependancy on the concentrator. Review ====== IAM Initial Address Message IFAM Initial and Final Address Message ISC International Switching Centre |--> Handle traffic to and from other countries |--> 5 ISCs in the UK: |--> Keybridge | |--> AXE10 |--> Kelvin | |--> AXE10 |--> Mondial | |--> 5ESS |--> Madley(A) | |--> AXE10 |--> Madley(B) | |--> DMS100 |--> Connected to Inland PSTN via DMSUs ISDN Intergrated Services Digital Network |--> ISDN lines tested from dedicated OMC line test system ISUP ISDN User Part IUP Interconnect User Part MTP Message Transfer Part PNO Public Network Operator PSTN Public Switched Telephony Network SCCP Signalling Connection Control Part TUP Telephony User Part ETSI European Technical Standards Institute NCRS Non Circuit Related Signalling GSM Global Systems for Mobile DSU Digital Switching Unit |--> DSUs are found in London |--> Trunk and Network traffic switched to London Boundrys |--> DSUs are only connected to DMSUs in London DMSU Digital Main Switching Unit |--> Parent of DLSU |--> Fully Integrated Trunk Network |--> Switch Traffic to and from Catchment Area |--> System X |--> Manufactured by GEC/Plessey (GPT) DLSU Digital Local Switching Unit |--> Digital Local Processor Exchange |--> Either a DCCE (+) DLE |--> System X (+) AXE10 DCCE Digital Cell Centre Exchange |--> Acts as parent for RCUs |--> Acts as parent for Analouge Exchanges | |--> Terminates Analouge Line plants | |--> Executes call charging |--> Switches Tandem traffic between DLSUs |--> Access to DMSU via DDF (Digital Distribution Frame) DJSU Digital Junction Switching Unit |--> Used in High Density Telecom Areas |--> Tandem Switching Functions |--> No Direct Customer Connections |--> Only in London |--> System X Exchange Type DSSS Digital Subscribers Switching Subsystem |--> Refered to as "DS3" or "D triple S" |--> Consists of Several Concentrators |--> P-Switch (Part of DSSS) DDSN Digital Derived Services Network |--> Connected to the PSTN |--> Provides LinkLine Services 0800/0345 etc |--> Controled by DDSSCs | (Digital Derived Services Switching Centre) | |--> which in turn are controled by: | |--> INDB (Intelligent Network Database) |--> Connects Service Providers to PSTN via DMSUs ALSU Analogue Local Switching Unit |--> Analogue Switched Functions to Customers | |--> Strowger (TXS) | |--> Crossbar (TXK1) | |--> Electronic (TXE2, TXE4) |--> TXE4 system enhanced to implement digital |--> CCS (Common Channel Signalling) ALE Analouge Local Exchange |--> No direct DMSU Links |--> Traffic Forwarded to DMSUs via DCCE DLE Digital Local Exchange |--> A DLSU that hosts RCU on a hierarchial relationship |--> Parent of ALEs replaced by RCUs RCU Remote Concentrator Unit |--> Part of the DLSU via connection |--> System X Concentrator | |--> Concentates large number of lines | |--> Lesser number of PCM channels |--> Maximum number of Terminated lines: 2048 (0-2047) |--> MUST be connected to DLSU RSS Remote Subscriber Switch |--> Same as RCU, but AXE10 RCC Remote Concentrator Centre |--> Multiple RCUs (+) RSSs at one location |--> (RMCC) Remote Multi Concentrator Unit UXD5 CDSS1 (Monarch PABX) Derived Digital Exchange |--> Some depraved Rural areas (being phased out) 5ESS Used in DDSN Network and International Gateway Exchanges DMS Used in International Gateway Exchanges and BT Featurenet AXE10 System Y |--> IOG (Input Output Group) | |--> SPS (Support Processor Subsystem) | |--> MCS (Man-Machine Communication Subsystems | | |--> Alarm Panels, Terminals etc | |--> DCS (Data Communications Subsystem) | | |--> Communications over digital links | | | |--> OMC | | |--> Same as AUS/NIS in System X | |--> FMS (File Management Subsystem) |--> APT | |--> SSS (Subscriber Switching Subsystem) | | |--> Same Concentrator function as DSSS | | |--> LSS Local Subscriber Switch | | |--> RSS Remote Subscriber Switch | |--> TSS (Trunk and Signalling Subsystem) | | |--> Handles connections to other nodes | |--> GSS (Group Switching Subsystem) | | |--> Connects speech paths like DSS | |--> CCS (Common Channel Subsystem CCITT7) |--> APZ |--> RPS (Regional Processing Subsystem) | |--> Scan subscriber lines | |--> Operation of Switches |--> CPS (Central Processing Subsystem) |--> 2 Central Processors (like PUS) TE Terminating Equipment |--> Terminate Customer Line, isolate Exchange |--> Terminating point for each junction OMC Operations and Maintenance Centre |--> Serves Partitioned Digital Exhanges |--> Data collected from Exchages and served to users |--> OMU |--> Traffic OMU Operations and Maintenance Unit |--> Office with engineers monitoring Exchanges | |--> Recieving data from the OMC |--> Multiple OMUs connected to 1 OMC |--> Connected to UXD5 Exchanges via stand-alone terminal |--> Terminals directly connected to the OMC | |--> Control over Exchanges connected to the OMC |--> Bottom of hierarchy |--> System Manager |--> UM (Operations and Maintenance Unit Manager) |--> MCO (Maintenance Control Officer) | |--> TCD (Task Co-ordinating Duty) |--> MO (Maintenance Officer) |--> MOA (Maintenance Officer A) |--> MOB (Maintenance Officer B) DSG District Support Group |--> Bellow PSG |--> BT District Staff PSG Product Support Group |--> Bellow ISOS |--> BT Headquaters ISOS In Service Operational Support |--> Top of hierarchy |--> Manufactures EIR Exchange Incident Report database SLU Subscriber Line Unit TNS Test Network Subsystem LCP Local Control Point GPA General Purpose Test Auxiliaries |--> Monitor |--> Looping and Ring Trip P-Switch Part of DSSS. Analouge switch for concentrating physical test access from the subscriber line to the: Q-Switch Second Switching Stage for subscriber access |--> built from 8x8 2 wire reed switch SIUs R-Switch Access to short handeling time access auxilaries |--> built from 3x3 2 wire reed switch SIUs D-Switch Connects the user into the Test Network |--> Access gained via dedicated DIAL BACK circuit MTN Mini Test Network |--> Test subscriber lines and equipment TAC Test Access Connection PLA Per Line Auxiliarys MDF Main Distribution Frame TOS Temporarily Out Of Service |--> Will still give dialtone when siezed | |--> only 999 and 151 calls can be made |--> To confirm line has been TOS'd make test call to TOS'd line and listen to message, or from the TOS'd line call an un-charged number and await NU tone FT Failure Tone |--> 600Hz at 260Ms on, 260Ms off, 1400Hz at 260Ms off) CPI Cable Pair Identification PCP Primary Cross connect Point |--> SCP (Secondary Cross connect Point) FAS Flexible Access System TRS Transmission Repeater Station PSN Public Switched Network OSC Operator Services Centre VPN Virtual Private Network |--> BT Featurenet PC Private Circuit Network ACE Automatic Cross-connection Equipment PDN Public Data Network |--> PSS (Packet Switch Stream) OLO Other Licenced Operator VAN Value Added Network |--> BT Cellnet PCM Pulse Code Modulation |--> CAS (Channel Associated Signalling) |--> CCS (Common Channel Signalling) PAM Pulse Amplitude Modulation TDM Time Division Multiplexing MML Man Machine Language PUS Processor Utility Subsystem (System X main Processor) DSS Digital Switching Subsystem (Main System X Switch) MTS Message Transmission System (C7 Messages) SIS Signalling Internetworking Subsystem (Analouge) AUS Access Utility Subsystem (System X) NIS Network Interworking Subsystem (OMC Access and Admin) COU Central Operations Unit |--> National Operational functions are performed here |--> Network Traffic Managment NOU Network Operations Unit |--> UK Network is divided into 9 zones | |--> Scotland | |--> North East | |--> North West | |--> N.Ireland | |--> Midlands | |--> Wales and West | |--> Southern Home Counties | |--> Northern Home Counties | |--> London |--> Remote Manipulation of the Network |--> Circuit Provisioning |--> Control of work between NOU and NFU boundarys NFU Network Field Unit |--> Physical Network access (Engineers etc) NACC Network Administration Computer Center |--> 11 in NOU Zones around the UK |--> 1 in Martlesham for emergency fallback |--> NOMS (Network Operations Managment Systems) |--> CSS (Customer Services System) CTU BT Circuit Termination Unit |--> located within the building housing the Operator |--> G703 interface on two 75 ohm coaxial cables direct to either the Operator Switch or an Operator Digital Distribution Frame (DDF) which is co-located with the Operator Switch. ---------------------- http://hybrid.dtmf.org