Establishing a basic voice call between two telephony agents is the main call processing task. A basic voice telephone call has the following characteristics: Each of the two telephony agents involved in the call can be either a line or a trunk. No custom calling features are active during the call. The functional steps required to process this call can be expressed in general terms as follows : Detecting the incoming call, that is, detecting a line origination or the seizure of an incoming trunk Receiving the digits, that is, determining what type of signaling the originating agent is using, and collecting the digits Translating the digits, that is, analyzing them to determine the call destination Selecting a terminating agent, that is, finding the best available route for the call Establishing the telephony connection, that is, setting up a speech path between the originating and terminating agents Signaling the terminating agent: if the terminator is a line, apply ringing and give audible ring-back tone to the originator if the terminator is a trunk, out-pulse the digits Detecting an answer by detecting an answer signal from the terminating agent recording the answer time in the billing information for the call, if a billing record is required Detecting disconnect by detecting a disconnect signal from either the originator or the terminator recording the disconnect time in the billing information for the call, if a billing record is required taking down the telephony connection idling the originating and terminating agents
In the U.S., the telephone company will be providing its BRI customers with a U interface . The U interface is a two-wire (single pair) interface from the phone switch. It supports full-duplex data transfer over a single pair of wires, therefore only a single device can be connected to a U interface. This device is called an Network Termination 1 (NT-1). The situation is different elsewhere in the world, where the phone company is allowed to supply the NT-1, and thereby the customer is given an S/T interface. The NT-1 is a relatively simple device that converts the 2-wire U interface into the 4-wire S/T interface. The S/T interface supports multiple devices (up to 7 devices can be placed on the S/T bus) because, while it is still a full-duplex interface, there is now a pair of wires for receive data, and another for transmit data. Today, many devices have NT-1s built into their design. This has the advantage of making the devices less expensive and easier to install, but often reduces flexibility by preventing additional devices from being connected. Technically, ISDN devices must go through an Network Termination 2 (NT-2) device, which converts the T interface into the S interface (Note: the S and T interfaces are electrically equivalent). Virtually all ISDN devices include an NT-2 in their design. The NT-2 communicates with terminal equipment, and handles the Layer 2 and 3 ISDN protocols. Devices most commonly expect either a U interface connection (these have a built-in NT-1), or an S/T interface connection. Devices that connect to the S/T (or S) interface include ISDN capable telephones and FAX machines, video teleconferencing equipment, bridge/routers, and terminal adapters. All devices that are designed for ISDN are designated Terminal Equipment 1 (TE1). All other communication devices that are not ISDN capable, but have a POTS telephone interface (also called the R interface), including ordinary analog telephones, FAX machines, and modems, are designated Terminal Equipment 2 (TE2). A Terminal Adapters ( TA ) connects a TE2 to an ISDN S/T bus. Going one step in the opposite direction takes us inside the telephone switch. Remember that the U interface connects the switch to the customer premises equipment. This local loop connection is called Line Termination (LT function). The connection to other switches within the phone network is called Exchange Termination (ET function). The LT function and the ET function communicate via the V interface .
The ISDN Physical Layer is specified by the ITU I-series and G-series documents. The U interface provided by the Telco for BRI is a 2-wire, 160 kbps digital connection. Echo cancellation is used to reduce noise, and data encoding schemes (2B1Q in North America, 4B3T in Europe) permit this relatively high data rate over ordinary single-pair local loops. 2B1Q (2 Binary 1 Quaternary) is the most common signaling method on U interfaces. This protocol is defined in detail in 1988 ANSI spec T1.601. In summary, 2B1Q provides: two bits per baud, 80 k-baud per second, transfer rate of 160 kbps The input voltage level can be one of 4 distinct levels ( 00 –3 –2.5v, 01 –1 -0.833v, 10 1 0.833v, 11 3 2.5v).(note: 0 Volts is not a valid voltage under this scheme).These levels are called Quaternaries. Each quaternary represents 2 data bits, since there are 4 possible ways to represent 2 bits, as in the table above. Each U interface frame is 240 bits long. At the prescribed data rate of 160 kbps, each frame is therefore 1.5 ms long. Each frame consists of : Frame overhead - 16 kbps, D channel - 16 kbps, 2 B channels at 64 kbps - 128 kbps The Sync field consists of 9 Quaternaries (2 bits each) in the pattern +3 +3 -3 -3 -3 +3 -3 +3 -3. (B1 + B2 + D) is 18 bits of data consisting of 8 bits from the first B channel, 8 bits from the second B channel, and 2 bits of D channel data. The Maintenance field contains CRC information, block error detection flags, and &quot;embedded operator commands&quot; used for loop-back testing without disrupting user data. Data is transmitted in a super-frame consisting of 8 240-bit frames for a total of 1920 bits (240 octets). The sync field of the first frame in the super-frame is inverted (i.e. -3 -3 +3 +3 +3 -3 +3 -3 +3).
Flag (1 octet) - This is always 7E16 (0111 11102) C/R (Command/Response) bit indicates if the frame is a command or a response EA0 / EA1 (Address Extension) bit indicates whether this is the final octet of the address or not TEI (Terminal Endpoint Identifier) 7-bit device identifier (see below) Control (2 octets) - The frame level control field indicates the frame type (Information, Supervisory, or Unnumbered) and sequence numbers (N(r) and N(s)) as required. Information - Layer 3 protocol information and User data CRC (2 octets) - Cyclic Redundancy Check is a low-level test for bit errors on the user data. SAPI : Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point where Layer 2 provides a service to Layer 3. ( TEI ) : Terminal Endpoint Identifiers are unique IDs given to each device (TE) on an ISDN S/T bus. This value may be assigned statically when the TE is installed, or dynamically when activated.
Establishing the Link Layer The Layer 2 establishment process is very similar to the X.25 LAP-B setup, if you are familiar with it. The TE (Terminal Endpoint) and the Network initially exchange Receive Ready (RR) frames, listening for someone to initiate a connection The TE sends an Unnumbered Information (UI) frame with a SAPI of 63 (management procedure, query network) and TEI of 127 (broadcast) The Network assigns an available TEI (in the range 64-126) The TE sends a Set Asynchronous Balanced Mode (SABME) frame with a SAPI of 0 (call control, used to initiate a SETUP) and a TEI of the value assigned by the network The network responds with an Unnumbered Acknowledgement (UA), SAPI=0, TEI=assigned. At this point, the connection is ready for a Layer 3 setup.
The ISDN Network Layer is also specified by the ITU Q-series documents Q.930 through Q.939. Layer 3 is used for the establishment, maintenance, and termination of logical network connections between two devices. Service Profile IDs (SPIDs) They are used to identify what services and features the Telco switch provides to the attached ISDN device. SPIDs are optional; when they are used, they are only accessed at device initialization time, before the call is set up. The format of the SPID is defined in a recommendation document, but it is only rarely followed. It is usually the 10-digit phone number of the ISDN line, plus a prefix and a suffix that are sometimes used to identify features on the line, but in reality it can be whatever the Telco decides it should be. If an ISDN line requires a SPID, but it is not correctly supplied, then Layer 2 initialization will take place, but Layer 3 will not, and the device will not be able to place or accept calls. See ITU spec Q.932 for details. Information Field Structure The Information Field is a variable length field that contains the Q.931 protocol data. Protocol Discriminator (1 octet) - identifies the Layer 3 protocol. If this is a Q.931 header, this value is always 0816. Length (1 octet) - indicates the length of the next field, the CRV. Call Reference Value (CRV) (1 or 2 octets) - used to uniquely identify each call on the user-network interface. This value is assigned at the beginning of a call, and this value becomes available for another call when the call is cleared. Message Type (1 octet) - identifies the message type (i.e., SETUP, CONNECT, etc.). This determines what additional information is required and allowed. Mandatory and Optional Information Elements (variable length) - are options that are set depending on the Message Type.
Layer 3 – Initialization These are the steps that occurs when an ISDN call is established. In the following example, there are three points where messages are sent and received; 1) the Caller, 2) the ISDN Switch, and 3) the Receiver. 1. Caller sends a SETUP to the Switch. 2. If the SETUP is OK, the switch sends a CALL Proceeding to the Caller, and then a SETUP to the Receiver. 3. The Receiver gets the SETUP. If it is OK, then it rings the phone and sends an ALERTING message to the Switch. 4. The Switch forwards the ALERTING message to the Caller. 5. When the receiver answers the call, is sends a CONNECT message to the Switch 6. The Switch forwards the CONNECT message to the Caller. 7. The Caller sends a CONNECT Acknowledge message to the Switch 8. The Switch forwards the CONNECT ACK message to the Receiver. 9. Done. The connection is now up.
Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination necessary works also for analog signals Disadvantages: waste of bandwidth if the traffic is distributed unevenly Inflexible guard spaces
A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users Disadvantages: precise synchronization necessary
A channel gets a certain frequency band for a certain amount of time Advantages: Better protection against tapping Protection against frequency selective interference Higher data rates compared to code multiplex But: precise coordination required
The audio signal of each channel is encoded using a unique pair of psuedo random bit sequences (PRBS). The output is then used to modulate the given carrier and sent over the radio interface. All channels use the same spectrum at the same time Advantages: Bandwidth efficient No coordination and synchronization necessary Good protection against interference and tapping Disadvantages Lower user data rate More complex signal regeneration Implemented using spread spectrum technology, also called Spread Spectrum Multiple Access ( SSMA )
The Network Switching System (NSS) The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units. home location register (HLR)—The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator. mobile services switching center (MSC)—The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others. visitor location register (VLR)—The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time. authentication center (AUC)—A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world. equipment identity register (EIR)—The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node. Base Station Sub-System (BSS) All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs). BSC—The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC. BTS—The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC. Additional Functional Elements Other functional elements shown in Figure 2 are as follows: message center (MXE)—The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, e-mail, and notification. mobile service node (MSN)—The MSN is the node that handles the mobile intelligent network (IN) services. (GMSC)—A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC. GSM interworking unit (GIWU)—The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VLR.
There is a problem with any copper wire transmission system -- Cross-talk . This is caused by the electrical signal in one wire being induced onto an adjacent wire, a problem that is especially prevalent in central offices, where large bundles of wires enter the building. The solution was relatively simple. Experiments showed that balancing out the undesired induced currents by "turning over" or transposing the relative positions of the disturbing and disturbed circuits could solve the inductance problem. Physically, this can be accomplished very easily on wire pairs (two wires) by giving them a twist every few inches. The careful manufacture of twisted-pair wires effectively eliminated this problem. Twisted pair wires are the 22- or 24-gauge subscriber line wires in your house that connect your telephone or computer modem to the telephone central office.
The increase in the cost cabling will be directly proportional to the increase in size of the network. Number of links required for connecting “n” subscribers is given as n!/(2*(n-2)!) The increase in the number of devices at CPE will be directly proportional to the increase in the size of the network. The cost of maintaining such a setup will spiral up as the size of the network increases. 1 2 3 4 5
What is the Solution ? to/from other locations Local Exchange (switch) A switched Network
The Structure of a typical Telephone Network EO EO EO Access tandem TAX TAX Access tandem EO EO EO AT AT AT AT
POTS Access P O T S • • • • Line cards subscriber loop Max : 150 miles RSC RLU RCC T1 / DS1 Line unit RSC Matrix Central Control
Business Access Joe's Small Business Department of Injustice Kathy's home Business Betty's Bigger Business subscriber line subscriber lines lines or trunks IBN (Centrex) lines KTS PBX Centrex Call Processing POTS Call Processing Digital Class 5 Local Office
"Digital end-to-end connectivity through a limited set of network interfaces providing a wide range of service features evolving from the telephone IDN to meet market needs into the 21st century.” - From ITU definition of "ISDN".
Flag (8) Address ( 8/16 ) Control (16) Information ( Layer –3) CRC (16) Flag (8) SAPI ( 6 ) 1 2 3 4 5 6 7 8 Address Field C/R EA0 TEI ( 7 ) EA1
Layer – II - Initialization Receive Ready (RR) frames Unnumbered Information (UI) frame with a SAPI of 63 and TEI of 127 TEI (in the range 64-126) Set Asynchronous Balanced Mode (SABME) frame with a SAPI of 0 and TEI TE Unnumbered Acknowledgement (UA), SAPI=0, TEI=assigned ISDN Network
Layer – III Message Type 1 2 3 4 5 6 7 8 Information Field Length of CRV Protocol Discriminator 0 0 0 0 Call Reference Value (1 or 2 octets) 0 Mandatory & Optional Information Elements (variable)
Digital subscriber line, which operates at a maximum of 144 Kb/s for ISDN subscriber, lines. ISDN is used for voice and data communications. Asymmetric digital subscriber line, which operates at 32Kb/s to 8.19 Mb/s downstream to the customer and 16 to 640 Kb/s upstream to the network over existing twisted-pair copper wire. ADSL is envisioned for use for Internet access, video on demand (VOD), simplex video, remote LAN access, and interactive multimedia. High-bit rate digital subscriber line delivers data symmetrically at rates up to at 1.544 Mb/s full-duplex for equivalent T1/E1 service, or at 2.48 Mb/s duplex (requires two pairs of wire) for subscriber lines. It delivers at 2.49 Mb/s duplex (requires three pairs of wire) for feeder plant, WAN services, LAN access, or server access. Rate adaptive ADSL is a version of ADSL where the ADSL modems test the line at start up and adapts the data rate to within 32 Kb/s of the maximum throughput the line is capable of supporting. Very high-bit-rate asymmetric digital subscriber line, which operates at a subset of speeds of VDSL when it supports symmetric operation. It describes a form of ADSL that does not require a splitter at the customer location to separate voice signals from digital signals in the data stream. This approach leads to a "plug-and-play" ADSL where the user can simply connect the line to a PC and be in service. Universal ADSL will operate at lower bit rates than "existing" ADSL systems, but it is up to 25 times faster than today's 56Kb/s modems and just as easy to install. Very high-bit rate digital subscriber line is under development for twisted-pair access service at 12.9Mb/s to 52.8Mb/s downstream and 1.5 to 2.3Mb/s upstream. However, the maximum reach will be reduced from 4,500 to 1,000 feet and it will need fiber-optics cable. Applications are the same as ADSL, plus high-definition TV.
Inter-register signaling Seize Seize Ack Answer Forward group I signal (called party digit) Forward Group II signal (regular) Forward Group III signal (end of digits) Forward Group III signal (calling party digit) Forward group II signal (regular) Backward group A-1 signal (next digit) Backward Group A-6 signal (req_dn_cat) Backward Group C-1 signal (next ANI digit) Backward group C-1 signal (next ANI digit) Backward group A-3 signal (req_bill_cat) Backward Group B signal (connect_call_chg) Forward group I signal (called party digit)
Overall Architecture of CCS7 Message Transfer Part ( MTP ) ISUP TUP SCCP TCAP DUP 1 - 3 1-3 4-7 4 - 6 7 User Parts OSI Layer Mapping OSI Layer Mapping
CCS7 Network Components Signal Transfer Point (STP) is node in the Network that routes messages between nodes. It does not originate any CCS7 messages other then NM messages Service Control Point(SCP)provides network access to transaction services ( Database queries ) Service Switching Point (SSP) is a node in the network that originates & terminates CCS7 messages ( both connection oriented and connectionless ) SSP A SCP SSP B STP - II STP - I Voice Signaling Point(SP) is a node in the network that provides CCS7 trunk signaling only Quasi Associated Associated Mode SP Trunks
CCS7 Signaling Link-Sets STP STP STP STP SCP SSP SP SSP a a a a e f b b b b c c a a f Access links connect SP, SSP & SCP to STPs Bridge links connect mated STP pairs to other mated STP pairs Cross links connect two STP nodes creating a mated pair Fully Associated links connect SP, SSP & SCP nodes using associated signaling Extended links connect an SP, SSP & SCP to an STP of a different region. Diagonal links connect STP quads in different regions ( for instance primary to secondary STP )
Basic CCS7 ISUP Call Switch X - Originator Switch Y - Terminator IAM SAM ACM ANM REL RLC Talking Line Line
IN Components It is not a physical network but a set of software features packages It enhances switch call processing capabilities to use centralized operating company-provided service logic programs placed at SCP Queries & responses between DMS & SCP use CCS7 protocol . IP Service Creation Environment SMS STP SCP SS7 Network Upload Service Query Response Exchange
CONNECTION IS ESTABLISHED BETWEEN THE PSTN & MOBILE
Need for convergence of Voice and Data Networks
Attractive because of low cost , flat rate pricing of public internet
Optimization of bandwidth utilized in data network when compared to fixed bandwidth in telecom network
Growth in technology faster in data networks than in telecom networks
Demand for new types of integrated voice/data applications
Convergence of Telecom and Data Networks CALL SERVER T1/E1/ J1/T3 ISDN, R1/R2, CAS SS7 Signal & Trunk Access Gateway Fast Ethernet SS7 ATM IP T1/E1/ J1/T3 ISDN, R1/R2, CAS SS7 SS7 Fast Ethernet EO EO PBX PBX Network Management System Signal & Trunk Access Gateway
2000 1850 10 10 12 10 6 Mono-mode fibre 1,7,16 Gbs/s 3600ch M/W 60ch coax First telephone Ist telephone ch multi mode fibre 140 Mbs/s 10800ch over coaxial voice ch ~ 600bps voice ch ~1200 voice ch~4800bps PCM voice ch~56bps Strowger Crossbar Electronic switches Satcom High capacity Radios Bits/s The Telecom story