Introduction To Telecom

Editor's Notes

• #21: 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
• #28: 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 .
• #30: 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 "embedded operator commands" 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).
• #31: 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.
• #32: 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.
• #33: 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.
• #34: 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.
• #71: 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
• #72: 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
• #73: 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
• #74: 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 )
• #76: 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.