Introduction to ISDN Technology Tutorial TT 002 Introduction ISDN, which stands for Integrated Services Digital Network, is a system of completely digital telephone/telecommunications connections designed to carry voice, data, images, and video. It is a suite of internationally-adopted standards for end-to-end digital communication over the public telephone network. ISDN is basically the telephone network turned all-digital end-to-end, using existing switches and wiring (for the most part) upgraded so that the basic call is a 64 Kbps end-to-end channel. While ISDN differs fundamentally from conventional telephone service (called Plain Old Telephone Service or POTS), ISDN has been designed to allow end-to-end compatibility for voice services. There are two forms of ISDN, narrowband and broadband, distinguished by transport speed. Most of the this discussion will focus on narrowband applications, however major differences between the two will be noted in the remainder of this introduction Narrowband (1.544 Mbps and Less) Narrowband ISDN works over existing copper telephone wiring to the telephone company. Two primary service types are available. Circuit-switched service is used for either voice or data dial-up applications, which means it is only connected when it is being used. Leaving ISDN connected around the clock can end up being more expensive than a dedicated line or leased line, which has a flat monthly price. Packetswitched service is used for nailed-up (permanent virtual circuit) high speed data applications providing a 64 Kbps X.25 connection. Narrowband ISDN delivers up to a five-fold speed increase over analog communications. An analog speed barrier exists at about 30 Kbps, whereas ISDN can connect at either 64 Kbps or 128 Kbps. Each 64 Kbps channel is a pure bit pipe, with no rate negotiation or handshaking involved (since there are no modem speed or protocol differences to cause conflicts). In fact, because the negotiation phase with ISDN is so simple, ISDN takes only a second or two to dial and establish a connection. (Modems may takes as long as a minute to accomplish the same thing.) Narrowband ISDN utilizes a twisted pair of copper wires from the telephone company to the Customer Premises Installation (CPI). Broadband (Greater than 1.544 Mbps) The two speeds that are most often discussed for broadband ISDN are OC 1 which is 155 Mbps and OC 3 which is 622 Mbps. Speeds of Various Service Types Frame Relay service is a connectionless service, meaning that each data packet passing through the network contains address information. One of the unique facets of frame relay service is that the service supports variable size packets. Frame Relay can be provided with a variety of speeds from 56 Kbps to 25 Mbps, although the most common speeds for the service are 56 Kbps and 1.544 Mbps. Switched Multimegabit Digital Service (SMDS) provides a high speed path for permanent virtual circuits. The transport speed for SMDS is usually 155 Mbps. May 4, 2000 CTI Products, Inc. 1211 W. Sharon Road, Cincinnati, OH 45240 Phone: +1.513.595.5900 Email: info@ctiproducts.com Fax: +1.513.595.5983 Web: www.ctiproducts.com
Asynchronous Transfer Mode (ATM) which is also known as cell relay is considered to be the transport service of the future. The transport speeds of most ATM applications are 155 Mbps with 622 Mbps gaining more popularity. ATM is a flexible service made possible by the size of the packets (cells). The cell size for all applications is 53 bytes (which includes a 5 byte header). The small cell size allows a variety of applications to run on ATM networks including voice, video, and data. Broadband ISDN utilizes fiber optic lines from the Customer Premises Installation to the telephone company. Narrowband ISDN Interface Types There are two types of narrowband ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Basic Rate Interface (BRI) BRI is normally used for connections to small businesses and homes. It uses existing copper lines, and so the maximum bandwidth is 160 Kbps. This 160 Kbps bandwidth is divided into three channels. Two Bearer (B) channels are used for data, with each having 64 Kbps transport speed. (Some older telco equipment can only perform up to 56 Kbps.) Each B channel can carry a separate telephone call and usually has its own telephone number, called a (Directory Number (DN). In addition, a single Delta (D) channel with a transport speed of 16 Kbps is used for in-band signaling to control the attachment of a device to the B channels. Another name for this BRI service with two B channels and one D channel is 2B+D. BRI supports a bus configuration with up to 8 devices on a line. ISDN is intelligent enough to arbitrate the use of the two B channels between these devices (up to two devices can be in use simultaneously) and route incoming calls to the appropriate device. Primary Rate Interface (PRI) PRI is normally used for connections to businesses where requirements exist for multiple 64 Kbps channels or higher transport speeds than 64 Kbps. PRI is typically delivered by fiber optic, coaxial cable, transverse screen cable or Microwave. Although PRI does not directly support a bus configuration, a PBX can reallocate ISDN PRI resources onto multiple BRI buses. Two distinct PRI systems have emerged; one implemented in North America (U.S. and Canada) and Japan, and the other by most of Europe. The number of B channels is the biggest difference due to the size of the trunk lines used; T1 in North America and Japan, E1 in Europe. North America and Japan Since ISDN PRI service provides digital access via a T1 line with a bandwidth of 1.544 Mbps, this bandwidth is divided into 24 64 Kbps channels. This ISDN PRI service uses 23 of the T1 channels to provide B channel access and uses the 24 th channel for signaling purposes (D channel). Europe Since the interstation communication media is an E1 line with a bandwidth of 2 Mbps, this bandwidth is divided into 31 64 Kbps channels. This ISDN PRI service uses 30 of the E1 channels to provide B channel access and uses the 31 st channel for signaling purposes (D channel). Reference Configurations The remainder of this discussion refers to Basic Rate Interface (BRI or 2B+D). You can t talk about ISDN without knowing about the reference configurations. This section gives the basic vocabulary for talking about all of the pieces of ISDN. Figure 1 shows two of the configurations most Page 2 May 4, 2000
commonly referred to. The networks will actually look more complicated than this; the diagram just serves to apply standard labels to the different parts of the network that will be encountered. CUSTOMER PREMISES PHONE COMPANY TE1 S T U V NT2 NT1 LT ET TE2 R TA S NT2 T NT1 U LT V ET Figure 1. Common Reference Configurations The top configuration shows the configuration for a device (TE1) that is compatible with ISDN. The bottom configuration shows the configuration for a non-isdn compatible device (TE2). Device Glossary for Figure 1 TE1 Terminal Equipment type 1. These are communications devices that are designed for ISDN, such as an ISDN telephone, computer, or ISDN FAX. TE2 Terminal Equipment type2. These are communications devices that are not ISDN capable, but have a POTS telephone interface. This type of device includes ordinary analog telephones, FAX machines, and modems. They must be interfaced to the ISDN line using a Terminal Adapter (TA). TA Terminal Adapter. This adapts non-isdn equipment (such as Ethernet interfaces and older TE2 equipment) to connect to the ISDN line s S/T interface. NT1 Network Termination type 1. This is the end of the line for the local phone company, and the beginning of the CPI. In North America and Singapore, this is the responsibility of the customer. In Europe, this is supplied by the phone company. The NT1 translates the bit encoding scheme used on the lines between it and the telephone company (the U interface) to the bipolar encoding used between it and the TEs (the S/T interface). It provides diagnostic functions such as loopback mode, and it may have to provide power for European applications. The output of NT1 is the T interface. It s a good idea to install the NT1 in a permanent fashion. If NT1 is unplugged from the ISDN line (the U interface twisted pair), it shows up as a sign of line trouble in the central office. Some telephone companies respond to this so-called trouble by disabling the ISDN line at the central office, and require you to place a service call to get your ISDN service restored. NT2 Network Termination type 2. This is a device which provides multiple ISDN interfaces on the ISDN line. In most homes, this won t exist. In companies, this would be their internal telephone system or PBX. The output of the NT2 is the S interface. LT Line Termination. This is the physical connection to the phone company. Page 3 May 4, 2000
ET Exchange Termination. This is the local phone company s logical connection from the Terminal Equipment to the phone network. Reference Points for Figure 1 R This is the old POTS telephone interface between an old-style telephone and a Terminal Adapter. S (Subscriber) This is a passive bus (since there are no repeaters on the line between the NT1 and the devices) that uses a second kind of terminator, called an NT2, to allow as many as eight separate TE1s or TAs to share a single 2B+D circuit. This multipoint bus requires a Bus/Stub topology (not star). See S/T interface below for more details. T (Termination) When only a single TE exists on the ISDN line at the CPI, the NT2 is not required, and the S and T interfaces are one and the same, referred to as the S/T interface. See S/T interface below for more details. S/T The S and T interfaces are electrically identical and can be implemented using the same cable and connectors as 10baseT Ethernet (22 gauge Unshielded Twisted Pair cable, and RJ45 connector). Two pairs of wires are used, one each for transmit and receive signals. For a single device (point-to-point) installation, this interface can be 1 km in length. For a multipoint installation, this bus can be 200 m in length, with 10 m stub lengths. The connector used for the 4-wire S/T interface is the RJ45. This is slightly wider than the RJ11 standard analog phone jack, and has 8 conductors. For North America, this style connector is used between the NT1 and the TE1 or TA. For Europe, this style connector is used between the wall jack and the TE1 or TA. U The characteristics of this interface are necessary due to the long local loop lengths (especially in the U.S.). Maximum length of U-Loop is 18000 ft. (about 3.4 miles or 5.5 km). The equipment on both sides of the U loop has to be carefully designed to deal with the long length of the U loop and the noisy environment it operates in. 2B1Q (2 Binary 1 Quaternary) is the most common signaling method on the U interface. It provides: Two bits per baud 80 kbaud per second Transfer rate of 160 Kbps This means that the input voltage level can be one of 4 distinct levels. 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 below. The line code groups two bipolar bits together and codes them in voltage levels of +3, +1, -1, -3 volts. Bits Quaternary Symbol Voltage Level 00-3 -2.5 01-1 -0.833 10 +3 +2.5 11 +1 +0.833 For North America, since NT1 is not supplied by the telephone company, the U interface is carefully defined so that products from different vendors will all properly communicate. For Europe, since NT1 is supplied by the telephone company, this interface may vary depending on the service provider. In most cases, the media is 22 gauge Unshielded Twisted Pair (UTP) cable. Page 4 May 4, 2000
Note: ISDN users who are in close proximity to the local ISDN serving office may be provisioned on a T interface line. A T interface provides a direct 4-wire connection to the telephone company switch. The length restriction for a T interface line is approximately 3300 ft (about 0.6 mile or 1 km). The connector used for the 2-wire U interface in North America and Singapore is the RJ11. This is the standard analog phone jack that has 4 conductors. V This is the interface at the Central Office (CO) between the Line Termination (LT) and the Exchange Termination (ET). Power Phantom power may be derived from the NT1 s S/T interface (4-wire interface). In Europe the NT1 receives up to 1.2W from the central office. In North America there is no provision for emergency mode operation. In any case, 2 or 4 additional wires may be used to provide power to operate telephones when normal power fails. Signaling There are two different types of signaling used for BRI ISDN in North America. (Europe has different but equivalent standards.) For communicating with your local phone company, ISDN uses the Digital Subscriber Signaling System #1 (DSS1). DSS1 defines the format for the data on the D-channel, how it is addressed, etc. It also defines message formats for a variety of messages used for establishing, maintaining, and dropping a call. These include SETUP messages, SUSPEND and RESUME messages, and DISCONNECT messages. Once the DSS1 signal makes it to the phone company, their own signaling system takes over to pass the call information within their system, and between other phone companies. SS7 defines a communications protocol and data format similar to DSS1, however SS7 is designed in a broader, more general way. DSS1 is specific to ISDN, however SS7 will handle the signaling needs of ISDN as well as other older signaling systems. Bearer Service Different bearer services provide different types of guarantees about the reliability and synchronization of the data. There are currently at least ten different bearer services for circuit-mode and at least three services for packet-mode. The attributes of the bearer service are encoded into a Bearer Code (BC), that is sent every time a new connection is being set up. Configuring a TA or TE1 Each ISDN device must be configured with the following telephone company supplied information: Switch Type Most ISDN hardware adapters need to know what type of switch they are connected to. The switch type simply refers to the brand of equipment and software revision level that the telephone company uses to provide you with ISDN service. There are only a few types of switches in the world and usually just one in countries other than the United States. Directory Numbers In some cases, each B channel on an ISDN line has its own number, while in other cases both B channels share a single number. The telephone company will tell you how many numbers your ISDN line will have. Separate numbers may be useful if you plan to take incoming calls on an ISDN line that has multiple TE s. Page 5 May 4, 2000
SPIDs (North America and Singapore only) Since some analog switches still exist at the phone company, it is necessary to specify the type of service required by Terminal Equipment for installations in North America and Singapore. In order to facilitate this, phone companies use an optional part of the ISDN standard to identify each TE1 or TA in use. The phone company assigns a Service Profile IDentifier (SPID) to each ISDN channel, and this SPID must be manually entered into each device in use on this channel. The phone company then stores this data, and when Terminal Equipment is first connected to the network, it sends its SPID to the nearest phone company switch. The phone company switch identifies what type of connection the device needs and (therefore) how to route the calls for this device. The SPIDs must refer to a configuration that matches one of the two B-channels on the ISDN line. The SPID usually consists of the phone number with some additional digits added to the beginning and end. See Appendix for SPID format examples. The SPID helps the switch understand what kind of equipment is attached to the line, and if there are multiple devices attached, helps route calls to the appropriate device on the line. The SPID is only sent to the phone company switch when the Terminal Equipment is first physically attached to the phone line, and allows the device to perform layer 3 initialization with the telephone company switch. The switch will then dynamically assign a TEI (Terminal Endpoint Identifier) to the device, which is used for all future connection requests from that piece of equipment. This allows the switch to look at the TEI (as well as other information in the Bearer Code), determine the SPID, and verify if all parameters match up. Page 6 May 4, 2000
Appendix - Glossary 2B+D The way a Basic Rate ISDN line is configured, using 2 B-channels and one D-channel. B-Channel Bearer Channel The 64 Kbps B channels can carry high quality audio, video, or data communications. In addition, the B channels can be bonded together for increased bandwidth. B channels can be used for circuitswitched voice or for circuit-switched or packet-switched data Basic Rate Interface (BRI) BRI enables the use of existing copper telephone wire (a single pair of twisted wires) to obtain ISDN service consisting of two 64 Kbps B channels and one 16 Kbps D channel. Central Office (CO) The telephone company's local facility that provides telephone service in your area. Demarc The "demarcation point", or the point where the telephone company's wiring stops and the customer s wiring begins. D Channel - Delta Channel The D channel (16 Kbps for BRI service and 64 Kbps for PRI service) handles the out-of-band signaling, call supervision and control, and the activation/deactivation of ISDN features IDN - Integrated Digital Network. The telephone network using digital switching. ISDN Integrated Services Digital Network: A completely digital telephone/telecommunications network for carrying voice, data, images, and video at high speed by sending digitally-encoded signals. ISDN provides "end-to-end" digital service and can work on the copper wiring phone lines that are in most homes and businesses today. ISDN Ordering Code A predefined number that tells the phone company how to provision your ISDN line based on the requirements of your ISDN hardware. Line or Loop Qualification A test that the phone company runs to make sure that an ISDN line meets the distance and quality requirements of 18,000 feet from the central office. NI-1 (National ISDN-1) An older specification for a "standard" ISDN phone line in North America. National ISDN-1 is intended to be a set of standards which every manufacturer can conform to. For example, ISDN phones that conform to the National ISDN 1 standard will work, regardless of the central office the customer is connected to. NI-2 (National ISDN-2) A specification for an ISDN phone line with more advanced features than NI-1 in North America. "National ISDN-2" also refers to a standard switch type. Other common (but older) and acceptable options are "AT&T 5ESS" and "Northern Telecom DMS-100". NT-1 (Network Termination 1) NT-1 is the device which converts the two-wire U Interface into the four-wire S/T Interface. This enables standard ISDN terminal equipment to plug into a standard eight-pin RJ-45 connector. Some terminal adapters have the NT-1 built into them for an easier installation. NT-2 - Network Termination 2. NT-2 is the device which converts the T Interface into the multi-point S Interface. Page 7 May 4, 2000
PBX Private Branch Exchange. A PBX is a private telephone switch that provides switching (including a full set of switching features) for an office or campus. PBXs often use proprietary digital-line protocols, although some are analog-based. POTS Plain Old Telephone Service: an analog (non-digital) telephone line. Primary Rate Interface (PRI) A type of ISDN service that offers 23 B channels at 64 Kbps and one D channel at 64 Kbps (23B+D). In Europe, PRI provides for 30 B channels and one D channel (30B+D). (See also Basic Rate Interface.) PRI is typically only required by ISPs or other telecommunications organizations providing third party dialup access. Provisioning The combination of device and service options that make up an ISDN line. You order your ISDN line, but the telephone company provisions the line, i.e.-- it configures the ISDN service according to the physical capabilities of the switch, as well as the options chosen. RJ-11 The most common telephone jack in the world, this is a six-conductor modular jack wired with 4 wires. RJ45 An 8-pin connector jack used with standard telephone lines, and required by some ISDN hardware. A little larger than an RJ-11 jack. Service Profile IDentifier (SPID) A number or set of numbers assigned to an ISDN line by the phone company. In the U.S., one SPID is assigned to each channel. The switch uses SPIDs as unique identification numbers for each ISDN line, so it can determine where to send calls and signals. S/T-Interface The S/T Interface carries the signals between the terminal adapter and the NT-1. Switch Type The brand of equipment and software revision level that the telephone company uses to provide ISDN service. There are only a few types of switches in the world and usually just one in countries other than the United States. (See NI-2) In Europe, "Euro ISDN (ETSI)" is the standard ISDN switch type, but there are still some variations in use. Examples are TR6 (Germany) and VN6 (France). Australia, Japan and Korea have their own "standard" switch type. Other countries usually adopt the European standard. T1 - Transmission 1. A transmission link in North America that has a data rate of 1.54 Mbps. T3 - Transmission 3. A transmission link in North America that has a data rate of 45 Mbps. TEI Terminal Endpoint Identifiers are unique IDs given to each device (TE1 or TA) on an ISDN S/T bus. This identifier is normally dynamically assigned when a device is attached to the S/T bus. Every TE operated in a bus configuration must have a different TEI so that the network can identify the correct device. Terminal Adapter - a.k.a "ISDN modem." The terminal adapter is the hardware used to connect a computer or network to the ISDN line. It may be internal (a card that fits into a slot inside your computer), or external (a separate piece of hardware that plugs into one of the serial ports on the back of your computer). A terminal adapter may be built Page 8 May 4, 2000
into an ISDN telephone for dumb terminals. Terminal adapters may also be used to convert an analog protocol into the ISDN interface, enabling the use of an existing analog modem or fax machine. U Interface A two-wire (single pair) interface from the phone switch to the customer premises. A terminal adapter with a U Interface has a built-in NT-1. Unshielded Twisted Pair (UTP) Another term for regular telephone wiring. Each telephone "wire" is actually a pair of wires. Page 9 May 4, 2000
Appendix - SPID Format Examples Below is a list of SPID format examples. Ameritech: 5ESS-Custom: 01 "7 digit" 0 example: phone # 555-555-1234 SPID: 01-555-1234-0 5ESS-NI1 (5E8 ver software): 01 "7 digit" 011 example: phone # 555-555-1234 SPID: 01-555-1234-011 5ESS-NI1 (5E9 ver software): "10 digit" 0111 example: phone # 555-555-1234 SPID: 555-555-1234-0111 DMS100-NI: "10 digit" 0111 example: phone # 555-555-1234 SPID: 555-555-1234-0111 DMS100-Custom: "10 digit"?? (??= 0, 1, 01, or 11) example: phone # 555-555-1234 SPID: 555-555-1234-0 Siemens EWSD-NI1: "10 digit" 0111 example: phone # 555-555-1234 SPID: 555-555-1234-0111 Bell Atlantic: 5ESS-NI1: example: phone # 555-555-1234 SPID: 01-555-1234-000 DMS100-NI1: example: phone # 555-555-1234 SPID: 555-555-1234-100 Custom ISDN: example: phone # 555-555-1234 SPID: 01-555-1234-0 Bell Canada: DMS100-NI1: example: phone # 555-555-1234 SPID: 555-1234-00 (7 digit number followed by 2 zero's) Bell South: 5ESS NI-1 example: phone # 555-555-1234 SPID: 555-555-1234-0100 DMS100: "10 digit" with last two digits repeated example: phone # 555-555-1234 SPID: 555-555-123434 DMS100: "10 digit" with last digit repeated example: phone # 555-555-1234 SPID: 555-555-12344 DMS100 NI1: "10-digit" 0100 example: phone # 555-555-1234 SPID: 555-555-1234-0100 NI1: "10-digit"-0 (or -00, -000) example: phone # 555-555-1234 SPID: 555-555-1234-0 example: phone # 555-555-1234 SPID: 555-555-1234-00 example: phone # 555-555-1234 SPID: 555-555-1234-000 GTE: (North Carolina) DMS100 NI1: example: phone # 555-555-1234 SPID: 555-555-1234-0100 example: phone # 555-555-1234 SPID: 555-555-1234-0000 (Oregon) NI-1 AT&T example: phone # 555-555-1234 SPID: 01-555-1234-000 Page 10 May 4, 2000
(In this case there was a separate SPID and DN for an ISDN Phone which is not for the ProShare system and will cause an Invalid SPID if entered into the ProShare setup..) (Oregon) AT&T Custom example: phone # 555-555-1234 SPID: 01-555-1234-0 (In this case there was a separate SPID and DN for an ISDN Phone which had no effect if in the ProShare setup.) Nynex: 5ESS-NI1: (10 digit number followed by 4 zero's) example: phone # 555-555-1234 SPID: 555-555-1234-0000 DMS100 NI-1: "10-digit" 0001 example: phone # 555-555-1234 SPID: 555-555-1234-0001 Pac Bell: DMS100: "10 digit" 1 or 2 example: phone # 555-555-1234 SPID: 555-555-1234-1 (or 10, or 100, or 1000) example :phone # 555-555-1234 SPID: 555-555-1234-2 (or 20, or 200, or 2000) DMS100: "10-digit" 1(for both DN's) example: phone # 555-555-1234 SPID: 555-555-1234-1 DMS100: "10-digit" 01 and 02 example: phone # 555-555-1234 SPID: 555-555-1234-1 (or 01, 0100) example: phone # 555-555-1234 SPID: 555-555-1234-2 (or 02, 0200) *5ESS-Custom - 01 "7 digit" 0 example: phone # 555-555-1234 SPID: 01-555-1234-0 *5ESS-NI1: 01 "7 digit" 000 example: phone # 555-555-1234 SPID: 01-555-1234-000 *Note: In either of these configurations a second number may be provided but should not be input into ProShare Video. Southern New England Telephone: 5ESS-NI1: 01 "7 digit" 000 example: phone # 555-555-1234 SPID: 01-555-1234-000 SouthWestern Bell: DMS100: "10 digit" 01 example: phone # 555-555-1234 SPID: 555-555-1234-01 Siemens-NI1: "10 digit" 0100 example: phone # 555-555-1234 SPID: 555-555-1234-0100 5ESS-NI1: 01 "7 digit" 000 example: phone # 555-555-1234 SPID: 01-555-1234-000 Note: You may need to add 2 zeros to SPID given by SW Bell. US West: 5ESS-NI1: 01 "7 digit" 000 example: phone #555-555-1234 SPID: 01-555-1234-000 5ESS-Custom: 01 "7 digit" 0 example: phone # 555-555-1234 SPID: 01-555-1234-0 5ESS NI1: "7 digit" 1111 example: phone # 555-555-1234 SPID: 555-1234-1111 Page 11 May 4, 2000