Transcript
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ISDN, Integrated Services Digital Network.
What is ISDN? ISDN (Integrated Services Digital Network) is the all-digital equivalent of the conventional telephone network PSTN (Public Switched Telephone Network), or POTS (Plain Old Telephone System). ISDN technology is standardized according to recommendations of the International Telecommunications Union (ITU),
Signalling Instead of the phone company sending a ring voltage signal to ring the bell in your phone (”In-Band signal”), it sends a digital packet on a separate channel (”Out-of-Band signal”). The Out-of-Band signal does not disturb established connections, and call setup time is very fast. The signalling also indicates who is calling, what type of call it is (data/voice), and what number was dialed. Available ISDN equipment is then capable of making intelligent decisions on how to direct the call.
Services Logically, ISDN consists of two types of communications channels: bearer service Bchannels, which carry data and services at 64 kbit/s; and a single D-channel, which usually carries signalling and administrative information which is used to setup and tear down calls. The transmission speed of the D-channel depends on the type of ISDN service subscribed to. ISDN services available today can be divided into two categories: Basic Rate Interface (BRI) service, which gives the subscriber access to two B-channels and a 16 kbit/s D-channel; and Primary Rate Interface (PRI) service, which provides a 64 kbit/s D-channel and 30 B-channels in Europe and most of Asia, in North America and Japan the PRI service gives 23 B-channels. When more than one device is connected through a single ISDN BRI connection, individual devices are distinguished from one another through the use of multiple subscriber numbers, (MSN) whereby a different ISDN number is assigned to each device served by the ISDN subscription. Up to eight ISDN devices can be connected on single bus, as signals on the Dchannel automatically take care of contention issues, and route calls and services to the appropriate ISDN device. Alternatively, a separate sub-address (SUB) value can be used to differentiate between devices.
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Speed The modem was a big breakthrough in computer communications. It allowed computers to communicate by converting their digital information into an analogue signal to travel through the public phone network. There is an upper limit to the amount of information that an analogue telephone line can hold. Currently, it is about 56 kbit/s. Commonly available modems have a maximum speed of 56 kbit/s., but are limited by the quality of the analogue connection and practically 45–50 kbit/s is reached. The high throughput offered by ISDN 2 x 64 kbit/s, rapid call setup, less than 2 s and the high level of accuracy inherent to digital transmission, are the main attractions to ISDN technology. The two channels can be bundled to give a virtual 128 kbit channel or used as two separate channels enabling simultaneous data and voice calls.
ISDN Components/Interfaces ISDN components include terminals, Terminal Adapters (TA), NetworkTermination devices (NT), line-termination equipment (LT), and exchange-termination equipment (ET). ISDN defines terminals of two types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals, such as DTE are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a 4-wire, twisted-pair digital link. TE2s connect to the ISDN network through a TA. The TE2 connects to the TA via a standard physicallayer interface such as RS-232/V.24 or RS485/V11. Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination devices that connect the 4-wire subscriber wiring to the conventional 2-wire local loop. In North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device that typically is found in digital private branch exchanges (PBXs) and that performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists as a single device that combines the functions of a NT1 and a NT2.
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ISDN specifies a number of reference points that define logical interfaces between functional groupings, such as TAs and NT1s. ISDN reference points include the following: • R---The reference point between non-ISDN equipment and a TA. • S---The reference point between user terminals and the NT2. • T---The reference point between NT1 and NT2 devices. • U---The reference point between NT1 devices and line-termination equipment in the carrier network. The U reference point is relevant only in North America, where the NT1 function is not provided by the carrier network. Network Termination. Used to convert U to S/T interface Supplied in Europe by Telco
ISDN equipment that can connect directly to ISDN line
S/T interface Termination point in Europe
TE1 V
U
NT-1
TE1
S/T
TA
Switch
R
TE2
ISDN equipment that can connect NOT directly to ISDN line
Used to connect TE2 devices to ISDN line
Equipment at phone company switch
Standard PSTN equipment has an R interface
Layer 1 Physical layer The signalling between the telecom switch and the user is according to the U-interface and the signalling in the user building is normally according to the S-interface. The U-interface uses frames of 240 bit length. At a rate of 160 kbit/s, each frame is therefore 1.5 ms long. Each frame consists of:
Frame structure U-Frame when 2B1Q coding 240 bits, 1.5 ms
12 words, 216 bits
S O/M W12
W11
W2
W1
S
S = Synchronitation pattern 18 bits O/M = Operation and Maintance 6 bits
B2 8 bits
40
B1
D
8 bits 2 bits
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Frame Format S interface ISDN physical-layer (Layer 1) S frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal). Both physical-layer interfaces are shown below. The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical-layer frame are used as follows: 48 bits 250µs
NT to TE
TE to NT
1 1
8
1 1 1 1 1
FL
B1
E DA FN
D L F L B1
8
1 1 1
1
1 1 1
8
B2 E D M B1 E D S
LDL FL
B2
L D L B1
E F L S N M
A = Activation bit B1 = B1 channel (2 x 8 bits / frame) B2 = B2 channel (2 x 8 bits / frame) D = D channel (4 x 1 bit / frame)
= = = = = =
B2
LDL
1 1 1
ED L –––
B2 L D L – – –
Echo of previous D bit Framing bit DC balancing S-channel Inverted F from NT to TE Multiframing bit
Layer 2 – Data Link Layer The ISDN Data Link Layer is specified by the ITU Q.920 through Q.923. All of the signalling on the D channel is defined in the Q.921 spec. Link Access Protocol – D channel (LAP-D) is the Layer 2 protocol used. This is almost identical to the X.25 LAP-B protocol. Here is the structure of a LAP-D frame: Flag
Address
Control
Information
CRC
Flag
Flag (1 octet) This is always 7E16 (0111 11102) Address 8
(2 octets) 7
6
5 SAPI (6 bits) TEI (7 bits)
4
3
2 C/R
1 EA0 EA1
SAPI (Service access point identifier), 6-bits (see next side) C/R (Command/Response) bit indicates if the frame is a command or a response EA0 (Address Extension) bit indicates whether this is the final octet of the address or not TEI (Terminal Endpoint Identifier) 7-bit device identifier (see next side) EA1 (Address Extension) bit, same as EA0 41
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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. Flag (1 octet) Closing flag, always 7E16 (0111 11102)
SAPI The Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point where Layer 2 provides a service to Layer 3. SAPI value 0 1–11 12 13–15 16 17–31 63 All others
The figure above gives a view of usage of the SAPI field,where SAPI = 0 is used for switch control and SAPI = 16 is used for package routing when X.31, X.25 over D-channel is used
Related layer 3 or management entity Call control procedures Reserved for future standardization Teleaction communication Reserved for future standardization Packet communication conforming to X.25 level 3 procedures Reserved for future standardization Layer 2 management procedures Not available for Q.921 procedures
Package data via D-channel
SAPI-16 Package data via B-channel
TE
ET SAPI-0
PH
TE
Switch control
TEIs Terminal Endpoint Identifiers (TEIs) are unique IDs given to each device (TE) on an ISDN S/T bus. This identifier can be dynamic; the value may be assigned statically when the TE is installed, or dynamically when activated.
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TEI Value
User Type
0–63 64–126 127
Non-automatic TEI assignment user equipment Automatic TEI assignment user equipment Broadcast to all devices
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Layer 3 – Network Layer The ISDN Network Layer is specified by the ITU Q.930 through Q.939. Layer 3 is used for the establishment, maintenance, and termination of logical network connections between two devices. Information Field Structure The Information Field is a variable length field that contains the Q.931 protocol data. Information Field 8 7 6 5 4 3 Protocol Discriminator 0 0 0 0 Length of CRV Call Reference Value (1 or 2 octets) 0 Message Type Mandatory & Optional Information Elements (variable)
2
1
These are the fields in a Q.931 header: Protocol Discriminator (1 octet) Identifies the Layer 3 protocol. For a Q.931 header, this value is 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.
CAPI COMMON-ISDN-API (CAPI) is an application programming interface standard used to access ISDN equipment connected to basic rate interfaces (BRI) and primary rate interfaces (PRI). By adhering to the standard, applications can make use of well defined mechanism for communications over ISDN lines, without being forced to adjust to the hardware vendor implementations. To reflect on the actual situation it can be stated that the international protocol specification is finished and almost every telecommunication provider offers BRI / PRI with protocols based on Q.931 / ETSI 300 102. CAPI Version 2.0 was developed to support all Q.931 based protocols. CAPI is designed to be the base of a whole range of new protocol-stacks for networking, telephony and file-transfer and is embodied in European standard ETS 300 838 ”Integrated Service Digital Network (ISDN); Harmonized Programmable Communication Interface (HPCI) for ISDN”.
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