ISDN Integrated Services Digital Network

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ISDNIntegrated Services Digital Network

• Jarett Boxberger
• Data Comm. II
• Fort Hays State University

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What is ISDN?

• Can be used to transport voice, data, or video.
• ISDN uses a system of digital phone connections
  for communication and data transfer.
• Been available for over a decade
• Allows data to be transmitted simultaneously
  across the world using end-to-end digital
  connectivity

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Overview

• ISDN uses different channels to send and receive
  information.
• B-Channel(s) is used to send and receive
  information.
• D-Channel is used for signaling purposes only.
• Two types of service
• Basic Rate Interface (BRI)
• Primary Rate Interface (PRI)

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Types of ISDN Service

• Basic Rate Interface (BRI)
• Consists of 2 64 kbps B-Channels and one 16 kbps
  D-Channel, for a total of 144 kbps
• Intended to meet the needs of most customers
• Primary Rate Interface (PRI)
• Consists of 23 B-Channels and one 64 kbps
  D-Channel, for a total of 1536 kbps
• Intended for users with greater capacity needs

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BRI

• BRI service at an office or a home, ISDN line
  consists of the same twisted pair of wires used
  for analog phones.
• Up to 8 ISDN devices can connect to this single
  line.
• Devices are distinguished through the use of
  multiple subscriber numbers.

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Whats Needed for BRI ISDN?

• Subscribe to an ISDN phone line
• Must be within 18000 feet or about 3.4 miles of
  the CO for BRI service.
• Must purchase ISDN Terminal Adapters (Sometimes
  incorrectly called ISDN Modems), and ISDN routers.

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PRI

• Includes 23 B-channels in North America and
  Japan, 30 in most of the rest of the world.
• The number is limited due to the T-1.
• Still only one D-channel, but its bandwidth is
  increased to 64 kbps.
• Only one device can be connected to a PRI line
• PBX, however, can reallocate the ISDN PRI
  resources onto multiple BRI buses.

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PRI continued

• A single PRI connection is usually much less
  expensive than multiple BRI connections.
• Main benefit of PRI is that its bandwidth can be
  dynamically allocated among applications.
• Example, certain channels can be allocated for
  voice channels, can be reallocated for
  videoconferencing.

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B-Channel

• These are logical pipes in a single ISDN line.
• Two can be combined to download data at high
  speeds, or one can be freed up for other devices
  such as a telephone or fax machine.
• The B-Channels 64 kbps uses its entire bandwidth
  for data.

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B-Channel continued

• Four types of connections can be set up.
• Circuit-Switched
• Packet-Switched uses X.25
• Frame Mode uses LAPF
• Semipermanent Leased Line
• Can be used to transport any layer 2 or higher
  protocol.

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Types of ISDN B-Channel Connections
(Circuit-switched B-Channel Connections)
(Packet-switched B-Channel Connections)
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D-Channel

• Used for administrative signaling
• Instruct carrier to setup or terminate a
  B-Channel.
• Insure that a B-Channel is available to receive a
  call.
• Provide signaling information for such features
  as Caller ID.

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D-Channel continued

• Uses packet-switched connections
• Associated with protocols at layer 2 (Q.921) and
  3 (Q.931).

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ISDN and the OSI Model

• ISDN deals with the bottom 3 layers of the OSI
  model.

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Layer 1 Devices

• Terminal Equipment 1 (TE1)
• Device thats compatible with ISDN.
• Terminal Equipment 2 (TE2)
• Device thats not compatible with ISDN.

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Layer 1 Devices cont.

• Network Termination 1 (NT1)
• Connects 4 wire ISDN wiring to the conventional
  2 wire local loop facility.
• Network Termination 2 (NT2)
• Intelligent device that performs switching among
  the attached devices to the NT1.

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Layer 1

• 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.

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U Interface

• Each U interface frame is 240 bits long.
• At the prescribed data rate of 160 kbps, each
  frame is therefore 1.5 msec long.
• Each frame consists of
• Frame overhead - 16 kbps
• D channel - 16 kbps
• 2 B channels at 64 kbps - 128 kbps

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U Interface cont.

• 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 superframe consisting of
  8 240-bit frames for a total of 1920 bits (240
  octets). The sync field of the first frame in the
  superframe is inverted.

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Layer 2 - Q.921

• Use only the D-Channel.
• Establishes and maintains communication between
  the ISDN terminal and the ISDN switch.
• Uses LAPD.

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LAPD and ISDN

• Plays an important role in terms of low-level
  signaling to ISDN devices.
• Frames contain information which ensures that
  incoming calls are routed to the appropriate ISDN
  device.
• Done using Terminal Endpoint Identifier (TEI) and
  Service Access Point Identifier (SAPI) fields
  within the LAPD frame.

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TEI - Terminal Endpoint Identifiers

• 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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SAPI - Service Access Point Identifier

• Identifies the point where Layer 2 provides a
  service to Layer 3.

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LAPD Frame
Flag (1 octet) - This is always 7E16 (0111 11102)
SAPI (Service access point identifier), 6-bits
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 EA1
(Address Extension) bit, same as EA0
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LAPD Frame cont.

• 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) - This is always 7E16 (0111 11102)

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Establishing the Link Layer

• The Layer 2 establishment process is very similar
  to the X.25 LAP-B setup.
• 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), SAPI0, TEIassigned.
• At this point, the connection is ready for a
  Layer 3 setup.

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Channel Signaling Layer 3

• ITU Q Series of Documents
• Relies on a link layer protocol to transmit
  messages over the D-channel.
• Specifies procedures for establishing
  connections on the B-channels that share the same
  physical interface to ISDN as the D-channel.
• Provides user-to-user control signaling of
  D-channel.

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SPID Service Profile ID

• Used to identify what services and features the
  Telco switch provides to the attached ISDN
  device.
• Optional when they are used, they are only
  accessed at call setup time.
• The format of the SPID 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.
• 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.

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Q.931 Specification

• Messages all contain 3 common fields.
• Protocol discriminator Used to distinguish
  messages for user-network call control from other
  message types.
• Call reference Identifies the user-channel call
  to which this message refers.
• Message type Identifies which I-451 message is
  being sent.

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Layer 3 Call Setup

• The following are the steps that occur when an
  ISDN call is established. In the following
  example, there are three points where messages
  are sent and received
• The Caller
• The ISDN Switch
• The Receiver.

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Call Setup continued

• Caller sends a SETUP to the Switch.
• If the SETUP is OK, the switch sends a CALL
  PROCEEDING to the Caller, and then a SETUP to the
  Receiver.
• The Receiver gets the SETUP. If it is OK, then it
  rings the phone and sends an ALERTING message to
  the Switch.
• The Switch forwards the ALERTING message to the
  Caller.
• When the receiver answers the call, is sends a
  CONNECT message to the Switch
• The Switch forwards the CONNECT message to the
  Caller.
• The Caller sends a CONNECT ACK message to the
  Switch
• The Switch forwards the CONNECT ACK message to
  the Receiver.
• Done. The connection is now up.

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Summary

• Types of ISDN Service
• BRI
• PRI
• B-Channel
• D-Channel
• ISDN and the OSI Model
• Layer 2 and Layer 3 setup