ISDN Integrated Services Digital Network

Developed by ITU-T, ISDN is a set of protocols that combines digital telephony and data transport services to digitise the telephone network to permit the transmission of audio, video and text over existing telephone line. ISDN is an effort to standardise subscriber services, provide user or network interface and facilitate the inter-networking capabilities of existing voice and data networks. The goal of ISDN is to form a wide area network that provides universal end-to-end connectivity over digital media by integrating separate transmission services into one without adding new links or subscriber links.

1. Integrated Services

Digital Network

(ISDN)


2. Prof. Madhumita Tamhane
GOAL
■To use existing infrastructure of telephone lines and
networks and to be able to transmit
✓Voice
✓Digital data
✓Other services like reservations, alarm etc.
!
■To form a wide area network that provides universal
end-to-end connectivity over digital media, by
integrating all transmission services into one without
adding new links.

3. Prof. Madhumita Tamhane
ISDN Services

4. Prof. Madhumita Tamhane
ISDN Services
■Bearer Services: Provides means to transfer information between
users without network manipulating content of information.
■Belongs to first 3 layers of OSI model.
■Can be provided using circuit-switched, packet-switched,
frame-switched or cell-switched networks.
■Tele-services: Network may change or process the data.
■Corresponds to layers 4-7 of OSI model.
■Rely on the facilities of bearer services.
■designed to accommodate complex user needs.
■Includes telephony, teletext, telefax, telex, teleconferencing.
■Supplementary Services: Provides additional functionality to
above.
■E.x. Reverse charging, cell waiting, message handling..

5. Prof. Madhumita Tamhane
History:Voice Communication over an 

Analog Telephone Network
■Used for transmission of analog information in
form of voice.
■Local loops connecting the subscriber’s handset to
telephone company’s central office were also
analog.

6. Prof. Madhumita Tamhane
History:Voice and Data Communication 

over an Analog Telephone Network
■With advent of digital processing, subscribers
needed to exchange data as well as voice.
■Modems were developed to allow digital exchanges
over existing analog lines.

7. Prof. Madhumita Tamhane
History:Analog and Digital Services 

over the Telephone Network
■To reduce cost and improve performance, digital
technologies added with backward compatibility.
■Three types of customers:
■Traditional costumers using local loops for analog purposes most
prominent.
■Customers using analog facilities for digital information via modem.
■Customers using digital facilities for digital information.

8. Prof. Madhumita Tamhane
IDN: Integrated Digital Network

9. Prof. Madhumita Tamhane
IDN: Integrated Digital Network
■To meet need for packet-switched and circuit-switched
networks.
■A combination of networks available for different purposes.
■Access to these networks by digital pipes( time-multiplexed
channels sharing very high speed paths).
■Customers can use their local loops to transmit both voice
and data to telephone central office.
■Central office directs these calls to appropriate digital
networks via digital pipes.

10. Prof. Madhumita Tamhane
ISDN

11. Prof. Madhumita Tamhane
ISDN
■Integrates customer services with IDN.
■Fully digital services are more efficient and flexible.
■Need to replace analog local loop with digital subscriber
loop.
■Voice transmission can be digitised at source.
■Possible to send data, voice, image, facsimile etc. over it.
■With all services digital, flexibility allows services available
on demand.
■Allows all connections in home or building via single
interface.
■Digital pipes allow different transmission rates and support
different subscriber needs.

12. Prof. Madhumita Tamhane
ISDN Architecture – Digital Bit Pipe
■Bidirectional conceptual pipe through which bits flow
between end user and CO/ ISDN exchange.
■Bits may correspond to any of the services.
■Supports TDM
■Two categories defined.
■Home user – Low bandwidth
■Business user – High bandwidth
■Total BW divided into Channels.
■Each channel equal to one home user channel.
■Business users can have multiple bit pipe each having
multiple channels.

13. Prof. Madhumita Tamhane
ISDN Architecture – ISDN channel Type
BEARER CHANNEL B
■64 kbps data rate.
■Used for digitized voice, data or other low data rate
information.
■Full duplex.
■8000 samples/s X 8 bits/sample = 64kbps.
■One B channel per subscriber per exchange of
information.
■Subscriber will contend for B channel.
■For higher data rates , two B channel can be combined to
give 128kbps.

14. Prof. Madhumita Tamhane
ISDN Architecture – ISDN channel Type
DATA CHANNEL D
■Contrast to name, does NOT carry data.
■Carries controlling signals as establishing a call, ringing,
call interrupt etc.
■Carries control signals for all using Out-band signalling.
(Protocol-Signalling system Number 7, SS7)
■16 / 64 kbps
■Common channel signalling.
■Subscriber secures a B connection by using D channel.
■In case of no signalling, it can be used to carry data as
videotext, tele-text, emergency services alarms etc..

15. Prof. Madhumita Tamhane
ISDN Architecture – ISDN channel Type
HYBRID CHANNEL H
■Used at high BW requirements.
■384 / 1536 / 1920 kbps.
■Used for video, video-conferencing, high speed data/audio
etc.
■Can be sub divided as per need.
■Can be used as B channel for high BW needs.

16. Prof. Madhumita Tamhane
ISDN Interfaces - BRI
■Basic rate interface, used for home users.
■Specifies a digital pipe with 2 B channels and 1 D channel
(16kbps).
■2 X 64 + 16 = 144kbps
■In addition, BRI services requires 48 kbps of management
overheads.
■Total data rate - 192 kbps.
■User can use one B channel for a call and other for
browsing.
■Both B channel can be combined for faster connection to
internet.

17. Prof. Madhumita Tamhane
ISDN Interfaces - BRI

18. Prof. Madhumita Tamhane
ISDN Interfaces - PRI
■Primary rate interface, used for business users.
■Specifies a very big digital pipe with 23 B channels and 1
D channel (64kbps).
■In addition, BRI services requires 8 kbps of management
overheads.
■23 X 64 + 64+ 8 = 1.544Mbps.
■Compatible with T1 line
■ A User can use more than one B channel.

19. Prof. Madhumita Tamhane
PRI

20. Prof. Madhumita Tamhane
Functional Grouping

21. Prof. Madhumita Tamhane
Functional Grouping--Terminal Equipment
■TE1
➢All ISDN equipments as digital telephone, digital fax,
digital voice and data terminals
➢Can be directly connected to ISDN.
■TE2
➢All non-ISDN equipments as normal analog telephone,
analog fax etc.
➢Helps backward compatibility.
➢Can not be directly connected to ISDN.

22. Prof. Madhumita Tamhane
Functional Grouping--Network Termination
■NT1
➢Controls electrical and physical termination of ISDN at
user’s premise.
➢Analogous to physical layer .
➢Organises data streams into frames and back.
➢Though not a MUX, it interleaves bytes to act like a
MUX.
➢Connected to ISDN using twisted pair wires of
telephone network.
➢Can connect up to 8 devices at one premise.
➢It is the boundary of ISDN

23. Prof. Madhumita Tamhane
Functional Grouping--Network Termination
■NT2
➢For large business, need to support more telephone
conversation at a time..
➢Analogous to EPABX or LAN.
➢It multiplexes multiple incoming links to be given to
NT1.
➢Works in three layers.
➢Multiplexing - layer 1
➢Flow control - layer 2
➢Packetising – layer 3

24. Prof. Madhumita Tamhane
Functional Grouping--Terminal Adapter
■TA
➢Converts information from non-ISDN
equipments to ISDN format.
➢Acts as converter
➢Kept at user’s premise.

25. Prof. Madhumita Tamhane
ISDN Layers
■7 layer OSI model can not be applied to ISDN because…
■ISDN specifies two different channels (B and D) with different
functionalities requiring different protocols.
■B channel - user to user communication.
■D channel - user to network signalling.
■ISDN also differs from OSI in management needs.
■Global integration, maintaining the flexibility required to keep
the network truly integrated using public services requires huge
management.
■ITU-T has devised an expanded model for ISDN layers in
defining three separate planes:
■User plane
■Control plane
■management plane.

26. Prof. Madhumita Tamhane
ISDN Layers

27. Prof. Madhumita Tamhane
ISDN Layers
■At physical layer, B and D channels are same, use either
BRI/PRI interface.
■At datalink layer, B/D channel uses LAPB/LAPD.
■At network layer, B channel has many options in
connecting to circuit switched/ packet switched/Frame
relay/ATM networks.
■User plane options for layers 4 through 7 is left to user.

28. Prof. Madhumita Tamhane
Simplified Layers of ISDN

29. Prof. Madhumita Tamhane
Physical Layers
■Specified by ITU-T standard: I.430 for BRI and I.431 for
PRI access.
■Primary aspects defined by these standards are:
■The mechanical and electrical specifications of
interface R, S, T and U.
■Encoding.
■Multiplexing channels for BRI and PRI digital pipes.
■Power supply.

30. Prof. Madhumita Tamhane
Reference points for physical layer

31. Prof. Madhumita Tamhane
Physical Layer
BRI Interfaces —R
R interface
•Local, not defined by ISDN.
•Any EIA standard as EIA232 or X.21 etc.

32. Prof. Madhumita Tamhane
Physical Layer
BRI Interface-S
Signal Encoding- Pseudo
ternary

33. Prof. Madhumita Tamhane
Physical Layer
BRI Interface-S
S- Interface ( ISO 8887) - 2 or 3-pair twisted cable
•4, 6 or 8 wire connection for full duplex
•c , f – Transmit,
•d, e – Receive.
•Three methods for power supply—
1.NT1 supplies power to TE from battery, power outlet or ISDN centre.
•4 connections needed between TE and NT1—- c ,d ,e and f.
2.Power supplied by NT1 but two separate lines relay it to TE.
•6 wires are used— c ,d ,e, f, g and h.
•Power from NT1 via g, h
3. TE supplies power itself and to other TEs using a and b.
•ISDN does not use this option.
!

34. Prof. Madhumita Tamhane
Physical Layer BRI Interface-U
2B/1Q Encoding
•Between NT1 and ISDN exchange.
•Single pair twisted pair cable in each direction.
•2 binary 1 quaternary Encoding,
•4 voltage levels for two bits 00, 01, 10 and 11.
• Lowers baud rate, high efficiency in using available BW.

35. Prof. Madhumita Tamhane
BRI Frame
•Each B channel is sampled twice and D channel four times in a frame.
•12 Overhead bits are for framing and synchronizing.
•48 bits long frame helps in making ATM cell.

36. Prof. Madhumita Tamhane
BRI
Topology

37. Prof. Madhumita Tamhane
BRI
Topology
■A bus or a Star based on distance of devices from NT1.
■Point-to-point Bus connection —1000 meters maximum.
■Multipoint Bus connection — less than 200meters if devices
are spaced apart.
■Distance limitation to ensure synchronization.
■Propagation delay between first and last device can
deteriorate synchronisation during multiplexing.
■If devices are clustered, distance can be 500 meters.
■Propagation delay will be almost same for all.
■Star topology link can be 1000 meters long as each act as
PoP connection.

38. Prof. Madhumita Tamhane
BRI
Topology
■Maximum 8 devices.
■Only 2 devices can access B channel at a time, one
exchange per channel.
■All devices can contend for D channel using CSMA.
■Winning device then requests for B channel.
■On availability of B channel, connection is made by D
channel for user to send data.

39. Prof. Madhumita Tamhane
Physical Layer for PRI
■ 23 B channels and 1 D channel.
■Interface used are R, S, T, U.
■R and S standards same as BRI.
■T standard is identical to S standard with substitution of
B8ZS encoding.
■U interface is also same except PRI rate is 1.544 Mbps
instead of 192 Kbps of BRI.

40. Prof. Madhumita Tamhane
PRI Interfaces
T interface – Same as S interface. Encoding is 8BZS

41. Prof. Madhumita Tamhane
PRI Frame and Topology
■B and D channels multiplexed using synchronous TDM to
create PRI frame.
■PRI frame samples each B channel and D channel only
once per frame.
■Connection and topology between devices and NT2 can be
same as that described between devices to NT1 in BRI.
■Depending on specific application it can change as —
■If NT2 LAN, topology specified by LAN.
■If NT2 PBX, topology specified by PBX…
■Link from NT2 to NT1 must always be point to point.

42. Prof. Madhumita Tamhane
PRI Frame

43. Prof. Madhumita Tamhane
DATA LINK LAYER
■B and D channels use different data link protocols.
■Link Access Protocol for B channel LAPB.
■Link Access Protocol for D channel LAPD.
■LAPD is same as HDLC with few modifications.
1.LAPD can be used in either unacknowledged(without
sequence numbering) or acknowledged(with sequence
numbering) formats.
2.Addressing: 2 bytes address field in LAPD…
Flag Address Control Data… FCS Flag
8 bits 16 bits 8/16 bits Variable
multiple of 8
16 bits 8 bits

44. Prof. Madhumita Tamhane
DATA LINK LAYER
■ SAPI – Service access point identifier.
■ Defines layer 3(network Layer) protocol entity within a
user device.
■Unique within TE1.

45. Prof. Madhumita Tamhane
DATA LINK LAYER
■Indicates intended use of D channel. Can define 64
different service access points.
• 0 – Call control procedures for managing B channel
circuits.
• 1 – Packet mode communication on D channel using Q.
931. End to end signalling.
•16 – Packet mode communication on D channel using
X.25. Data use of D channel
• 32-61 --- Frame relay communication on D channel.
• 63 – Exchange of layer 2 management information.
•TE1 and SAPI together define logical connections and
give Data Link Connection Identifier DLCI.

46. Prof. Madhumita Tamhane
DATA LINK LAYER
•C/R – Whether command (“0”) or response(“1”) frame.
•8
th
bit “0” indicates continuation to next frame.
•Extended addressing 7 bits + 1 bit
•TEI- Terminal Equipment Identifier is unique address of TE.
•7 bits can identify 128 different TEs.
•Last bit “1” indicates end of address.

47. Prof. Madhumita Tamhane
Network Layer Packet Format
•After connection establishment by D channel, B channel sends
data using circuit switching, X.25, or other similar protocols.
•Network layer functions of D channel defined by ITU-T Q.931.
•Network layer packet, called message, is encapsulated in
information field of a LAPD I-frame for transport across link.
•Various fields are..

48. Prof. Madhumita Tamhane
Network Layer Packet Format
• Single 1 byte field - Protocol Discriminator identifies
protocol in use—
•00001000 for Q.931.
•00000010 for X.25 call user data.
•00000000 for user-specific protocol.
•00000001 for OSI higher layer protocol……..
•2 or 3 bytes field Call Reference as—-

49. Prof. Madhumita Tamhane
Call Reference Field
•Call reference is sequence number of the call.
•Length – Length of the remainder of sub field.
•BRI – 8 bits and PRI – 16 bits.

50. Prof. Madhumita Tamhane
Call Reference Value Field
•Call reference value – number assigned to this call.
•This number should be quoted while future operation on this
call.
•Number is assigned by TE1 if requesting connection OR assigned
by NT if incoming call.
•First bit is called Flag.
•Flag – 0 – Message from the originator.
•Flag – 1 – Message To the originator.
•Flag needed in case both NT and TE1 allot same number.
•Call reference value is local between NT and TE1 at each end.

51. Prof. Madhumita Tamhane
Message Type Field
•Message Type – Define application they support and functions
they perform.
•4 types : Call establishment, Call information, Call clearing
messages and miscellaneous messages .
•Defines Circuit mode connection control, Packet mode
connection control etc.

52. Prof. Madhumita Tamhane
Call Establishment Messages
•Setup: Sent by calling user to network or by network to called user to
initiate a call.
•Setup Acknowledgement: Sent by called user to network or by network to
calling user to initiate a indicate— setup received.
•Connect: Sent by called user to network or by network to calling user to
indicate acceptance of the call.
•Connect Acknowledgement: Sent by network to called user to say that
desired connection has been achieved..
•Progress: Sent by network to called user to indicate that call
establishment is in progress. “Please standby” if needs more time.
•Alerting: Sent by called user to network or by network to calling user to
indicate that call user alert (ringing) has been alerted.
•Call Processing: Sent by called user to network or by network to calling
user to indicate that requested Call Establishment has been initiated.

53. Prof. Madhumita Tamhane
Call Information Messages
•Resume: Sent by a user to the network to request resumption of a
suspended call.
•Resume Acknowledgement: Sent by network the user to
acknowledge a request to resume the call.
•Suspend: Sent by a user to request that the network suspends a
call.
•Suspend Acknowledgement: Sent by the network to the user to
acknowledge the requested suspension of the call.
•Suspend Reject: Sent by the network to the a user to reject the
requested suspension.
•User Information: Sent by the user to the network to be delivered
to the remote user. Allows user information sending using out-of-
band signalling.

54. Prof. Madhumita Tamhane
Call Clearing Messages
•Disconnect: Sent by the calling user to network or by network to
the called user to clear end-to-end connection.
•Release: Sent by user or network to indicate the intention to
disconnect and release the channel.
•Release Complete: Sent by a user or network to show that the
channel has been released.
!
!
•Miscellaneous: Protocol specific. Not used in routine
communication.

55. Prof. Madhumita Tamhane
Information Elements
■Information elements – Used in setup message to request a bearer service.
■Contains specific details about required connection.
■Address of sender and receiver.
■Routing information.
■Type of network desired for B channel exchange.
■Circuit switched, X.25, ATM or frame relay.
■Contains specific details about the choice of bearer service.
■Eg. Unrestricted digital information.
■Contains information about terminal or intended call.
■Eg.- Destination terminal capability or possibility of inter- networking
with other NW.

56. Prof. Madhumita Tamhane
Information Element Types
■Information Element consists of one or more byte.
■One byte information element can be :
■Type 1:
■1st bit ‘0’,
■next 3 bits identify information being sent.
■Remaining 4 bits carry the specific content or attribute of
the element.

57. Prof. Madhumita Tamhane
Information Element Types
■Type 2:
■1st bit ‘1’,
■Next reserved for ID.
■Variable Length:
■1st bit of 1st Byte is ‘0’,
■next 7 bits ID.
■Second byte defines length of content in bytes.
■Remaining bytes are content.

58. Prof. Madhumita Tamhane
Addressing in ISDN
•NC – National Code-Service provider within country
•Subscriber Number – 832…
•Total 15 bits defines access to a subscriber NT1.
•Subaddress defines each of multiple devices (if any) connected to
NT1 through NT2.

59. Prof. Madhumita Tamhane
Bit Rates for Different Applications