Integrated ServicesDigital Network

(ISDN)

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neGOAL

■ 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.

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neISDN Services

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neISDN 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..

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neHistory: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.

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neHistory: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.

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neHistory: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.

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neIDN: Integrated Digital Network

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neIDN: 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.

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neISDN

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neISDN

■ 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.

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neISDN 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.

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neISDN 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.

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neISDN 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..

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neISDN 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.

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neISDN 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.

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neISDN Interfaces - BRI

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neISDN 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.

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nePRI

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neFunctional Grouping

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neFunctional 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.

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neFunctional 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

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neFunctional 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

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neFunctional Grouping--Terminal Adapter

■ TA ➢ Converts information from non-ISDN

equipments to ISDN format. ➢ Acts as converter ➢ Kept at user’s premise.

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neISDN 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.

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neISDN Layers

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neISDN 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.

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neSimplified Layers of ISDN

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nePhysical 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.

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neReference points for physical layer

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Physical Layer BRI Interfaces —R

R interface

•Local, not defined by ISDN.

•Any EIA standard as EIA232 or X.21 etc.

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Physical Layer BRI Interface-S

Signal Encoding- Pseudo ternary

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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.

!

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nePhysical 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.

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neBRI 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.

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BRI Topology

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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.

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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.

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nePhysical 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.

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nePRI Interfaces

T interface – Same as S interface. Encoding is 8BZS

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nePRI 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.

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nePRI Frame

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neDATA 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

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DATA LINK LAYER

■ SAPI – Service access point identifier. ■ Defines layer 3(network Layer) protocol entity within a

user device. ■ Unique within TE1.

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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.

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DATA LINK LAYER

• C/R – Whether command (“0”) or response(“1”) frame.

• 8th 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.

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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..

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neNetwork 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—-

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neCall 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.

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neCall 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.

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neMessage 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.

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neCall 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.

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neCall 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.

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neCall 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.

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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.

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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.

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neInformation 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.

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neAddressing 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.

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Bit Rates for Different Applications