The Integrated Service Digital Network (ISDN)

The Integrated Service Digital Network (ISDN) was first offered to the public in the 1980s,as telephone provider began deploying their SS7 networks.With ISN, many new services could be extended to the customer premises.By using ISDN subscribers and consolidate all their trunks to one DS1 facility.The ISDN protocols provide circuit allocation within the ISDN

When additonal badwidth is needed for high speed data communication, the protocol is capable of allocating additional channel within the DS1 to carry the others calls.This is often reffered to as dynamic bandwidth allocation and is one of the principle feature of ISDN.Beside transmitting voice,ISDN is also capable of transmitting data using the same facilities as the voice

ISDN signaling uses a separates channel and is compatibel with SS7.The signaling information is handled off to the SS7 network and tranfered to the distance end using the SS7 ISDN User Part (ISUP).The ISUP was developed for all call setup adn teardown, and replaced the SS7 Telephone User Part (TUP) protocol in ANSI network

The term of ISDN was originally used to refer to the entire of IN, including SS7.This later evolved to reference only the subscriber interface.Oroginally the creator of SS7 thought of extending the SS7 network all the way to the subscribers.This was abondoned, however, over concern of security and network fraud.The solution was to create an intelligent interface, compatoble with SS7, which could offer the same services and intelligeces as the SS7 network. It was this that spuured the creation of the ISDN protocol

Perhaps the most important application for ISDN is the concept of connecting PBXs within a private network

ISDN offers many services to the subscribers.The basic lavels of service are defined as:

1.Transport elements.Allow information to be transported through the telephone service provider network and its switches, routers, multiplexer and other network equipment tranparently without alteration to the original data

2.Control elements.Support real time operations of transport capabilities (connection establishment and database queries)

3.Network managemenet elements.Provide procedures and capabilities ad administer,maintain and operate the communication infrastructure.Include provisioning of transmission facilities, fault mangement,cogestion control and administration of database and routing table

4.Communication applications environment.Provide a development for programmers from which application can be developed, using other three elements

5.Tranport.Provide the lower thredd layers of OSI, providing allocation of badwidth,routing,ralaying and error detection/correction

TO understand how ISDN can be of significant benefit to PBX networks, consider this example.Many large corporation will own several PBXs at different location.The line are often used to tie the PBXs together.This allows users to access extension in any other company location by dialling adn access number (to access teh tie line) and dialing the extension.Many digital PBXs allow callers to dial extension numbers of remote extensions without dialing an access code.Automatic routing features provide software to determine which trunk the call must be routed to

There are two classes of ISDN service: Basic Rate (BRI) and Primary Rate (PRI).BRI service provides two 64kbps bearer channel (B channels) and one 16 kbps signaling channel.This service is designed for residential and small business usage.PRI offer 23 64 kbps bearer channels (B channel) and one 64 kbps siganling channel (D channel).PRI is designed for larger businnesses with large call volume.Many PBX manufacturers already provide ISDN compatible trunking interfaces for their equimpment, making ISDN a good choice for companies who need end to end voice and data communications

ISDN cannot be succesful by itself.As we have already seen, without SS7, ISDN remains a local digital service providing a limited numbers of features and applications.With the addtion of SS7, ISDN can become an extension of the telephone network to the customer premise, offering true end to end voice and data communication woth no boundaries

The ISDN standards can be founf in the ITU-TS"I" series. The siganling standards are defined in publication Q.921 (defines the Link Access Procedure -D Channel) and Q.931 publication (defined the ISDN call control procedures at layer three) Read More..

The SS7 protocol stack

The SS7 protocol differs somewhat from the OSI model.While the OSI moden consists of seven different layers, the SS7 standar uses only four level.The functions carried out by these four levels correspond with the OSI model's seven layer.Some of the functions called for in the OSI model have no purpose in the SS7 network and are therefore undifined

It should also be noted that the functions in the SS7 protocol have been refined over the years and tailored for the specific requirments of the SS7 network.For this reason,there are many discrepancies betwen the two protocol and their corresponding functions

Regardless of the differences, the SS7 protocol has proven to be a highly reliable packet switching protocol,providing all of the services and function required bt the telephone service providers.THis protocol continues to evolve as the network grows and the services provided by the telephone companies change

1.Level one -Physical level

it is virtually the same as that of the OSI model.THe OSI model does not specify any specific interface to be used, as that will always differ from network to network.In SS7,we can specify which interface will be used, since the Bellcore standard and the ANSI standard all call for one or two types or interface - the DS0A or V.35
THe DS0A interface is the most forward for the application with the V.35 acceptable as the second choice

2.Level two - data link level

it provides the SS7 network with error detection/correction and sequenced delivery of all SS7 message packets.A with OSI model,this level is concerned only with the transmission of data from one node to the next node in the network.It does not concerned it self with the final destination of the message.As the message travels from one node to node, each node examine the dialied digits (contained in level four) and use that information to determine the next route of the message
Level two is provided the inforamation by level three which determines message routing

3.Level three-network level

it rpovides three functions;routing,message discrimination and distributin.All three functions depend on the servicee of the level two.When a mesaage is received, it is passed by level two to level three for message discrimination
Message discrimination determines who the message is addressed to.If the mesage contains the local addess (of the receiving node), then the message is passed to message distribution.If the message isnot addressed to the local node,then it is passed to the message routing function.The message routing function reads the called and dialling party address int the message to determine which physical address to route to.The called and the calling party addresses can be considered logical addresses and the physical address the node address

4.Level four-user parts

it consists of several different protocols, all called user parts and applicaiton parts.For basic telephone call connection and disconnect, the Telephone User part (TUP) or ISDN user part protocol (ISUP) are used.TUP is used in Europe and other countries following ITU-TS standards, while ISUP is used primarily in Nort America Read More..

UMA vs IMS

Inmoving beyond superficial services bundling in pursuit of truly integrated FMC services,carriers are proceesing down two distinct main technology paths:unlicensed mobile access (UMA) and IP multimedia system (IMS)

UMA is preferred primarily by mobile carriers,as it operates effectively as an extention of the mobile network,keeping all control in the hands of the mobile carrier.When a mobile subscriber with a UMA-enabled,dual mode handset moves within range of a WLAN access point, it can attemp to roam onto WLAN.The handset contacts a UMA network controller via the IP access network, and if authenticated and authorized,has it current location information registered by the mobile network,which routes further calls via the WLAN

Most industry analysts view UMA as tactical,intermediary step on the part of mobile carriers who are not yet ready to convert their extensive global system for mobile communication (GSM) and general packet radio service (GPRS) core networks to IP, an IMS prerequisite

The UMA functional architecture are:

1. mobile station
2. WLAN access point
3. Broadband IP network
4. UMA network controller (UNA)
5. UNC security gateway
6. Public land mobile network (PLMN) : mobile switching center, serving GPRS support node, AAA proxy server, HLR
7. Home PLMN (roaming case) : AAA server, HLR

IMS is prefered primarily by carriers (typically fixed) that already have extensive IP backbones in place, as it keeps call control in their hands and runs VOIP and SIP without major impact on their networks.

Its primary advantages over UMA are its scalability and applicability to a much broader range of underlying access technologies.Its basis in IP yields an ability to leverage the much wider range of applications that already exploit IP based services and interfaces.Its reliance on SIP based signalling simplifies the introduction of multiservice combinations,e.g, instant messaging plus voice

Perhaps most important,it provides a basis for more sophisticated billing of new services,allowing carriers to more quickly monetize the considerable investments they will have to make in network infrastucture, OSS upgrades and system integration services to migrate to IMS

The three layer architecture of IMS are:

1. Transport layer : core network
2. Control layer : home subscriber service and call session control function
3. Service layer : application servers

What FMC means to enterprises today

Many enterprises may look at the timeline for the arrival of true FMC services and conclude that it can be safetly ignored for now. To the extent that FMC is in the hands of carriers, this is largely true.But enterprises do have one critical element of FMC in their control today: their WLANs. Furthure, most market researchers are forecasting rapid growth over the next few years in enterprise spending on WLAN equipment.

A majority of network managers expect to voice enable their WLANs in that timeframe.Even if enterprises discount the possible arrival of FMC service, they should be scrutinizing how well ther WLAN build outs can accomodate high quality mobile voice service

As it happens,the key WLAN capabilities required to deliver mobile voice over WLAN services - granular control of QoS and security,fast voice call handoffs when raoming,zero configuration access from a wide range of devices,graceful capacity scaling,tighter integration with wired LAN infrastructure and improved WLAN security - are perfectly congruent with those needed to facilitate FMC services.Thus investments in voice grade WLAN infrastructure today can yield a second payoff if enterprise eventually migrates to FMC services Read More..