The real power in an Integrated Services Digital Network or ISDN is the fact that the network can switch.
This inherent capability in an ISDN is often overlooked, especially in Australia where it enjoys a service known as the “semi-permanent circuit” (SPC).
An SPC is a permanently connected transmission facility or call between two end points across the public ISDN. In Australia the carrier, Telecom Australia, implemented SPCs to provide a leased line or tie line service across the ISDN.
The intent was to encourage the use of the public ISDN by providing a service that looked like a leased line, thus providing dedicated and switched capacity on the one termination.
This strategy proved to be extremely successful for Telecom Australia, with ISDN and the SPC becoming very popular services.
Much of the popularity has been due to the fact that many organisations adopted SPCs to do exactly what they had been doing pre the availability of the public ISDN.
In most cases, this involved running the corporate voice/data Bandwidth Managers across ISDN SPCs. In fact many Bandwidth Managers are connected to the public ISDN via Jtec’s equipment.
In order to understand the real power behind an ISDN, it is important to understand the concepts behind “Bandwidth Management”.
What is Bandwidth Management?
ISDN is built on a set of standards, especially signalling standards. Through this signalling capability, a device such as the Jtec J1000 series can signal to the public network — the carrier, and request transmission capacity between two locations.This is analogous to picking up the phone and making a call across the Public Switched Telephone Network (PSTN).
Prior to the advent of internationally agreed signalling standards, from whence ISDNs were born, many organisations had the desire to create private networks that could integrate voice and data communications onto facilities leased from the carrier, essentially they bought “pipes” through the network.
The motivation was simple, typically the carrier could achieve economies of scale with higher speed pipes, and if you could get a device that could interface voice and data onto the one high speed pipe then substantial savings could be realised over time. The concept of Bandwidth Management arrived.
The Bandwidth Manager products connected to fixed transmission capacity leased from the carrier, and typically provided voice and data interfaces to the “end user” devices. What is important to note is that the Bandwidth Manager did not actually implement and switching. That was done through the end user devices.
The Bandwidth Manager could however react to failure conditions in the transmission network with pre-programmed responses. For example, in a triangulated network, with sites A, B and C, if a transmission failure occurred between A and C, traffic between A and B could be “bumped”, and traffic to C re-routed via B.
This solution to a network transmission failure was marketed as “priority bumping”. Through priority bumping, traffic of lower priority was “bumped” (discarded) in order to make room for traffic of higher priority.
Some bandwidth managers catered for up to 15 levels of priority. The Bandwidth Managers accomplished their ‘magic’ by implementing a proprietary signalling scheme between their network nodes.
In sites where the traffic was deemed to be extremely critical, duplicated transmission facilities between sites were leased from the carrier. This was however, expensive. The goal was to provide the private network with a high degree of “meshing” or alternative paths between sites.
The private network was in fact sized on a “Just-In-Case” scenario, that is purchasing excessive bandwidth that was very unlikely to be used to allow for the rare “disaster” situation.
The Bandwidth Manager’s approached the problem of networking from the “pipe” perspective, that is they leased transmission from any source, and these devices additionally had to be attuned to synchronous clocking issues — clock fall-back hierarchy, pleisochronous clocking, what happens if the network gets partitioned, etc.
Some bandwidth managers provided this better than others, and the vendors competed with one another based on how flexible their clocking strategies were.
Bandwidth Management networks became large and complex, and therefore required complex network management and administration tools in order to make them a maintainable infrastructure.
Additionally, large numbers of network engineers were required to run the network. In many cases, these resources were required at several locations — every location, in some cases.
Some Bandwidth Managers are connecting to the ISDN today, however they are using the ISDN as just another source of transmission capacity.
Bandwidth Management and ISDN
An ISDN is a Bandwidth Management network because the backbone network can switch.While the public ISDN has the features of a Bandwidth Management network infrastructure, the challenge is to access it and leverage off its capabilities. This is precisely what Jtec’s J1000 series Access Controllers were designed to do.
The J1000 series Virtual eXchanges are designed to apply the principles of an ISDN to networks of Dedicated Data pipes, to create a Private ISDN, or a Bandwidth Management network with the intelligence of the Public ISDN.
While a failure in transmission infrastructure required the Bandwidth Manager to perform “bumping”, all that the J1000 has to do is request capacity from the carrier to handle this transient condition by placing a switched call.
For the traditional data communications speeds, (48kbit/s or below), the J1000 series can do this automatically if an SPC fails. The ISDN nodes (carrier exchanges or the Virtual eXchange) re-route around any failed link. As a result, no traffic has been “bumped” and no service level is degraded.
Additionally, it should be remembered that the carrier’s inter-exchange network (the IEN) which is the backbone network of the public ISDN, is a heavily meshed transmission network.
When a J1000 series accesses the ISDN, the users get a “free ride” on Telecom’s redundancy that is “in-built”. This is redundancy that the J1000 user does not have to budget for.
If redundancy is required in the customer access network (CAN), the J1000 can be configured with multiple ISDN interfaces, Primary Rate or Basic Rate or both. In the case of the traditional Bandwidth Manager, transmission redundancy if required, had to be implemented end to end. With ISDN all you have to do is put it in the access network. Redundancy in the IEN was done for you — free, by the carrier.
Voice traffic is handled much more elegantly between the sites. Capacity can be “nailed up”, as is the case with the traditional Bandwidth Managers to cater for voice, or voice can be handled on a demand basis by placing a switched call.
The J1000 understands all of the standard voice signalling eg: Loop, E&M, Channel Associated signalling etc. and can translate these to the signalling required by the ISDN. A switched call is then placed end to end.
Whereas the Bandwidth Management Networks are configured for a “Just-In-Case” scenario, the J1000 / ISDN combination can be configured for a “Just-In-Time” situation. Base load requirements can be handled by pipes, leased or SPC, while peak transmission requirements are handled by switched connections. This is just not possible with a traditional Bandwidth Manager.
The motivation is cost savings. The network does not have to be sized to cater for peaks that may happen twice a day or week or whatever. Having capacity infrastructure in place which is not in use for most of the time is expensive.
As the ISDN is an intelligent switching infrastructure, and all you have to do is access it, technical challenges such as the clocking issues that be-devil a traditional Bandwidth Management approach do not become an issue.
The reason is that the ISDN manages all of the clocking issues for you. All that the J1000 series has to do is extract the timing from the carriers back bone network — the exchanges. The carrier manages the backbone network, and that relieves the user from that onerous task.
Still further, because it is the carrier that is managing the backbone, the demands placed on the user network for complex network management tools and large numbers of engineers is greatly reduced. This is as a result of the problem being reduced in size.
The network management tools required are for an access network node — the J1000 series, and not a backbone network node such as an AXE exchange.
While the networking issues are reduced in size, they are also greatly reduced in terms of complexity. As a result, much larger networks can be contemplated without the network management tool and personnel costs becoming cost prohibitive — as can occur with the traditional Bandwidth Management Networks.
Itinerant traffic, switching, ease of management, ease of maintenance, “Just-In-Time” instead of “Just-In-Case” makes the ISDN based Bandwidth Management the infrastructure of choice.
Intelligent accessing of the public ISDN then becomes the issue. Here the J1000 series of ISDN Access Controllers and Virtual eXchanges have no peer.
Applications
Central site hub
Many would-be users of ISDN have so far been put off migrating to large dial-up networks based on the technology. Their objection has been a simple but nonetheless persuasive one – while ISDN Primary Rate Access offers a lower cost, more technically elegant central site connection, no equipment has existed to interface directly to it.
The J1000 can utilise leased circuits (ISDN SPC, E1 or Fractional E1) for its base load requirements and the ISDN for overflow, or peak load requirements with the Virtual eXchange option.
Additionally, the security of dial-up connections is ensured using the J1000’s Calling Line Identification VErification (CLIVE) option.
Channel Aggregation
Looking to satisfy a need for bandwidths mid-way between 64kbit/s and 2Mbit/s. – For example in LAN internetworking, videoconferencing and CAD/CAM applications – users often want to aggregate two or more 64kbit/s channels.
Currently the J1000 is able to combine up to eight 64kbit/s channels to the Australian standard – giving 504kbit/s of bandwidth including channel overhead.
Already widely used as the back-up medium of choice for leased digital circuits, ISDN is also a powerful vehicle for connections to the third party disaster recovery (DR) centres now maintained by many larger corporate organisations.
The J1000 is the equipment of choice for the DR centre, enabling ISDN PRA incoming connections to be used in favour of the less economical and more equipment space-hungry ISDN BRA or redundant leased circuit alternatives. This is especially valid given that many DR facilities will serve multiple clients.
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