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TDM over IP

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In this 'Viewpoint' we consider the world of TDM over IP via PacketBand and how to optimise existing network investment, particularly in legacy infrastructure.

Enabling the integration of legacy equipment into modern IP networks thereby optimising the existing network infrastructure and supporting systems when expenditure is under such scrutiny, is a challenge that many organisations face. The reality is we need to 'squeeze' every last drop out of existing technology or better still, find technically advanced solutions that save us money now! One such technology is TDM over IP (Time Division Multiplexing over a packet switched network such as IP), with the ability to deliver clock-locked switched data channels across packet networks thus allowing E1 and serial based devices to communicate over Ethernet / IP networks. Our PacketBand range of TDM over IP products are helping to meet this challenge, often being used to inter-connect PBX's together. But what are Packet Networks and exactly what does PacketBand offer you?

A packet-switched network is an interconnected set of networks that are joined by routers and switches, and they've been with us now for some time. The most common packet-switching technology is TCP/IP, and the Internet is the largest packet-switched network.

The concept of a packet-switched network is that any host connecting to the network can, in theory, send packets to any other hosts. The network is said to provide any-to-any service. The network typically consists of multiple paths to a destination that provide redundancy. Packets contain header information that includes a destination address. Routers in the network read this address and forward packets along the most appropriate path to that destination.

Packet networks have no concept of clocks or synchronisation. Data is transmitted at one location and arrives at the other, at some unknown time. You will have seen when downloading files on the public Internet that the data rate varies and the arrival of the data goes faster/slower. This is due to loadings on the network and the number of devices the data has to go through. This variation is known as "jitter" or PDV ("Packet Delay Variation"). Jitter can be far more pronounced and noticeable on the Internet compared to private networks, but the concept is the same.

Data applications are designed to handle this varied and non-deterministic arrival rate. They will also normally deal with any lost information by automatically requesting a retransmission, so the Internet looks to users as though it is error-free, which it generally is not.

All of the above works brilliantly. But, not for some applications. There are many devices around that are designed primarily to work with traditional leased lines which are point-to-point non-switched circuits - just like a dedicated wire with no jitter, and such devices are very common in Local Government. Such equipment generally needs to transmit/receive data at a certain rate and maybe be synchronised with other devices so they all transmit or receive at the same rate, for example mobile phone transmitters. In these cases, the equipment expects to have a steady, accurate clock source (not the time of day but a steady "heartbeat") with which to align its transmit and receive data.

As packet networks have no concept of clocks or clocking, this raises the requirement for PacketBand to enable TDM over IP or TDM over Ethernet services. A PacketBand can be given a reliable clock source (external or internal) which is recovered by the PacketBands at the other side of the packet network. This means the attached TDM equipment receives the necessary steady clocks and all devices are synchronised together. The recovery and stability of these clocks, which is the whole basis and reason for using this technology, is a major strength of the PacketBand range.

In addition to the clocks, PacketBand also buffers the incoming data before it transmits to the attached device, so any slower packets will have arrived before their clock pulse. [Data only has one instant at which it must be passed to the attached device; miss its moment and it cannot be used again]. With PacketBand the TDM device sees what it believes to be a continuous steady clocked stream of traffic with no delays - just as if it was connected on a point-to-point wire or leased line.

Our PacketBand TDM over IP products excel in delivering stable and accurately clocked E1/T1 “leased lines” over packet networks, suitable for the most demanding applications. Clock recovery, accuracy and stability is key to many TDM applications. This clock recovery performance must be maintained when migrating to an un-clocked packet network solution. Many types of equipment expect similar performance to that of traditional leased lines which are generally referenced to the G.823 Synchronous Interface mask. The PacketBand ranges are specifically designed and optimised to excel in this area and when used on high-quality networks can meet and exceed the G.823 requirements. The clock recovery methods use a variety of mechanisms to match different network characteristics.

PacketBand is simple to configure and provides prioritised transport for all types of voice and data applications. The range includes a 2U chassis supporting 16 or 32 E1/T1 and/or V.35/X.21 circuits.


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