Moip: making pstn modems focus on ip systems

The proceed to an exciting-Ip address (IP) connection appears just like a logical solution for that voice world. Knowing that, developers have spent the final couple of years tweaking and perfecting IP technology therefore it can better handle voice traffic. But voice isn’t the only kind of data traveling over packet systems. For instance, in enterprise systems, corporations begin using these same lines for linking up fax machines and copiers and modems.

The issue was identified for fax machines and copiers in the past and resolved with a standard known as ITU-T T.38 fax relay. Other kinds of communication equipment, for example modems, was not formerly addressed through the standards community.

Fortunately, the landscape is altering with the aid of the Worldwide Telecommunication Union (ITU). Lately, the ITU approved V.150.1, which will help bridge the space between modems and also the IP world by permitting designers to construct modem-over-IP (MoIP) solutions. Let us have a much deeper consider the important elements that comprise V.150.1.

Leaving the PSTN
The V.series modem standards (V.90, V34, etc.) produced through the ITU are equipped for use within the PSTN. The maturity from the PSTN and connected modem technologies have produced a reliable atmosphere where modem performance is reliable within the full suite of PSTN impairments. This stable setting continues to be upset by the development of IP packet-based systems.

During the introduction of V.150.1, the ITU’s primary goal was to achieve the packet systems seamlessly enhance or replace circuit-switched systems. For any packet network to become a ubiquitous substitute towards the circuit-switched PSTN, it has to reliably carry all kinds of voice band traffic, i.e. generated by terminals for facsimile, ISP access, remote use of corporate systems, point-of-purchase and text terminals.

The impairments of the packet-based network are essentially diverse from individuals from the PSTN.. PSTN impairments are basically noise superimposed around the modem signal. The signal quality can vary dramatically around the PSTN however the delay is constant and also the signal is definitely present. Modems are particularly made to be employed in systems with impairments that degrade the signal to noise ratio (SNR).

Packet network impairments include packet loss, jitter, and delay. These impairments are specifically dangerous to modems because modems specified for to operate exclusively around the PSTN, which does not have these kinds of impairment.

There’s two techniques to transport voice-band modems over packet systems:

  1. Pass-through: The voice band modem visitors are transported within the packet network inside a specific configuration of the G.711 VoP funnel.
  2. Relay: Modulation and re-modulation techniques are utilized to transport the voice band modem traffic within the packet network.

In November 2000, the ITU-T was contacted to build up a strategy to the voice band modem problem, much very much the same because it did for T.38 fax relay. From The month of january 2000 until December 2002, voice-band modem and IP transport experts collaborated to build up an answer that integrates both pass-through and relay functions right into a single standard. This solution, now known as V.150.1, was approved in The month of january, 2003.

The aim for MoIP is to make sure that connectivity and gratifaction over an IP network is just like the bond over PSTN. MoIP addresses the pass-through way of transport of appropriate audio encoding of modem signals between gateways by supplying a voice band data mode (VBD), and also the transport method utilizing a demodulation/re-modulation plan known as universal modem relay (MR). Let us take a look at both.

Understanding Pass-through, Universal MR

Modem pass-through transports the modem signal as voice, however the voice funnel might be reconfigured in the original voice settings to some setting favorable for that transport of modem signals. This process, as described in V.150.1, defines the funnel such that lots of the impairments can be taken off, but packet loss and bandwidth usage continue to be problematic.

The MR mode of operation is characterised through the termination of both physical layer and error-correction functions in the gateway. Universal MR supports almost all deployed modems, including high-speed modems and also the low-speed modems utilized by applications for example Reason for Sales Terminals, and modem over wireless.

Figure 1 illustrates the different layer termination points for that single transcompression kind of MR. Within this figure, designers can easily see the modulations are ended in the gateway. The figure also shows the mistake correction (EC) layer.

Figure 1: Diagram detailing the transcompression layer kind of MR.

Within the universal MR mode, data compression is shipped between your gateways within an efficient manner that enables data compression within the packet network only necessitates the sources of merely one compression or decompression engine. The answer is flexible too the information compression abilities from the finish-point modems could be the same or might be different. The compression parameters for example dictionary, codebook, or history sizes might or might not function as the same. When the negotiated finish-to-finish compression parameters are identical, transcompression could be disabled and therefore be a transparent pass-through. However, when the data compressions will vary, the gateway will transcompress and permit both modems to make use of their preferred compression schemes.

Figure 2 (obtained from V.MoIP) shows the gateway reference model utilized in MoIP. This reference model illustrates the 3 entities that comprise a MoIP Gateway. There’s the modem, IP-network interface using the IP transport layer, and media switching protocol, that are proven around the right and left. Above them may be the MoIP application.

Figure 2: Diagram from the gateway reference model utilized in the MoIP specs.

As Figure 2 highlights, the MoIP application provides the control and functionality as defined in V.150.1 to handle the modem connection and also the telephony and IP systems. Included in this are the modem call discrimination, modulation selection, error control, and compression management.

MR versus. Pass-through: An Evaluation

While V.150.1 provides some standard methods for linking modems using the IP world, it will leave some questions unanswered. The greatest, and many questionable, is whether or not it’s easier to make use of the pass-through method or MR method for delivering modem traffic over an IP link. Let us consider the issue further.

The performance of modem relay and modem pass-through could be characterised through the call rate of success (CSR) of finish point modem along with the abnormal disconnect rate. With respect to the packet lack of packet systems, the amount of modem calls effectively linked to a modem on the other hand can be quite low.

Figure 3 shows some sample data with four anonymous modems (types A, B, C and D) connecting more than a packet network with various levels of burst packet loss. The packet loss was random between 5 and 25 packets. The chart reveals vast improvement within the call rate of success with universal modem relay.

Figure 3:Call rate of success data from 4 sample modems.

An advantage from the MR mode is being able to adapt the quantity of IP bandwidth it consumes to the quantity of real information which will be transported. In universal MR, packets are just transported when there’s information to become sent. When there’s no information, packets aren’t transmitted, instead of G.711 pass-by which is definitely passing packets at 64,000 bit/s.

To show the variations in bandwidth utilization between pass-through and MR, let us consider a typical IP transport application. The typical utilization for applications for example Internet browsing is 12.5% based on a typical throughput rate of 7Kbit/s. G.711 pass-through transports a modem signal as audio encoded samples across a network in a constant rate of 8,000 eight-bit samples per second. This means a continuing rate of 64,000 bit/s. If reliability against IP impairments is incorporated by way of redundancy, IP bandwidth needs can double or perhaps triple while using the pass-through method. However, since universal MR transports effective user data across a network, the IP bandwidth needs will drop.

Figure 4 characterizes the quantity of IP bandwidth savings for various modem modulations when compared with G.711 pass-through. The options are based on a predicament where for just 25% of times (considered heavy usage) real information is being transported where once redundancy is added for reliability. The quantity of IP bandwidth savings could be dramatic for this typical scenario (90% for V.90 downstream and 99.5% for V.22 bis 2400 bps), as highlighted in Figure 5.

Figure 4: With the universal MR mode, MoIP system designers can dramatically reduce bandwidth needs inside a system design.

Figure 5: Number of bandwidth saved using universal MR.

Additional Protocols

Additionally towards the pass-through and MR modes, the V.150.1 spec defines two other protocols which will ease the movement of modem traffic of IP links. Included in this are: the straightforward packet relay transport (SPRT) protocol and also the condition signaling event protocol (SSEP).

1. SRPT: Among the primary characteristics of modem relay as defined in V.150.1 is using a new reliable IP transport protocol (IP-TLP) known as SPRT. SPRT is really a windowed error correcting protocol which has its origins in V.42. Enhanced to be used on IP systems, SPRT provides the way to control IP bandwidth and guarantees trustworthy transport of demodulated data across an IP network. SPRT is packet preserving, distinctively identifiable and seamless with RTP, has low latency, and it is lightweight in implementation.

2. SSEP: Throughout the modem discrimination, there are a variety of strict timing needs, when it comes to recognition and response, that should be met to avoid call failure. Also, the MoIP gateway needs so that you can switch between different operational modes on a single UDP port. The mechanism to do this is known as the SSE. SSE is definitely an in-band mechanism that operates like and it is distinguishable to RFC 2833 occasions.


While the majority of the networking world concentrates on IP-enabling voice services, the ITU has silently come up with a highly effective spec for bridging modems and IP connections. Through V.150.1 and universal modem relay, designers can effectively fill the space left within the proceed to IP, connecting modems, text devices, and much more towards the IP infrastructure, thus allowing companies to create a true leap for an all-IP network.

Concerning the Authors

Keith Chu may be the manager of modem digital signal processing at Mindspeed Technologies. He’s the Rapporteur of ITU-T Question 11/16 and also the Chairman of TIA TR30.1. Keith can also be the editor of V.150.1 Keith could be arrived at at

Michael Metzger may be the director of products management for access products at Mindspeed Technologies. He received his master’s degree in electrical engineering from College of Ulm, Germany. Michael could be arrived at at


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