MoIP: Making PSTN Modems Work on IP Networks

MoIP: Making PSTN Modems Work on IP Networks
The move to an all-Internet protocol (IP) connection seems like a logical solution for the voice world. With that in mind, developers have spent the last few years tweaking and perfecting IP technology so it can more effectively handle voice traffic. But voice is not the only type of data traveling over packet networks. For example, in enterprise networks, corporations use these same lines for linking up fax machines and modems.

The problem was identified for fax machines several years ago and resolved by a standard called ITU-T T.38 fax relay. Other types of communication equipment, such as modems, had not been previously addressed by the standards community.

Fortunately, the landscape is changing with the help of the International Telecommunication Union (ITU). Recently, the ITU approved V.150.1, which helps bridge the gap between modems and the IP world by allowing designers to build modem-over-IP (MoIP) solutions. Let's take a deeper look at the key elements that make up V.150.1.

Moving Away from the PSTN
The V.series modem standards (V.90, V34, etc.) created by the ITU are designed for use over the PSTN. The maturity of the PSTN and associated modem technology has created a stable environment where modem performance is reliable over the full suite of PSTN impairments. This stable setting has been upset by the introduction of IP packet-based networks.

During the development of V.150.1, the ITU's main goal was to have the packet networks seamlessly enhance or replace circuit-switched networks. For a packet network to be a ubiquitous replacement to the circuit-switched PSTN, it must reliably carry all types of voice band traffic, i.e. generated by terminals for facsimile, ISP access, remote access to corporate networks, point-of-sale and text terminals.

The impairments of a packet-based network are fundamentally different than those of the PSTN.. PSTN impairments are essentially noise superimposed on the modem signal. The signal quality may vary dramatically on the PSTN but the delay is constant and the signal is always present. Modems are specifically designed to operate in networks with impairments that degrade the signal to noise ratio (SNR).

Packet network impairments include packet loss, jitter, and delay. These impairments are especially harmful to modems because modems were designed to work solely on the PSTN, which doesn't have these types of impairment.

There are two methods to transport voice-band modems over packet networks:

1. Pass-through: The voice band modem traffic is transported over the packet network in a specific configuration of a G.711 VoP channel.
2. Relay: Modulation and re-modulation techniques are used to transport the voice band modem traffic over the packet network.

In November 2000, the ITU-T was approached to develop a solution to the voice band modem problem, much in the same manner as it did for T.38 fax relay. From January 2000 until December 2002, voice-band modem and IP transport experts collaborated to develop a solution that integrates both the pass-through and relay functions into a single standard. This solution, now called V.150.1, was approved in January, 2003.

The goal for MoIP is to ensure that connectivity and performance over an IP network is as good as the connection over PSTN. MoIP addresses the pass-through method for transport of appropriate audio encoding of modem signals between gateways by providing a voice band data mode (VBD), and the transport method using a demodulation/re-modulation scheme called universal modem relay (MR). Let's look at both.

Understanding Pass-through, Universal MR
Modem pass-through transports the modem signal as voice, but the voice channel may be reconfigured from the original voice settings to a setting conducive for the transport of modem signals. This method, as described in V.150.1, defines the channel such that many of the impairments can be removed, but packet loss and bandwidth usage are still problematic.

The MR mode of operation is characterized by the termination of both the physical layer and error-correction functions at the gateway. Universal MR supports virtually all deployed modems, including high-speed modems and the low-speed modems used by applications such as Point of Sales Terminals, and modem over wireless.

Figure 1 illustrates the various layer termination points for the single transcompression type of MR. In this figure, designers can see that the modulations are terminated at the gateway. The figure also shows the error correction (EC) layer.

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

In the universal MR mode, data compression is distributed between the gateways in an efficient manner that allows data compression over the packet network but only requires the resources of a single compression or decompression engine. The solution is flexible as well; the data compression capabilities of the end-point modems may be the same or may be different. The compression parameters such as dictionary, codebook, or history sizes may or may not be the same. If the negotiated end-to-end compression parameters are the same, transcompression can be disabled and thus become a transparent pass-through. On the other hand, if the data compressions are different, the gateway will transcompress and allow both modems to use their preferred compression schemes.

Figure 2 (extracted from V.MoIP) shows the gateway reference model used in MoIP. This reference model illustrates the three entities that make up a MoIP Gateway. There is the modem, IP-network interface with the IP transport layer, and media switching protocol, which are shown on the left and right. Above them is the MoIP application.

Figure 2: Diagram of the gateway reference model used in the MoIP specs.

As Figure 2 points out, the MoIP application contains the control and functionality as defined in V.150.1 to manage the modem connection and the telephony and IP networks. These include the modem call discrimination, modulation selection, error control, and compression management.

MR vs. Pass-through: A Comparison
While V.150.1 provides a set of standard ways for linking modems with the IP world, it does leave some questions unanswered. The biggest, and most controversial, is whether it's better to use the pass-through method or MR approach for sending modem traffic over an IP link. Let's look at the issue further.

The performance of modem relay and modem pass-through can be characterized by the call success rate (CSR) of end point modem as well as the abnormal disconnect rate. Depending on the packet loss of packet networks, the number of modem calls successfully connected to a modem on the other side can be very low.

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

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

One of the advantages of the MR mode is its ability to adapt the amount of IP bandwidth it consumes to the amount of real information that is to be transported. In universal MR, packets are only transported when there is information to be sent. When there is no information, packets are not transmitted, as opposed to G.711 pass-through which is always passing packets at 64,000 bit/s.

To demonstrate the differences in bandwidth utilization between pass-through and MR, let's look at a typical IP transport application. The average utilization for applications such as Internet browsing is 12.5% based upon an average throughput rate of 7Kbit/s. G.711 pass-through transports a modem signal as audio encoded samples across a network at a constant rate of 8,000 eight-bit samples per second. This equates to a constant rate of 64,000 bit/s. If reliability against IP impairments is included by means of redundancy, IP bandwidth requirements can double or even triple when using the pass-through method. However, since universal MR transports effective user data across a network, the IP bandwidth requirements will drop.

Figure 4 characterizes the amount of IP bandwidth savings for different modem modulations compared to G.711 pass-through. The characteristics are based upon a scenario where for only 25% of the time (considered heavy usage) real data is being transported and where one time redundancy is added for reliability. The amount of IP bandwidth savings can be dramatic even for this typical scenario (90% for V.90 downstream and 99.5% for V.22 bis 2400 bps), as illustrated in Figure 5.

Figure 4: Through the universal MR mode, MoIP system designers can dramatically reduce bandwidth requirements in a system design.

Figure 5: Percentage of bandwidth saved using universal MR.

Additional Protocols
In addition to the pass-through and MR modes, the V.150.1 spec defines two other protocols that will ease the movement of modem traffic of IP links. These include: the simple packet relay transport (SPRT) protocol and the state signaling event protocol (SSEP).

1. SRPT: One of the main characteristics of modem relay as defined in V.150.1 is the use of a new reliable IP transport protocol (IP-TLP) called SPRT. SPRT is a windowed error correcting protocol that has its origins in V.42. Optimized for use on IP networks, SPRT gives the means to control IP bandwidth and guarantees the reliable transport of demodulated data across an IP network. SPRT is packet preserving, uniquely identifiable and seamless with RTP, has low latency, and is lightweight in implementation.

2. SSEP: During the modem discrimination, there are a number of strict timing requirements, in terms of detection and response, that need to be met in order to prevent call failure. Also, the MoIP gateway needs to be able to switch between different operational modes on the same UDP port. The mechanism to achieve this is called the SSE. SSE is an in-band mechanism that operates like and is distinguishable to RFC 2833 events.

Wrap Up
While most of the networking world focuses on IP-enabling voice services, the ITU has quietly put together an effective spec for bridging modems and IP connections. Through V.150.1 and universal modem relay, designers can effectively fill the gap left in the move to IP, connecting modems, text devices, and more to the IP infrastructure, thus allowing companies to make a true leap to an all-IP network.

About the Authors
Keith Chu is the manager of modem digital signal processing at Mindspeed Technologies. He is the Rapporteur of ITU-T Question 11/16 and the Chairman of TIA TR30.1. Keith is also the editor of V.150.1 Keith can be reached at keith.chu@mindspeed.com.

Michael Metzger is the director of product line management for access products at Mindspeed Technologies. He received his master's degree in electrical engineering from University of Ulm, Germany. Michael can be reached at michael.metzger@mindspeed.com.