Modem Communications An Overview of Analog Dialup Modem Performance, Environments, and Impairments.
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EXECUTIVE SUMMARY .................................................................................................................................. 3 MODEM PERFORMANCE ............................................................................................................................... 3 CONNECT SPEED .................................................................................................................................................. 3 BANDWIDTH AND ATTENUATION.........................
Executive Summary The modem is the one personal computer device that, probably more than any other, is dependent upon an external environment (public switched telephone network, or PSTN) in order to operate. Modems are designed in such a way as to maximize their performance over these conditions – conditions which can vary considerably from one location to the next.
It is also important to note that the overall throughput of the connection is dependent not only on the connect speed that is maintained, but also upon whether there are any data errors on the connection which require that data be resent. If a modem tries to force a connect speed higher than what the prevailing line conditions will actually support, errors will be introduced and ultimately lower the effective throughput of the connection, or the call could be dropped due to excessive errors.
From this model, an industry-standard test method was derived and documented in TIA (Telecommunications Industry Association) bulletin TSB-38. Using this type of testing, it is possible to predict how well a modem may perform over a variety of network conditions, and makes it possible to perform meaningful comparisons between different modem products.
Error Correction In order to correct for any errors that may occur on the line while the modems are connected, the modems will negotiate an error-correcting protocol. In most applications, the protocol used will be LAPM, although MNP4 may also be negotiated – both protocols are specified in the ITU-T Recommendation V.42.
Central Office The Central Office, or CO, is the telephone company switching location that directly services the customer. Depending upon the size of an area, there may be several – or dozens – of CO locations to serve all of the customers and telephone lines in an area. Central office provisioning has improved considerably in the last several years, enabling such technology and features as Caller ID, voice messaging, ISDN, call forwarding/call return, and other such features.
Inter-Office Trunking Inter-office trunking is the part of the network that connects the CO facilities to each other, as well as to other network systems and facilities. This is typically done over digital fiber-optic links which are capable of carrying enormous amounts of data.
Echo Echo levels are described in terms of near-end (listener) and far-end (talker) echo levels. This is the reflection of the signal from the hybrid balances in the network, and must be cancelled out in order for the modems to work. On a normal voice call, the telephone network will utilize its own echo cancellers so that the echoes are not distracting to normal voice conversation. On a modem call, the modems will set up their own echo cancellers and do not utilize the ones on the network.
PCM (56k) Dependencies and Performance Due to improvements in the PSTN network, and the increasing number of service providers that are digitally connected to the network, it became possible to develop a technology that would extend the possible data rates of a modem call beyond 33.6K bps. The technology for doing so is referred to as PCM (pulse-coded modulation), and is fundamentally different from the technology used to achieve data rates up to 33.6K bps.
Connect Speed Explained One final note on 56K technology is that at the current time, FCC limitations on transmit power prevent the modems from being able to actually connect at 56K. FCC Part 68 regulation limit the transmit power to the network to –10dBm; in order for current PCM technologies to reach 56Kbps, the required transmit power is approximately –8dBm.
Interoperability n Data Interoperability This testing verifies that the Compaq modem is able to successfully connect with a broad variety of 3rd party modem products – including both client-end modems and host-end modem racks, concentrators an other type of high-density RAS equipment. This type of testing helps to ensure maximum compatibility with other modem products in the field. All of the applicable modulation, error correction, and data compression protocols are tested.
Bps – Bits Per Second. This is the unit of measurement used when describing data rate of a modem. CO – Central Office. This is the telco facility that directly provides telephone service to the customer. ITU-T – International Telecommunications Union. This is the international standard-setting body that publishes the V.xx Recommendations for modem communications. K56flex™ – proprietary PCM protocol developed jointly by Rockwell and Lucent. interoperable with x2 or V.90.
