Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1997-11-25
2001-01-23
Nguyen, Chau (Department: 2739)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S523000
Reexamination Certificate
active
06178185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data communications over a telephone network, and more particularly to a network interface device, method, and article of manufacture providing for high bit rate data communication over robbed bit trunks.
2. Description of the Prior Art
Recently V.pcm modem technology has emerged which uses pulse amplitude modulation (PAM) techniques to transfer data at 56,000 bits per second. This technology is based on the premise that the 56K transmitter is digitally connected to the public telephone network and can control the pattern sent to the telephone network codec (digital to analog converter) driving the analog link to the receiver.
The digital portion of the telephone network transfers data as 8,000 8-bit samples per second, or 64,000 bits per second per channel (i.e., one channel of a T1 trunk using a DS-1 structure). However, due to a variety of factors, prior art transfer rates are limited to 56,000 bps. This leaves 8,000 bits per second capacity that does not get utilized for communication.
There are two main reasons why current technology is limited to 56,000 bits per second. The first concerns granularity and discrimination. These 56K modems use Pulse Amplitude Modulation (PAM) for transmission with eight bits per sample. Current signal processing technology cannot resolve the least-significant bit in the recovered signal, so only 128 different codes are sent, out of 256 possible codes, lowering the maximum bit rate to 56,000 from 64,000 bits per second. This problem is particularly difficult with codes corresponding to low signal amplitudes, since telephony codecs use a logarithmic (&mgr;-law) characteristic. For these codes, not only are the absolute levels lower, but discrete levels are closer together, compared to higher amplitudes. This problem does not apply to the ISDN technology where no telephony codec is involved.
The second reason why current technology is limited to 56,000 bps is so-called “robbed-bit” signaling. On certain trunks, the digital portion of North American telephone systems ‘robs’ the least-significant bit (LSB) of every sixth sample, replacing it with signaling information. Signaling generally means the passing of call related information such as hookswitch status, address, and number being called. This signaling information does not change during the useful portion of a telephone call and thus the robbed-bit signaling results in having the bit stuck or set to one value, and hence correct half the time. This is unnoticed in voice telephone conversations, and only marginally affects conventional (QAM, DPSK, FSK, etc.) modems, but irrevocably corrupts PAM transmissions. With robbed bit signaling even though only one bit out of every six samples is used for signaling, there is no way to communicate between ends which bit is corrupt, so all LSBs must be unused.
Robbed-bit signaling is also a problem for ISDN links where both ends of a connection are digitally connected. ISDN devices coupled through a robbed-bit trunk can only transfer data at 56,000 bits per second, avoiding all use of the LSB, even though only one bit out of six samples is corrupt. Therefore, maximum throughput for ISDN devices coupled through a robbed-bit link is 56,000 bits per second. Thus, robbed-bit signaling limits throughput on ISDN systems using a robbed-bit trunk.
These unresolved problems and deficiencies are clearly felt in the art and are solved by the invention in the manner described below.
SUMMARY OF THE INVENTION
It is an object of the invention to provide high data access to the internet from a host computer.
It is yet another object of the invention to provide a modem capable of high speed access to the internet using standard telephone lines.
It is yet a further object of the present invention to provide a network interface device that supports highest possible data communications over robbed trunks.
It is an object of the invention to increase the maximum possible transmission rate over robbed-bit lines by identifying which frame positions may be corrupt, and avoiding only those positions for a pair of devices attached through the Public Switched Telephone Network, where at least one of the devices is attached digitally.
It is a further object of this invention to increase maximum possible transmission rates by identifying when no bits are robbed, for a pair of devices attached through the Public Switched Telephone Network, where at least one of the devices is attached digitally.
The above-mentioned needs have been met in accordance with the present invention by providing a communication system for increasing data throughput on robbed bit trunks. A method, apparatus and article of manufacture for transmitting and/or receiving data from a robbed trunk with higher data throughput is provided. A communication system is provided having two network interface devices that are connected over a path that includes a digital trunk using robbed-bit signaling. The network interface devices use a technique to identify the presence of robbed-bit signaling. The network interface devices synchronize to bytes that are subject to robbed-bit signaling. The network interface devices must synchronize to the robbed-bit signaling to avoid transmitting information or receiving information in these signaling positions. The network interface devices then take advantage of the least significant bit positions of frames that are not used for robbed-bit signaling. This results in an increase in data carrying capacity from 56 kps to 62.7 kps.
REFERENCES:
patent: 4571736 (1986-02-01), Agrawal et al.
patent: 4937839 (1990-06-01), Motley et al.
patent: 5038366 (1991-08-01), Motley et al.
patent: 5392291 (1995-02-01), Berkovich et al.
patent: 5408524 (1995-04-01), Reum
patent: 5515426 (1996-05-01), Yacenda et al.
patent: 5528679 (1996-06-01), Taarud
patent: 5640387 (1997-06-01), Takahashi et al.
patent: 5761247 (1998-06-01), Betts et al.
patent: 5793809 (1998-08-01), Holmquist
patent: 5859872 (1999-01-01), Townshend
patent: 5875229 (1999-02-01), Eyuboglu et al.
patent: 5995558 (1999-11-01), Betts et al.
Flynn John D.
International Business Machines - Corporation
Nguyen Chau
Nguyen Danny
LandOfFree
Network interface device, method and article of manufacture... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Network interface device, method and article of manufacture..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Network interface device, method and article of manufacture... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2460782