Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1999-11-30
2001-01-30
Felber, Joseph L. (Department: 2732)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S503000, C370S508000
Reexamination Certificate
active
06181716
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to cable modems. More specifically, the present invention relates to voice transmission over cable modems. The Data Over Cable Systems Interface Specification (DOCSIS) describes a standard for the cable modem interface between a cable TV operator and a computer. DOCSIS has been accepted as the standard for devices that handle incoming and outgoing data signals across this interface since DOCSIS 1.0 was ratified by the International Telecommunication Union (ITU) in March of 1998. Cable modems conforming to DOCSIS are available in many areas where cable operators operate.
The DOCSIS 1.1 specification provides for Unsolicited Grant service as one of several data transmission mechanisms for grabbing bandwidth for packets. This service is set up ahead of time between a cable modem and head-end equipment (CMTS), and is parameterized by the desired data packet arrival rate, packet size, and jitter tolerance. Once set up, the cable network head-end equipment will give unsolicited transmission opportunities (grants) to the cable modem at the requested rate. This service provides an efficient mechanism for transporting voice packets but does not provide for maximizing voice quality.
Unlike callers in a circuit-switched telephone network who are allocated a dedicated connection, cable modem users do not occupy a fixed amount of bandwidth during their online sessions but instead share the network with other online users, only using network resources when sending and receiving data in quick bursts. Users typically grab what they need from the available bandwidth at the instant they need to upload their packets. Delays in transmitting data packets do not affect data quality. However, for voice packets, delays in transmission degrade voice quality so there is a need to closely time voice packet availability with opportunities for transmission, especially in cable networks.
DOCSIS is an evolving standard which specifies modulation schemes and protocols for exchange of bidirectional signals over cable, allowing version 4 IP traffic to achieve transparent transfer between the Cable Modem Termination System-Network Side Interface (CMTS-NSI) and the Cable Modem to Customer Premise Equipment Interface (CMCI). Voice over IP (VoIP) is a new service capable of being supported by DOCSIS cable networks. Upgrades to existing cable modems and DSPs to maximize VoIP quality can be achieved by changing the programming in their EEPROM flash memory.
SUMMARY OF THE INVENTION
The present invention relates to a method for synchronizing packet availability with the Unsolicited Grant data transfer opportunities in a DOCSIS 1.1 cable. The goal of this synchronization is to ensure that packet transmission delay in a cable modem is minimized by having the voice or data processed, packetized and ready to be transmitted by the cable modem as close to the grant opportunity as possible. This is an important consideration in voice over packet networks in general, and voice over cable networks specifically, due to the impact of delay on voice quality.
The major issue with implementation of packet/grant synchronization is that the module generating packets (digital signal processor or DSP) is typically separate from the module handling the cable data transmission/reception (cable modem). Thus, any synchronization scheme will need to account for latencies as well as minimizing packet wait time in issuing re-synchronization instructions, especially when the different modules reside on different physical hardware and are not running synchronized clocks. This invention describes a mechanism that is largely independent of these latencies, thus facilitating implementation.
Other advantages and features of the invention will be made apparent in connection with the following detailed description.
The present invention synchronizes availability of packets for transmission with the Unsolicited Grant data transfer opportunities in a DOCSIS 1.1 cable.
The present invention obtains a moving average of packet transmission waiting time in a DOCSIS 1.1 cable modem.
The present invention provides an apparatus for re-synchronizing packet transfer between a DSP and a cable modem that samples packet wait times and re-synchs if this time exceeds a presettable tolerance.
The present invention provides an apparatus for re-synchronizing packet transfer between a DSP and a cable modem that reaches a steady state within few sequential re-synchronization requests from a cable modem to the DSP in a DOCSIS 1.1 cable.
An exemplary embodiment of the invention, as described below includes a low pass filter and a device driver that samples the filter output, for a DOCSIS 1.1 cable modem such that the device driver determines average packet waiting time and maximum propagation delay and communicates their difference to the DSP as a re-sync amount. The DSP then re-syncs packet transfer by delaying transmission of the next packet in sequence by the re-sync amount. Thereafter, the DSP sends packets continuously, separated in time by the grant period. The net result is that subsequent packets arrive at the modem just-in-time for transmission over the cable, i.e., within a small time tolerance prior to the unsolicited grant opportunities in a DOCSIS 1.1 cable.
REFERENCES:
patent: 6064652 (2000-05-01), Buckland et al.
Felber Joseph L.
Levy & Grandinetti
Telogy Networks Inc.
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