Multiplex communications – Channel assignment techniques – Carrier sense multiple access
Reexamination Certificate
1997-09-26
2001-08-07
Chin, Wellington (Department: 2664)
Multiplex communications
Channel assignment techniques
Carrier sense multiple access
C370S461000
Reexamination Certificate
active
06272147
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to data networks.
In the last 20 years, Local Area Networks (LANs) have revolutionised the way in which computers have been used in the work place, and have allowed personal computers (“PC”s) and the client/server paradigm to replace to a large extent the mainframe/terminal technology prior to LANs. In a similar way, in the last 10 years, the MIDI network has revolutionised the production and performance of music, and created a huge market in MIDI-compatible equipment.
In a similar way, a home LAN could help to revolutionise home equipment, by bringing together computing and electronic entertainment. Such a network might link around the home digital TVs, a set-top box, digital VCR, CD player producing digital audio, PC, printer and ISDN router. Currently, there are multiple standards bodies examining home LANs, but no really strong contender for home LAN technology has emerged, and most proposals seem to have some drawbacks. The proposals tend to concentrate on carrying just video traffic and are not compatible with the Ethernet interfaces which are already installed or being installed in large numbers in the home, to connect PCs to printers and ISDN routers. These proposals ignore the fact that Ethernet is very widespread.
SUMMARY OF THE INVENTION
The present invention, or at least certain embodiments of it, is concerned with enabling Ethernet compatible video transmission, allowing the same network to be used for piping video around the home as the transport of computer data, so that existing Ethernet cards of the home network can be used without modification. The present invention, or at least certain embodiments of it, achieves this by enabling video and the like to be transmitted on the Ethernet with higher priority. The present invention, or at least certain embodiments of it, is also concerned with allowing video to be transmitted with very low jitter. This is particularly important in interfaces to home appliances, where the extremely cost sensitive market means only minimal buffering at the receiver can be afforded to overcome network jitter. The present invention, or at least certain embodiments of it, is also concerned with providing a protocol which is simple, and/or implementable purely in hardware, and/or robust.
In accordance with a first aspect of the present invention, there is provided a method of operating a data network having a bus and a plurality of stations operable to transmit data on the bus, wherein: those stations which wish to transmit on the bus have respective rankings (e.g. 1, 2, 3, . . . ) and perform a contention resolution process whereby they transmit in cycles and within each cycle they transmit in an order determined from their rankings; and each station which wishes to transmit further data immediately after it has transmitted data in a current cycle refrains from the contention resolution process until it has determined that the current cycle has been completed. Thus, this aspect of the invention allows the stations to transmit in turn. It will be appreciated from the following description that not all of the stations on the network are necessarily arranged to operate in accordance with this aspect of the invention, but only what will hereinafter be referred to as “high-priority” stations, such as those transmitting digital video and audio.
It should be noted that in this specification, unless the context otherwise requires, “bus” is not limited to a bus employing a cable connection, and also includes any kind of shared communications channel, such as wireless buses employing, for example, radio or infra-red as a means of communication.
Preferably, the contention resolution process performed by each station prior to transmission of data on the bus comprises the steps of: transmitting a jam on the bus for up to a period of time dependent on the current ranking of that station; detecting whether there is a collision on the bus while transmitting the jam; and, if not, enabling transmission of data; and each station is operable to perform a fairness process, which comprises the steps of: detecting whether one of the stations has transmitted data succeeding a jam without a collision; and, if not, disabling its contention resolution process until such a detection has been made. This therefore provides an easily implementable protocol for contention resolution and fairness.
In order to deal with the possibility that a station may have taken part unsuccessfully in contention resolution, but then some change occurs which means that it no longer requires to transmit data, for example if that station is switched off, the fairness process of each station preferably comprises the further steps of: detecting whether its contention resolution process has been disabled for a predetermined period of time; and if so, re-enabling its contention resolution process.
The ranking of each station may be fixed, and this may provide reasonably efficient contention resolution if all of the stations are active all or most of the time. However, in order to allow the efficiency of the contention resolution process to adapt as stations become active and inactive, each station preferably performs a ranking allocation process prior to an initial transmission on the bus, which comprises the steps of: determining, from the bus, rankings currently in use by the other station(s); and allocating to itself a current ranking to one extreme of the determined rankings. In this case, the ranking allocation process performed by each station preferably comprises the further steps of: determining whether its current ranking is the same as that of the other station, or another of the stations; and, if so, recommencing its ranking allocation process. In order to deal with the problem that, if the error comes to light at the time of contention resolution, the rankings reallocated by the stations may also be the same, preferably the method further comprises the step of waiting for a pseudo-random period of time before the recommencement of the ranking allocation process. In order to reduce the delays which take place during contention resolution for stations with particular (e.g. “high”) rankings, the ranking allocation process performed by each station preferably comprises the further steps of: detecting whether the ranking next to that station's current ranking in the direction towards the other extreme of the rankings (e.g. the lower direction) continues to be in use; and, if not, re-allocating to itself that other ranking as its current ranking.
A second aspect of the present invention provides a data network comprising a bus and a plurality of stations operable to transmit data on the bus, the stations being arranged to perform a method according to the first aspect of the invention.
A third aspect of the present invention provides a station adapted to form part of a network according to the second aspect of the invention.
As mentioned above, the stations may be thought of as high-priority stations, and the method of the first aspect of the invention may also involve at least one low-priority station which is operable to transmit data on the bus, the low-priority station(s) performing a different contention resolution process (for example in accordance with IEEE standard 802.3 CSMA/CD) to the high-priority stations.
In the case where the or each low-priority station is operable to perform a contention resolution process prior to transmission of data on the bus, the contention resolution process commencing when that station does not sense any activity on the bus, or a predetermined period of time after that station senses a transition from carrier to silence on the bus, or a predetermined period of time after that station has finished transmitting data, each high-priority station is preferably operable, immediately after transmission of data, to transmit a filler on the bus for a period of time exceeding said predetermined period(s) of time for the low-priority station(s).
This latter feature may be employed in methods other th
Sharon Oran
Spratt Michael Peter
Chin Wellington
Duong Frank
Hewlett--Packard Company
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