Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1998-10-22
2001-01-30
Lim, Krisna (Department: 2758)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S342000, C370S348000, C370S319000
Reexamination Certificate
active
06181687
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a digital data, multi-access computer communication system where a number of stations communicate with each other over a common frequency spectrum, by sharing a sequence of transmission time intervals.
2. Description of Prior Art
With the ever increasing penetration of digital communication into everyday business and personal lives, and a demand for an ever increasing accumulation of image and graphics oriented data communication services, like remote access to corporate computer resources, telecommuting, Internet access and Web surfing, remote and interactive education and entertainment, interactive video services, etc., service providers and network operators are striving to satisfy the demand.
The only way for the vast majority of residential customers to access data services is through voice-grade telephone modems. Currently, the fastest of modems perform at transmission speeds of at most 56 kilobits per second, however, commonly actual transmission speeds are much lower. Today's modems are capable of adequately supporting only narrow-band text-oriented services like e-mail and are inadequate in supporting applications requiring much higher speeds. These new applications can only be supported marginally, if at all, by today's voice-grade telephone modems.
A new breed of communication technologies was introduced to support the higher transmission speeds required by new broadband data services. This higher transmission speed could be in excess of 1 megabits per second. Telephone service providers, Cable TV (CaTV) service providers, wireless and cellular service providers, and even electric utility service providers are exploring and developing alternative, sometimes supplemental and sometimes competing, communication technologies and networking solution offerings for their customers to assist them in gaining access to the bandwidth-demanding new services.
The CaTV broadband data network provides two-way communication between subscriber computer systems and the CaTV head-end (H/E). From there, the two-way communications continue to the rest of the digital data network to which the CaTV plant is connected. Subscriber computer systems are connected to the CaTV H/E via a device called the cable modem, which can be located internally or externally with respect to the subscriber computer system. The cable modem connects to the H/E via the same CaTV cable used for regular analog TV viewing.
FIG. 1
shows the typical topology of a two-way capable CaTV system. It consists of the CaTV H/E
101
, and subscribers
104
. The network topology is a tree-and-branch, which is an optimal topology for the distribution of a one way TV broadcast. The H/E
101
is located at the root of the tree while the subscribers
104
are located at the leaves of the tree. For increased signal quality, low maintenance, etc., the majority of the distance between the H/E
101
and the subscribers
104
is covered by fiber optic cables
106
. A series of fiber nodes
102
are responsible for converting light signals to electromagnetic signals and vice-versa, for transmission over the coaxial cable region of the network
107
. These dual medium CaTV plants are called hybrid fiber/coaxial (HFC).
Each fiber node
102
is at the root of a sub-tree that spans the subscribers in a logical neighborhood
105
. A neighborhood
105
covers a few hundred subscribers
104
. The coaxial cables that arrive at subscribers' homes carries signals in the frequency range of 5 MHz to about 750 MHz, or about 1 GHz in newer plants.
Data transmissions from cable modems to the H/E
101
typically occupy frequency bands in the range of 5 MHz to 42 MHz and are usually referred to as the upstream transmissions. Each such frequency band is also referred to as an upstream or return channel. Data transmissions from the H/E
101
to the cable modems typically occupy frequency bands in the region above 450 MHz and are usually referred to as the downstream transmissions. Each such frequency band is also referred to as a downstream channel.
During cable-modem power-on, the H/E
101
assigns to each cable-modem
104
a specific upstream channel, where they attempt all their message transmissions, and a specific downstream channel to receive all transmissions from the H/E
101
. The message transmissions in both directions continue until H/E
101
decides otherwise. For ease of system management, cable-modems
104
that are assigned the same upstream channel are also assigned the same downstream channel.
To guarantee good signal quality and sufficient signal strength, bidirectional amplifiers
103
are added along the path from the CaTV
101
to subscribers
104
. These amplifiers are bidirectional in the sense that operate over two distinct frequency regions, one that relates to upstream channels and one that relates to downstream channels.
While transmissions from the H/E
101
to stations, i.e., cable modems, on the downstream channel are under the exclusive control of the H/E
101
, and thus well behaved and contention-free, coordinating transmissions from multiple stations to the H/E on an upstream channel is a more challenging task.
Unidirectional taps at each subscriber, disallow upstream transmission to be heard by other subscribers. Thus, on the one hand this increases the system security and privacy, on the other hand it guarantees that stations cannot communicate directly with each other, but they need to communicate through the H/E station via the help of an appropriate Medium Access Control (MAC) protocol, e.g., comprising a contention resolution component. Since, various stations cannot hear each other's upstream transmissions, during contention resolution, stations cannot listen for other transmissions prior to or during a message transmission of their own. As such, stations can coordinate their transmissions only under the explicit assistance from the H/E.
In order to communicate with the H/E, a cable modem needs to follow pre-established rules commonly referred to as a protocol suite. These rules regulate how the cable modem
a. may format the digital information into meaningful information messages,
b. will transmit these messages on the cable plant,
c. will share the communication resource, i.e., the cable, with other subscribers' modems, etc.
A protocol suite comprises protocol layers like:
1. transport, describing the way that information from or to a subscriber forms data messages,
2. network, describing how messages travel and are routed through a communications network,
3. medium access (MAC), describing how messages share common communication resources,
4. physical, describing how a message is prepared for transmission on the communication medium, and etc.
The network topology and architecture of a CaTV system, does not allow subscribers' cable modems tc listen to each other's transmissions. Thus, the possibility exists that two or more messages transmitted by different cable modems to overlap in time and frequency with each other resulting in destruction of the information that these messages carry. Message transmission, free from interference from other message transmissions, is achieved by incorporating in the MAC protocol:
a. a contention/collision, resolution algorithm that is agreed upon and consistently executed by the cable modems, possibly facilitated by the H/E in order to resolve the message collisions; or
b. allowing the H/E to explicitly or implicitly poll stations to transmit; or
c. a combination of the a) and b).
Typically a data station is idle with nothing to transmit most of the time, thus polling stations for message transmissions could result in inefficient use of the communication resources, while active stations could experience a high delay until their turn comes to transmit. The low performance of a polling system, especially with low traffic loads, is exaggerated by the large round-trip propagation and message processing delays in a CaTV system, and the large number of subscribers at
International Business Machines - Corporation
Lim Krisna
Scully Scott Murphy & Presser
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