Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1999-10-09
2003-02-25
Chin, Wellington (Department: 2665)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S458000, C370S468000, C370S490000, C725S121000, C725S131000
Reexamination Certificate
active
06526070
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to methods and apparatus for synchronizing data transmission in computer networks and, in particular embodiments, to methods and apparatuses for synchronizing upstream network transmission in cable modems.
2. Description of Related Art
Data networks have different forms to serve different purposes. An example of a simple network is a network in an office that allows several computers to use one printer. Such a network is commonly known as a Local Area Network (LAN). Other networks may be more complex. The Internet is an example of a more complex network, in which many smaller networks from all over the world may be interconnected. The Internet allows worldwide transmission of many types of data, including text files, graphics, audio, and video data. A network that extends over a large geographical area is commonly known as a Wide Area Network (WAN).
WANs have commonly used the telephone system to transmit data over long distances. The telephone system is a convenient data transmission media because it has an established infrastructure which can reliably transmit data worldwide. A major drawback to data transmission via the telephone network, however, is the limited rate at which it can transmit data (its low bandwidth).
For users requiring higher bandwidth, sophisticated but expensive WAN systems have been built. For example, large businesses have utilized satellites orbiting the earth and microwave linked ground stations for the transmission of data. For the general user, however, the telephone system remains a prevalent choice as an acceptable tradeoff of cost and performance. In other words, the general user has accepted low bandwidth because the cost of obtaining greater bandwidth has been high.
Recently, cable television networks have become available. A typical cable TV system can carry many television stations, which are the equivalent of a large amount of data (high bandwidth), simultaneously. Because of the increasing availability of cable television infrastructure, using television cables as the medium for computer data networks has the potential for giving users high bandwidth at a reasonable cost. A cable TV system, however, requires several enhancements in order to function as a data network.
In its classic form, a cable TV system carries information in only one direction, from the cable system head end, to the individual user. The user's interface to the system generally comprises a receiver, for example, a television or a stereo. The head end transmits television or stereo channels simultaneously. In general the user has no influence on what is transmitted and can only choose among the channels the head end is transmitting.
In contrast, a data network must carry data from the head end to the user (the downstream path) and from the user to the head end (the upstream path). The individual user requires equipment, such as a cable modem, that can both receive from the head end and transmit to it. A cable data network must be able to handle many individual users simultaneously, each of whom have control over what they receive and transmit.
In order for a cable TV network to operate as a data network, it requires a head end capable of both transmitting and receiving data as well as a user end equipped with the capability of both receiving and transmitting data through the use of equipment such as a Cable Modem (CM). To assure that each user receives the data they require, a network protocol must be implemented to allow independent users of the network to utilize the shared head end and the distribution network without interference from or receiving the data of other users.
The network protocol places requirements on both the head end and the user end. Generally, the head end serves as the network controller, and the user's cable modem must be able to respond to commands from the head end. In order to support a number of independent users, the network protocol divides the system's resources using two basic methods.
In a cable TV system the head end can transmit several TV channels simultaneously by placing them in different channels in the radio frequency (RF) spectrum. Similarly the network protocol divides the cable network's bandwidth into frequency channels. Each user's cable modem then can be tuned to receive and transmit on one or more of the channels. Generally, in a cable data network, the downstream transmissions are segregated from upstream transmissions by placing them on different RF channels. Such a method is termed Frequency Division Multiple Access (FDMA).
In order to accommodate a number of users, RF channels can be further divided into time slots and each user allotted a timeslot to transmit and receive. This method is commonly known as Time Division Multiple Access (TDMA).
The time slots for the downstream messages are determined by the head end network controller. The reception of data by users is determined by an addressing scheme. The head end transmits a unique address for each cable modem along with the data for that user; the individual modem is configured to accept only the data intended for it.
Allocating time slots for upstream messages generated by users is complicated by the fact that the upstream messages are initiated by independent units. In general, two types of schemes have been developed to control transmissions by the users: arbitration methods and allocation by the controller.
In a common arbitration system, the user's modem initiates transmissions. The system includes a method for detecting collisions between user messages; i.e., more than one user attempting to transmit an upstream message at the same time. When a collision is detected the users must then retransmit their messages, usually adjusting the times at which they retransmit in a attempt to reduce the chances of another collision with messages from the same unit. This method has a drawback in that bandwidth is wasted when the messages that collided are retransmitted. As the channel becomes more crowded, the number of collisions tend to increase.
A method of utilization of the channel is to have the system controller assign a time interval for each user's modem transmission. To implement such a method, the user's transmission must be synchronized so as not to collide with each other. A common way to provide synchronization is to assign transmission time slots to each user. Each user can then transmit in a time assigned to them and collisions are avoided. The more precisely the user modems transmit at their assigned time, the more closely spaced the controller can schedule messages, and the greater the capacity of the network. Therefore, precise scheduling of user modem transmissions is desirable.
Precise synchronization between elements widely separated in space is not a trivial matter. Compensation for the skewing caused by the finite time required for the signals to travel time between elements must be added if correct synchronization is to be achieved. In addition, transmission of data over a cable may be accomplished by several different standards. One such standard is the MCNS or Multimedia Cable Network System standard, which has been promulgated primarily in North America by DOCSIS (the Data Over Cable System Interface Specification) which has become a de facto standard for compatible cable modems in North America. Multiple other standards have been promulgated; for example, the Digital Video Broadcasting (DVB) standard which is the standard produced by the European Broadcast Union (EBU) under the auspices of the European Telecommunications Standards Institute (ETSI). A system similar to the DVB system has also been proposed by DAVIC (Digital Audio Video Council). To address the synchronization problem MCNS systems may be implemented with a local clock in each Cable Modem, which periodically needs to be synchronized to a master clock within the cable system head end. DVB systems synchronize the local clock in the NIU (DVB terminology for Cab
Bernath Brett Alan
Brooks John Milford
Goldenberg Yoav Pesach
Mehta Manoj
Chin Wellington
Foley & Lardner
Phan M.
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