Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-11-24
2002-09-10
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C375S371000
Reexamination Certificate
active
06449291
ABSTRACT:
1. BACKGROUND OF THE INVENTION
a. Field of the Invention
The present invention relates to communication systems. In particular, the present invention relates to the synchronization of time signals amongst a plurality of communications system components such as cable modems.
b. Description of Related Art
As information becomes increasingly more available on communication networks such as a LAN or over the Internet, the development of new methods and apparatus for sending and receiving this information more quickly between communication system users has become an important issue. For instance, one-way and two-way cable modems, both internal and external, based on the Multimedia Cable Network System (MCNS) Data-Over-Cable Interface Specifications (DOCSIS) standard, are currently available to consumers to access data over the Internet at speeds far in excess of those previously attainable by standard analog telephone modems. An external cable modem is a complete, self contained unit which is housed in its own enclosure, separate from a personal computer (PC), as opposed to an internal cable modem which is designed as a peripheral card on a printed circuit board (PCB) inserted into a PC. Two-way cable modems receive modulated data from a head-end (H/E) controller over a 75-ohm coaxial cable (the same cable found in residential housing) and send back upstream data over this same cable to the headend controller. A one-way cable modem receives data from the headend on a 75-ohm cable, but transmits upstream data back to the headend using a standard analog telephone modem (i.e. 28/33/56 kbps). In each case the headend controller exists to serve a number of subscribers to the cable modem service.
Downstream (D/S) data for all subscribers is interleaved in time and continuously transmitted down the cable. The downstream data in one instance occupies a 6 MHz wide channel with a center frequency between 54-850 MHz. Raw D/S data rates may range between 30-40 Mbps. However, most subscribers will see much less than this since the downstream bandwidth needs to be shared with many other subscribers as stated earlier. A typical cable plant installation will have between 500 and 2000 subscribers on a particular downstream channel. In addition, there is some degree of overhead required for header data and forward error correction. This serves to lower the true raw data rate somewhat. If every subscriber were receiving data continuously, then the effective raw data rate seen by any one subscriber would be 1/500 or 1/2000 of the maximum D/S data rate possible after subtracting overhead. However, computer-computer data communications tend to be bursty in nature, and not every subscriber is logged on at the same time. This means that under nominal loading, each subscriber can expect to see effective D/S data rates in the range of 100's of kbps.
FIG. 1
illustrates the typical D/S data stream
100
for a cable modem system. Notice that the D/S data contains both data for each modem
102
and general management packets
104
.
In the case of D/S data, each cable modem continuously monitors the D/S channel. When data addressed to a particular modem is received, the modem takes appropriate action. All other data which is not addressed to that modem is ignored. In the case of the two-way cable modem system, all replies are transmitted on the upstream (U/S) channel of the coaxial cable back to the headend controller. In one instance of the typical two-way cable modem system, there is no contention (or collisions) on the D/S channel, because no modem ever uses the D/S data channel frequency for U/S data. For, in this system U/S data occupies channels of 200 kHz-3.2 Mz wide in the range of 5-42 MHz. The headend controller is the single system component which completely decides what data to what modem is sent when on the D/S channel.
However, in the case of the U/S data channel for a two-way system with a number of subscribers there are many cable modems which must compete with each other in some fashion to send their data back to the headend controller. Of course, if two modems try and send data at the same time to the headend controller, a collision can occur. Unlike a typical network such as an Ethernet, the individual cable modems can not “hear” (i.e. receive or monitor) data from other cable modems. This is due mostly to the one-way transmission property of the cable plant (due to directive circuit elements, such as power splitters, amplifiers and directional couplers) and also due to the large time delays inherent in the cable plant due to the large distances involved in the cable routing.
FIG. 2
shows a diagram of a typical cable plant. The typical cable plant includes a headend controller
200
which is coupled to the rest of the plant via, in one instance, fiber optic cable
210
. Data is passed from the headend
200
to the cable modems such as modems
1
,
2
,
3
,
4
, N, and N+1, via a network of combiners such as 2-way combiners
215
, and 4-way combiners
220
. Similarly, in a two-way system, data is passed from the cable modems to the headend
200
over the same network.
Therefore, it is up to the headend controller to decide which subscriber modem sends U/S data at what time. In one instance this is done by using a system of mini-slot time increments of around 6.25 usec each. Each modem is assigned a time in which it can transmit its signal so as to arrive at the headend controller in time-interleaved fashion, thereby not colliding with U/S data from other modem subscribers. For all of this to work, the headend controller performs a ranging operation to determine the time delay from each modem. The headend controller then figures out for each modem a time slot in which it can send its data so as to not collide with the U/S data from other modems at the headend controller. This sequence of events is illustrated by FIGS.
3
(
a
)-(
b
) in which FIG.
3
(
a
) shows a timing diagram illustrating how the U/S data from a number of cable modems looks by the time it reaches the headend controller associated with those cable modems. The blocks labeled Modem #1, Modem #234, Modem #57, Modem #465, Modem #1, and Modem #33 represent data sent from those respective modems to the headend
200
. The blocks of time labeled guard time represent time slots in which data is not expected to be sent from any of the modems to the headend
200
—this helps to minimize collisions of upstream data by minimizing the chance of data timeslot overlap. FIG.
3
(
b
) depicts a timing diagram that illustrates a hypothetical example of the actual time that each U/S packet was sent, so as to arrive at the headend controller at the proper time. The details of this process are complicated and are described more fully in the MCNS DOCSIS specifications referred to earlier and which are hereby incorporated by reference.
As can be seen from the above discussion, in order for the U/S data synchronization to work, each modem needs an accurate local clock reference which is precisely in synchronization with the headend clock. In general, accurate clock references are derived from an accurate crystal oscillator whose frequency is divided down to the desired time interval. Since there is typically only one headend controller per every 500-2000 subscribers, it makes economic sense to buy a highly precise and accurate crystal oscillator. For this reason, headend crystal oscillators are typically of the expensive temperature-compensated type with frequency (and hence time) variations of just a few parts-per-million (ppm). However, in the case of an individual cable modem intended for the consumer market, cost is of paramount importance. Therefore, a typical cable modem contains an inexpensive, non-temperature-compensated crystal oscillator with an accuracy of 50-100 ppm as illustrated in FIG.
4
(
a
) which shows a typical standard crystal oscillator configured to output a digital clock output to be used by the rest of the cable modem's integrated circuitry. Some of this variation comes from the wider
Boyd Edward
Burns Lawrence M.
3Com Corporation
Kizou Hassan
Yin Lu
LandOfFree
Method and apparatus for time synchronization in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for time synchronization in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for time synchronization in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2880501