Data processing: measuring – calibrating – or testing – Testing system – Of circuit
Reexamination Certificate
1998-12-09
2003-12-09
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Testing system
Of circuit
C702S120000, C702S057000, C702S076000, C702S182000, C725S107000, C725S111000, C725S129000, C375S222000, C375S221000, C370S264000, C370S252000, C370S207000
Reexamination Certificate
active
06662135
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communication systems. In particular, the present invention relates to the testing of communications system components such as cable modems.
2. 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 H/E controller. A one-way cable modem receives data from the H/E 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 H/E 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.
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 H/E 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 instance of the system, the U/S data occupies channels from 200 kHz-3.2 MHz wide in the range of 5-42 MHz. The H/E 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 H/E controller. Of course, if two modems try and send data at the same time to the H/E 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. 1
shows a diagram of a typical cable plant. The typical cable plant includes a headend controller
100
which is coupled to the rest of the plant via, in one instance, fiber optic cable
110
. Data is passed from the headend
100
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
115
, and 4-way combiners
120
. Similarly, in a two-way system, data is passed from the cable modems to the headend
100
over the same network. Additionally, in some existing cable modem plants, the U/S data is split-off from the cable at the fiber
110
junction.
Therefore, it is up to the H/E 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 use each. Each modem is assigned a time in which it can transmit its signal so as to arrive at the H/E controller in time-interleaved fashion, thereby not colliding with U/S data from other modem subscribers. For all of this to work, the H/E controller performs a ranging operation to determine the time delay from each modem. The H/E 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 H/E controller. The details of this process are complicated and are described more fully in the MCNS DOCSIS specifications referred to earlier.
As can be seen from the above discussion, in order for the overall cable modem based communication system to work properly, especially the two-way cable modem system, each cable modem in the system must be operating properly and according to the MCNS DOCSIS specifications. Thus, in order to ensure a robust cable modem based communication system, it is imperative that each individual cable modem in the system be properly tested to ensure that it is operating correctly.
Currently known methods of testing cable modems, either on the factory floor before home installation, or in the field at the end users installation site, generally make use of bulky, expensive and complicated headend test equipment as illustrated in
FIG. 2. A
headend unit
205
is coupled to a cable modem
200
which is in turn coupled to a computer
210
. The headend
205
is a complex computer controlled apparatus which can be placed in a test mode to send and receive data from a cable modem
200
being tested to analyze the cable modem's performance. Thus, in order to test a number of modems each modem would be brought to and connected to a headend where a series of tests would be run—the modem would then be disconnected and the next modem would go through the same process. Derivations of this headend testing methodology could involve connecting ten or more modems
200
up to the same headend
205
which could sequentially run the testing process on each of the modems.
There are a number of disadvantages with these testing approaches. First, the use of a headend test unit either in the field or on the manufacturing floor is very expensive because of the high cost of the relatively complicated headend test unit. Further, because of the complication of the headend test units, highly skilled test technicians are needed to operate them, even if only a relatively simple test needs to be performed. Finally, the use of a headend test unit takes a large amount of time to setup and perform the test, especially when only a simple functionality test of the unit under test is required. For instance, it is desirable to be able to perform a series of power on self tests which test a number of simple operations of the modem prior to performing any further detailed tests.
Therefore, what is needed is a new method and apparatus which is capable of performing operability tests on a cable modem in a quick, efficient, and cost effective manner which avoids the aforementioned problems of currently known testing methods.
SUMMARY OF THE INVENTION
As discussed above, currently known methods of testing the operability of the RF hardware components of a cable modem can be expensive and complicated, especially when only a simple power
Burns Lawrence M.
Ramachandran Ravi
3Com Corporation
Desta Elias
Hoff Marc S.
McDonnell Boehnen Hultbert & Berghoff
LandOfFree
Method and apparatus for reflective mixer testing of a cable... 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 reflective mixer testing of a cable..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for reflective mixer testing of a cable... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3115750