Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1998-07-29
2002-04-16
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S470000, C370S442000
Reexamination Certificate
active
06373860
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to telephone systems, and more particularly to Digital-Subscriber Lines (DSL) carrying both voice and data traffic.
BACKGROUND OF THE INVENTION
The telephone system was originally constructed for carrying voice calls. With the widespread acceptance of personal computers and wide-area data networks such as the Internet, telephone networks are carrying more and more data traffic. Digital telephone lines such as Integrated Services Digital Network (ISDN) and T
1
lines carry both data and voice traffic. Some higher-speed Digital-Subscriber Lines (DSL) also reserve some bandwidth for voice calls.
ISDN Bearer Channel Allocation—FIG.
1
FIG. 1
shows an ISDN line used for both voice and data traffic. The customer premises equipment (CPE) includes ISDN terminal adapter or modem
12
that receives a data stream from a computer. ISDN modem
12
may include one or two plain-old-telephone-service (POTS) voice ports that can be connected to standard telephone or fax equipment. ISDN modem
12
transmits data or digitized voice over telephone line
20
, which is an ISDN line using digital rather than analog signaling.
At the Phone Company's central office (CO), ISDN line card
14
terminates ISDN telephone line
20
. The data or digitized voice is sent over the public switched-telephone network (PSTN) that includes circuit switched network
22
. When ISDN modem
12
transmits a voice call, the call is sent over circuit switched network
22
to other voice telephones over voice lines
26
,
27
. However, when data is sent by ISDN modem
12
, the remote number called is connected to another computer (not shown) at Internet Service Provider (ISP)
21
.
ISDN telephone line
20
carries three independent channels: a low-bandwidth data channel D is primarily used for control signals, while bearer channels B
1
, B
2
are each 64 Kbps channels, each capable of carrying one voice call. When no voice calls are being made, ISDN modem
12
routes the data stream over both bearer channels B
1
, B
2
, providing a combined bandwidth of 128 Kbps. These two bearer channels remain as two separate calls within the PSTN, since ISDN line card
14
makes two connections
24
,
25
to circuit switched network
22
. These two calls are sent over two lines
28
,
29
to ISP
21
. At ISP
21
, upper-layer software
18
combines data from the two lines
28
,
29
into a single data stream. Thus the single data stream from the user is split into two separate calls to ISP
21
.
When the user makes or receives a voice call, one of the bearer channels is dropped and no longer carries data. The data bandwidth then falls to 64 Kbps since only one bearer channels B
1
in ISDN telephone line
20
is used for data, and only one connection
24
and only one of two lines
28
,
29
are used. The other bearer channel B
2
is used for the voice call, connecting the user's telephone or fax with a remote telephone or fax on voice line
26
. ISDN line card
14
uses the other connection
25
to connect with circuit switched network
22
and voice line
26
. When two voice calls are simultaneously made, then both bearer channels are used and no data can be transmitted. Thus ISDN modem
12
is able to drop one of the bearer channels for a voice call. Once the voice call ends, ISDN modem
12
reconnects the second bearer channel to ISP
21
.
While ISDN bearer channels can be dropped and reconnected as voice and data loads change, the low bandwidth of ISDN limits its future usage. Having to separate data connections through the PSTN that must be recombined by software at the ISP is also undesirable.
Fixed Allocation of Voice and Data Channels of T
1
Lines—FIG.
2
FIG. 2
highlights a high-speed T
1
phone line that has fixed allocations of its bandwidth to voice and data traffic. Larger corporations often connect to the telephone network over 1.5-Mps T
1
or 45-Mbps T
3
lines.
T
1
lines can be partitioned into a number of 64 Kbit DS
0
channels to carry several separate voice calls, with the remaining bandwidth used for data traffic. For example, the T
1
line can have 256 Kbps allocated for voice calls, with the remaining 1.25 Mbps for data traffic. The 256 Kbps allocated for voice can carry four separate voice calls at one time.
While it is useful to allocate some of the bandwidth of a T
1
line to voice calls, the allocation does not vary over time. Once the T
1
line is configured for four voice calls, the allocation cannot be changed when five or more voice calls are received. The fifth caller hears a busy signal. Since calls often occur together at peak times such as just after lunch, bandwidth must be reserved for these peak times. Data bandwidth is restricted for all times of the day and night by the peak voice bandwidth than occurs only infrequently.
DSL With POTS Band—FIG.
3
FIG. 3
is a graph of DSL frequency bands with a lower POTS band.
FIG. 3
shows frequency bands for asymmetric DSL, or ADSL (T
1
.413) service using frequency-division duplex and voice calls.
FIGS. 3
is not drawn with a linear scale. Plain-old-telephone service (POTS) voice calls are transmitted over low-frequency POTS band
2
, as they are for standard telephone lines. POTS band
2
operates from near D.C. to 4 kHz. Since this is the same frequency range as standard telephones, ordinary telephone equipment or voice-band modems can be used over POTS band
2
.
ADSL upstream channel
4
is for uploads from the customer, or for sending commands and user input from the customer to the central office side. Some embodiments may use a bi-directional channel in place of upstream channel
4
. Upstream channel
4
operates at up to 138 kHz, with the data rate up to 1 Mbps.
Wide-band
5
carries the bulk of the ADSL-line bandwidth. Wide-band
5
carries ADSL data downstream to the customer at up to 8 Mbps. Wide-band
5
is a frequency band typically from 140-200 kHz up to about 1.1 MHz. The lowest frequencies are reserved for POTS. Other kinds of DSL use different frequency bands, but all use relatively high frequency bands.
While ADSL can be configured to reserve some bandwidth for services such as ISDN basic rate, POTS band
2
is frequency-limited and can carry just one voice call. Other proposed DSL services do not have a POTS band at all, and provide transport without distinguishing between voice and data services.
DSL Equipment Includes Frequency Splitter—FIG.
4
Special equipment is needed at both the customer premises and at the phone company's central office where the customer's copper phone line ends. Analog devices called frequency splitters are typically used to separate the low-frequency POTS band from the high-speed data bands.
FIG. 4
is a diagram of a DSL phone line highlighting the frequency splitters.
Copper telephone line
20
is a pair of copper wires running from central office
8
to the customer. The phone customer has installed customer premises equipment
6
. Since DSL uses high frequencies for data traffic and POTS uses low frequencies for voice calls, the signal received over POTS telephone line
20
must be split into high- and low frequency components. Splitter
46
contains a low-pass filter that outputs the low-frequency components from copper telephone line
20
. These low-frequency components carry the voice calls that are sent to telephone set
10
. Telephone set
10
is a standard POTS analog telephone set. Additional phone sets, fax machines, or voice-band modem equipment can be connected to telephone set
10
as phone-line extensions as is well-known.
Splitter
46
also contains a high-pass filter that outputs the high-frequency components to DSL modem
48
. DSL modem
48
receives the high-frequency analog signal from splitter
46
and converts it to downstream digital data during the receiving window. During the transmitting window, it converts the upstream data into high-frequency analog signal. Splitter
46
mixes this high-frequency analog signal from DSL modem
48
with the low-frequency voice from telephone set
10
and transmits the combined signal
Aalaei Faraj
O'Toole Anthony J. P.
Auvinen Stuart T.
Centillium Communications Inc.
Hsu Alpus H.
Tran Thien
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