Pulse or digital communications – Testing – Data rate
Reexamination Certificate
1998-06-12
2002-10-08
Pham, Chi (Department: 2631)
Pulse or digital communications
Testing
Data rate
C375S221000, C370S473000
Reexamination Certificate
active
06463096
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to digital communication and more particularly to protocols for communicating data originating from sources having disparate data rates over a shared medium.
Trends in digital communication point toward a common transmission medium providing both high data rate services such as digital video and low data rate services such as voice. Internet access is inherently a mixed service. Upstream requests for information typically include minimal data while downstream traffic may include graphics or even live video.
Specific examples of such a common transmission medium include a wireless local loop (WLL) that substitutes for the local telephone loop and provides additional high data rate services such as video and Internet access. Another example is a CATV network that has been updated to provide high data rate services and voice service.
A key objective is maximizing efficiency in use of bandwidth. The available bandwidth is shared among multiple data communication devices. When a data communication device is allocated all or part of the available bandwidth, it should make efficient use of its allocation. Depending on the protocols and modulation systems used, a certain percentage of the data is devoted to network operation rather than customer service. This is referred to as overhead. Consider a network where packets of information are communicated in successive frames and:
d
p
=Payload data (bits)—the number of payload bits contained in a frame,
r=Data rate (bits/sec), proportional to spectrum used (Hz). Data rate refers to the rate at which information is communicated through the wireless medium. Information rate roughly represents the rate of generation of payload data.
t
f
=Frame time (sec)—the duration of the smallest unit of time that may be allocated to a data communication device for transmission on the shared medium. Note that a packet of like data, such as voice or data, may be transmitted in a single frame or may be divided among many frames.
t
g
=Overhead time (sec), including guard time, training, and synchronization that is required for each frame.
The system efficiency associated with the overhead given by t
g
is
eff
=
d
p
r
(
t
g
+
t
f
)
.
This value of efficiency reaches 100% when the overhead time is zero and the frame time equals the payload data divided by the data rate (when the frame time is exactly the time required to transmit the payload data at the transmission rate).
The network designer is left free to vary frame time to maximize efficiency. However, it is difficult to reconcile the needs of different traffic types. Consider choosing one frame time to accommodate both low information rate voice traffic and high information rate data traffic. Due to transmission latency requirements, voice traffic requires frequent frame transmissions to reduce latency. Hence, voice requires a short frame time. Furthermore, the amount of data to be sent in these frames is small since voice is low information rate. If long frames are sent, voice traffic is insufficient to fill each frame, resulting in wasted bandwidth. However, sending small frames incurs a different type of bandwidth loss. The fixed overhead associated with each frame substantially reduces spectral efficiency. This becomes particularly significant for high information rate traffic, where data must be divided over many frames instead of being efficiently transmitted in long frames. This conflict can be described in an example.
Consider choosing a frame time to efficiently transmit both a 64-byte voice packet and a 1000-byte data packet. Assume an overhead time of 3 us (t
g
=3 us), a data rate of 30 Mbits/sec, and two candidate frame times of 17 us and 267 us. For a frame time of 17 us, the efficiency for a 64-byte packet of data is,
eff
64
=
d
p
r
(
t
g
+
t
f
)
=
512
30
×
10
6
(
(
3
+
17
)
×
10
-
6
)
=
17
3
+
17
=
85
%
.
Since the frame time is exactly the amount of time required to transmit 64 bytes at a 30 Mbit/sec rate, this is the maximum efficiency at this data rate. A 1000-byte packet would be spread among 64-byte transmission opportunities corresponding to individual frames. Hence the efficiency for a 1000 byte packet is approximately the same as for the 64-byte packet. (To be precise, the efficiency is slightly less than 85% since the final frame is not fully utilized.)
Increasing the frame time can increase this efficiency by reducing the overhead. For example, a frame time of 267 us results in close to 99% efficiency for a 1000 byte packet,
eff
1000
=
d
p
r
(
t
g
+
t
f
)
=
8000
30
×
10
6
(
(
3
+
267
)
×
10
-
6
)
=
267
3
+
267
=
99
%
.
Unfortunately, this large frame time causes severe inefficiency for the 64-byte packet because a large portion of the frame is left unutilized. Here it is assumed that because of latency requirements, it is not feasible to collect multiple 64-byte packets to fill a frame. For example, it would take a 64 kbps voice source over 125 ms to fill a 1000-byte frame, which results in intolerable latency. Allowing 8 ms of latency for the collection of one 64-byte packet, the long frame capable of supporting 1000 bytes which carries one 64-byte packet has very poor efficiency:
eff
64
=
d
p
r
(
t
g
+
t
f
)
=
512
30
×
10
6
(
(
3
+
267
)
×
10
-
6
)
=
17
3
+
267
=
6
%
.
No single choice of frame time leads to efficient use of the spectrum for both high data rate and low data rate traffic.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for combining high data rate traffic and low data rate traffic on a common transmission medium while maximizing efficient use of available spectrum. Since spectrum is an economically valuable resource and transport of data generates revenue, the present invention directly leads to more profitable network operation. The systems and methods provided by the present invention are applicable to both wired and wireless transmission media. In one embodiment, a bandwidth reservation scheme provides that data rate may be varied so that when a particular data communication device is allocated a frame, it is also assigned a data rate for use in that frame. Because bandwidth usage scales with data rate, individual data communication devices will be assigned to possibly different spectrum bandwidth on a frame-by-frame basis.
A first aspect of the present invention provides a method for allocating access to a common transmission medium among a plurality of data communication devices. The method includes steps of: assigning a transmission frame to a particular data communication device, assigning a data rate for the particular data communication device to employ in the transmission frame, and transmitting the transmission frame assignment and the data rate assignment to the particular data communication device.
A second aspect of the present invention provides an alternative method for allocating access to a common transmission medium among a plurality of data communication devices. The method includes steps of: receiving access request messages from requesting data communication devices at a hub, the access request messages requesting access to the common transmission medium, in response to the access request messages, at the hub, allocating access to the common transmission medium in both the frequency and time domain among the requesting data communication devices, and thereafter transmitting from the hub to the requesting access devices, instructions for each requesting access device to transmit at particular times, and at particular data rates chosen according to the allocating step.
A third aspect of the present invention provides a digital communication network including a plurality of data communications devices transmitting via a common transmission medium, and a hub receiving signals from the data communications devices via the common transmission medium. The hub includes: a bandwidth manager that receives access request messages from requesti
Jones Vincent K.
Pollack Michael A.
Raleigh Gregory G.
Bayard Emmanuel
Cisco Systems, Inc
Pham Chi
Ritter Lang & Kaplan LLP
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