Note: Descriptions are shown in the official language in which they were submitted.
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Method and Apparatus for Extendin4 the Range of the Universal Serial Bus
Protocol
Field of the Invention
This invention relates to methods and apparatus for transmitting signals
between devices using Universal Serial Bus ports, and, in particular, to an
method for allowing communications between devices using such ports over an
extended range.
Description of the Related Art
Universal Serial Bus (USB) is a new technology designed to permit a
wide range of peripherals to be attached to personal computers by the average
user. Since the technology supports all of the common peripheral devices such
as keyboards, mice, speakers, modems, joysticks, cameras and many others, it
will replace the serial and parallel ports in use today. The new Apple iMac
(Trade Mark), for example, supports only USB ports. In addition, almost every
personal computer (PC) manufactured since 1997 has been equipped with USB
ports.
USB was created by an alliance of seven of the largest companies in the
computer and communication markets. Those companies were Intel, Compaq,
Microsoft, NorTel, NEC, Digital and IBM. The specifications defining USB (e.g.
Intel et al., Universal Serial Bus Specification, Revision 1.0, January 1996; -
hereinafter referred to as the "USB Specification") are non-proprietary and
are
managed by an open industry organization known as the USB Forum. The USB
Specification establishes a number of criteria which must be met in order to
comply to USB standards.
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For example, it is a current requirement of the USB Specification that a
single USB domain shall support up to 127 devices operating over a shared
medium providing a maximum bandwidth of 12 Mbps.
Further, the USB Specification limits the distance that a device can be
separated from its host PC to 5 meters. By using a series of USB Hubs -
devices that are intended to support increased populations rather than
increased distances - this distance limitation can be increased, in theory, to
30
meters. This multiple hub solution is both expensive and clumsy. For example,
to support a single device at a range of 30 meters the consumer must purchase
five hubs at a current cost of about $50 US each. In addition, at least two of
these hubs must be provided with electrical power. Since the individual cables
between hubs are limited to 5 meters each, it is also likely that some of the
hubs
would have to be positioned in very inconvenient and insecure locations.
There is therefore a need for methods and apparatus to allow USB
devices to be positioned at greater distances from the host PC. For example,
an
uninterrupted distance of at least 100 meters is required for compatibility
with
the standards governing the cabling of commercial buildings (see, for example,
TIA/EIA-568-A, Commercial Building Telecommunications Cabling Standard,
Telecommunications Industry Association, October 1995). Meeting this standard
must be accomplished without the need for intermediate repeaters since
distribution cabling is not normally accessible between its end-points at, for
example, the Telecommunications Closet and the Work Area. Furthermore,
even if the cable were to be accessible, the cabling standard does not allow
active devices to be inserted other than at the end-points.
Providing for an extended range capability would also create new
applications for USB devices as well as facilitating existing ones. For
example, a
simple residential or SOHO (small office, home office) surveillance system
could be constructed by connecting consumer quality cameras to a central PC.
An overhead mounted monitor could be controlled by a remote keyboard or
mouse. A door-phone entrance system could be monitored from any office in a
commercial building.
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Currently, however, the USB Specifications do not permit the use of
extended ranges. For example, it is a further requirement of the USB
Specification that the access of each device to the shared communications bus
is controlled by a single Host Controller. It is also specified that when the
Host
Controller instructs a particular device to place its information onto the
shared
bus, the requested information must be received by the Host Controller within
sixteen (16) bit-times of said Host Controller issuing said instruction. In
practise,
this ensures that the USB Specification provides for a high efficiency of
bandwidth utilization by limiting the period during which no information is
being
transmitted. However, these requirements also limit the physical range of USB
devices since one bit-time is equivalent to the time taken for an electronic
signal
to traverse approximately 17 meters of copper cable.
Further, although the USB device must respond to a request from the
Host Controller within 16 bit-times, 7.5 bit-times is allocated for delay
within a
USB device and its associated 5 meter cable. This allocation retains only 8.5
bit-times for additional cable delay. The time represented by 8.5 bit-times is
equivalent to the delay incurred by electronic signals in traversing
approximately
144 meters of cable. However, this cable length is insufficient to satisfy the
round-trip cable length of 200 meters required by the premise cabling
specification.
Thus, it is not currently possible to provide USB devices which are
separated over an extended distance.
However, it is a further feature of the USB Specification that the USB
Specification (or protocol) segregates access to the shared bus into discrete
units known as frames. Each frame is designed to last for a period of 1 ms.
Further, the current USB Specification also requires that at least four
separate types of data streams or "traffic" are recognized, namely isochronous
transfers, control transfers, interrupt transfers and bulk transfers. Of most
interest in the present application, isochronous data transfer ensures that
data
flows essentially continuously, and at a steady rate, in close timing with the
ability of the receiving mechanism to receive and use the incoming data. This
type of data transfer is distinguished from asynchronous data transfer, which
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pertains to processes that proceed independently of each other until a
dependent process has to "interrupt" the other process, and synchronous, which
pertains to processes in which one process has to wait on the completion ofi
an
event in another process before continuing. It should be noted that it is an
aspect of isochronous transfers that timely delivery of information is ensured
at
the expense of potential transient losses in the data stream. In particular,
there
is no attempt to retransmit any data that may have been lost in previous
transmissions. For example, with an isochronous video signal, loss of one
frame
of information is generally not significant, and there is no interest in
retrieving
the lost frame. Instead, the host controller is typically more concerned with
receiving the current frame.
The current invention uses the fundamental characteristics of
isochronous data, and more generally any data transmission where loss of data
in previous transmission is of minor consequence, and the existence of regular
protocol frames in order to provide methods and apparatus to enable data
transmission over extended distances.
Accordingly, while USB technology has proven to be useful, it would still
be desirable to provide improvements to the technology by providing a method
and apparatus for enabling data transmission equipment, and in particular,
data
transmission equipment utilizing the USB Specification, to be used over an
extended range.
Therefore, it is an object of the present invention to provide methods and
apparatus to enable devices, hubs and controllers and other devices that
conform to the USB Specification to communicate over distances greater than
that currently permitted under said USB Specification.
It is a further object of the present invention that such extended range be
achieved without the need for intermediate hubs, repeaters or other methods of
electronic signal regeneration.
It is a further object of the present invention that no hardware or software
changes need be made to the existing devices, hubs and controllers supported
by the system, and in particular, an isochronous system operating under the
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USB Specification. The invention, thereby, may be incorporated into networks
composed of both conventional range and extended range devices.
It is a further object of the present invention that the apparatus be very
cost effective, consistent with the broadest population of devices targeted by
the
USB industry.
These and other objects of the invention, which will become apparent
herein, are attained by the present invention as described hereinbelow.
Summar~~ of the Invention
Accordingly, the present invention provides a method for transmitting a
time relevant data stream, and in particular an isochronous data stream,
between a host controller and a peripheral device over an extended distance;
said method comprising:
a. feeding a first original, outgoing digital signal from a host controller to
a
local expander unit;
b. optionally converting said outgoing digital signals into a converted
outgoing signal having a format suitable for transmission over extended
distances;
c. transmitting either said outgoing digital signal or said converted outgoing
signal, as a outgoing transmission signal, over a signal distribution
system;
d. receiving said outgoing transmission signal at a remote expander unit;
e. optionally converting said outgoing transmission signal to said first
original outgoing digital signal;
f. delivering said first original outgoing digital signal from said remote
expander to at least one peripheral device;
g. receiving, at said remote expander, a reply digital signal from said
peripheral device;
h. optionally converting said reply digital signal into a converted reply
signal
having a format suitable for transmission over extended distances;
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i. transmitting said reply digital signal or said converted reply signal as a
reply transmission signal over said signal distribution system;
j. receiving said reply transmission signal at said local expander;
k. optionally converting said reply transmission signal to said original reply
digital signal;
1. storing said reply digital signal as a stored reply digital signal until
the
receipt of a subsequent original, outgoing digital signal from said host
controller, which subsequent signal is the same as, or similar to, said first
original outgoing digital signal; and
m. forwarding said stored reply digital signal to said host controller in
response to said subsequent original outgoing digital signal.
For the purposes of the present specification, the term "time relevant
data" is meant to relate to data streams such as isochronous data streams
wherein loss of a frame from a previous transmission is of minor consequence
and therefore, does not need to be retransmitted.
In a preferred embodiment, all digital signals conform to the USB
Specification (other than for the distance between devices), and all digital
signals, and in particular, the reply digital signal, represent isochronous
data.
Further, in a preferred embodiment, the extended distance exceeds 5 meters,
more preferably, exceeds 30 meters, and still more preferably, exceeds 100
meters. In particular, the distance between the local expander and the remote
expander exceeds 5 meters, more preferably, exceeds 30 meters, and still more
preferably, exceeds 100 meters.
While a number of different signal distribution systems might be used,
preferably, the signal distribution system utilizes unshielded twisted pair
(UTP)
wiring (or cabling).
In a further preferred embodiment, the present invention provides a
method for transmission of isochronous data according to the USB Specification
wherein isochronous data is transmitted from a peripheral device and is
received by a host controller, said method comprising:
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a. Transmitting a request for isochronous data from a host controller to a
local expander;
b. Forwarding said request for isochronous data from said local expander to
a remote expander over a signal distribution system;
c. Delivering said forwarded request for isochronous data to at least one
peripheral device;
d. Transmitting the requested isochronous data from said peripheral device
to said remote expander;
e. Forwarding said requested isochronous data from said remote expander
to said local expander over said signal distribution system;
f. Storing said requested isochronous data in a packet buffer at said local
expander;
g. Transmitting a subsequent request for isochronous data from said host
controller to said local expander;
h. Receiving said subsequent request for isochronous data at said local
expander; and
I. Retrieving the stored isochronous data from said local expander;
II. Delivering said stored isochronous data to said host controller;
III. Forwarding said subsequent request for isochronous data from
said local expander to said remote expander over said signal
distribution system; and
IV. Repeating steps (c) through (h) for said subsequent request and
any further subsequent requests for isochronous data.
In an even more preferred embodiment, the present invention provides a
method additionally comprising the following steps after item "a" described
hereinabove, namely:
i. Determining whether said local expander already possesses said
requested isochronous data;
ii. Generating a synthetic data packet if no such requested isochronous
data is present; and
iii. Delivering said synthetic isochronous data to said host controller.
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One method for generating the synthetic data packet is described
hereinbelow.
In another aspect, the present invention also provides an apparatus for
transmission of a digital signal over an extended distance comprising:
a local expander comprising means for receiving a request for a time
relevant data signal, and preferably an isochronous data signal, from a host
controller which host controller is connected to said local expander;
means in said local expander for generating an outgoing transmission
signal;
means in said local expander for sending said outgoing transmission
signal to a remote expander, which signals are sent over a signal distribution
system;
a remote expander comprising means for receiving said outgoing
transmission signal;
means in said remote expander for generating a digital signal from said
outgoing transmission signal;
means in said remote expander for forwarding said digital signal to at
least one peripheral device, which peripheral device is connected to said
remote
expander;
means in said remote expander for receiving inbound digital signals from
said peripheral device;
means in said remote expander for converting said inbound digital
signals to an inbound transmission signal;
means in said remote expander for sending said inbound transmission
signal to said local expander, which signals are sent over said signal
distribution
system;
means in said local expander for receiving said inbound transmission
signal;
means in said local expander for generating a digital signal from said
inbound transmission; and
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means in said remote expander for forwarding said digital signal to said
host controller.
Preferably, the local expander additionally comprises:
means for storing said inbound signal as a stored inbound signal;
means for analysing said digital signal from said host controller to
recognize a subsequent request for transmission of said time relevant digital
signal; and
means for sending said stored inbound signal to said host controller in
response to said subsequent request.
Description of the Preferred Embodiments
Although a number of signal distribution systems may be used, as
described hereinabove, preferably the signals are transmitted over a signal
distribution system which utilizes Unshielded Twisted Pair (UTP) copper wire.
Using this method of device connection provides a low cost, effective means
for
data transmission. However, in another embodiment of the system, signals can
be transmitted over coaxial cable.
While the methods and apparatus of the present invention have general
utility in a variety of applications, it is of primary importance that the
data
transmission methods and apparatus of the present invention allow for
compliance with the USB Specifications. Preferably, the original signal from
the
host controller is a request for data from a peripheral device. Additionally,
preferably the data requested is isochronous data from peripheral devices such
as cameras, keyboards, mice, monitors, speakers, and the like.
In particular, during operations utilizing the methods and apparatus of the
present invention in applications involving extended range transmissions, it
is
preferred that the apparatus be preferably capable of recognizing isochronous
transfers, when they are received. The data contained within the isochronous
transfer is then stored within the system for a period of time. Accordingly,
the
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data that is received during a particular frame may be stored and then
transmitted in a following frame. Additionally, a further preferred embodiment
of
the present invention is that isochronous transfers originating from a
plurality of
sources may be stored, and retransmitted.
In the operation of a preferred embodiment of the current invention, a
host controller (which preferably is a PC) may issue a request to a device for
the
transfer of isochronous data. The request is received by the apparatus of the
present invention, and retransmitted to the target device. When the requested
isochronous transfer response is received by the apparatus from the target
l0 device, the isochronous data is stored within the internal memory of the
apparatus. During a subsequent frame, the host controller will again issue a
request to the target device for the transfer of isochronous data. The
apparatus
will again retransmit this request to the target device. In addition, however,
the
apparatus recognizes that it currently has isochronous data from the target
device stored in its internal memory. The apparatus sends this data to the
host
controller within the 16 bit-time margin relevant to the current request
within the
current frame. In this manner, the apparatus uses data collected in a previous
frame to satisfy the response time requirement of a current frame.
When a packet is received from the target device, and no further request
for data is received from the host controller, the last data packet or packets
received and stored (hereinafter the "vestigial" packets) are preferably
removed
from the system so that they are not transmitted when and if a further request
is
received from the host controller. Preferably, this is achieved by
modification of
the method described hereinabove by additionally comprising the following
stages, namely:
i) Detecting when a new frame has begun;
ii) Examining the properties of each packet buffer;
iii) Determining whether the data packet contained in said examined
packet buffer has been stored for at least one complete frame period;
iv) Discarding said contained data packet if said contained data packet
has been stored for at least one complete frame period; and
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v) Repeating steps (i) through (iv) for each packet buffer in the system.
In an alternative embodiment of the invention, the apparatus handles the
first request for the inbound transfer of isochronous data in a unique manner.
This unique manner requires the apparatus to generate its own synthetic
inbound data packet, and generation of this synthetic data packet is described
hereinbelow.
It is possible that packets sent from the Remote Expander may not
arrived at the Local Expander in the order expected by the Local Expander. In
order to avoid difficulties which might be caused by this occurrence, the
method
l0 of the present invention also preferably comprises the following stages,
namely:
i) Storing the address of the requested peripheral device at said remote
expander unit after the local expander has delivered the forwarded
request for isochronous data;
and further comprising the following steps after transmitting the requested
isochronous data from the peripheral device to the remote expander, namely:
i) Retrieving the address of said requested peripheral device at said
remote expander unit; and
ii) Adding said retrieved address to said requested isochronous data.
By utilizing the method and apparatus of the present invention, it is
possible to have transfer of time relevant data, and isochronous data in
particular, over extended distances, and in particular, over distances greater
than specified in the USB Specification.
Brief Description of the Drawings
The invention, and various aspects thereof, will be described by
reference to the attached drawings wherein:
Figure 1 is a PC equipped with conventional USB Hub and USB Devices;
Figure 2 is a PC equipped with Extended Range Hub and USB Devices
according to the present invention;
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Figure 3 is a schematic drawing of an embodiment of the invention
designed to operate using UTP wiring as a signal distribution system;
Figure 4 is a timing diagram showing isochronous transfers according to
the USB protocol;
Figure 5 is a timing diagram showing isochronous transfers according to
the current invention;
Figure 6 is a schematic drawing of an embodiment of a Local Expander
according to the invention;
Figure 7 is a schematic drawing of an embodiment of a Remote Expander
according to the invention;
Figure 8 is a sequence diagram showing an isochronous input transfer
according to the invention;
Figure 9 is an alternative isochronous input transfer according to the
invention;
Figure 10 is a sequence diagram showing an isochronous output transfer
according to the invention;
Figure 11 is a logic diagram of a LEX controller according to the
invention;
Figure 12 is a logic diagram of an enhanced LEX controller according to
the invention;
Figure 13 is a logic diagram of a further enhancement to the LEX
Controller enabling vestigial packets to be removed;
Figure 14 is a logic diagram of a REX controller according to the
invention; and
Figure 15 is a logic diagram of an enhanced REX controller according to
the invention.
In the drawings, Figure 1 shows a PC (1 ) equipped with two conventional
USB Hubs (2a & 2b), and four standard USB Devices (3a, 3b, 3c & 3d). The
length of cable between the two hubs (2a and 2b) cannot exceed 5 meters
according to the current USB Specification.
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Figure 2 shows a PC (1 ) equipped with an apparatus according to the
present invention, which is termed as an "Extended Range Hub" (7), and four
standard USB Devices (3a, 3b, 3c & 3d). The Extended Range Hub is
composed of two separate units, a "Local Expander" (4) and a "Remote
Expander" (5) connected by cable (6). In one embodiment of the invention,
units
4 and 5 can be separated by, for example, up to 200 meters of Category 5 UTP
cable (6).
Figure 3 illustrates an embodiment of the invention designed to operate
over UTP. In this embodiment, the functions normally provided by a USB Hub
are provided by two separate units connected by a length of Unshielded
Twisted Pair (UTP) copper wiring (6a). A Host PC may be connected to the first
unit {4), referred to as Local Expander (LEX). The second unit (5) is referred
to
herein as Remote Expander (REX). REX (5) may be connected to a plurality of
USB devices. In this embodiment said plurality is chosen to be four, but it
will be
clear to those skilled in the art that other choices may be made within the
scope
of the invention.
Operation over extended distances is preferably achieved by placing said
LEX Unit {4) close to said host PC, placing said REX unit (5) close to said
plurality of USB devices, and connecting LEX unit (4) and REX unit (5) by the
required extended length of UTP cabling (6a).
Figure 4 provides a timing diagram showing isochronous transfers
according to the USB protocol. The diagram is constructed from the point of
view of a USB Host Controller, normally included on a PC motherboard (Host
PC). The USB protocol divides time allocation on the shared bus into regular
"frames", each of 1 ms in duration. The start of each frame is identified on
the
diagram as F1, F2, F3 & F4. When a Host Controller is engaged upon an
isochronous transfer with a device, the Host Controller issues regular
requests
for data transfer to said device. These requests are identified in Figure 4 as
packets R1, R2 & R3 (10, 12 & 14). Under the USB protocol, a USB device must
respond to said request within 16 bit-times. The responses are shown in the
diagram as packets Is1, Is2 & Is3 (11, 13 & 15). It is commonly expected that
transfer Is1 (11 ) will be delivered in response to request R1 (10), transfer
Is2
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(13) will be delivered in response to request R2 (12), and so on until the
requests are terminated.
Figure 5 provides a timing diagram showing isochronous transfers
according to the present invention. The diagram shows the progression of
packets through the various subsystems comprising the invention. Timelines are
presented for the Host PC (1 ), Local Expander (LEX) (4) and Remote Expander
(REX) (5) components which were shown in Figure 2.
An isochronous transfer is initiated from a Host PC (1 ) by emitting a
request for input data R1 (20) to a particular USB address and end-point. Said
request R1 (20) is received by the LEX (4) and retransmitted as R1 (25) over
the
external cabling to the REX (5). Said retransmitted packet R1 (25) is received
by
the REX (5) and forwarded as R1 (31 ) to the target device (e.g. one of items
3a,
3b, 3c or 3d of Figure 2).
The target device generates an input data packet Is1 (32). According to
the USB protocol, a device without an integrated cable must generate a
response within 6.5 bit-times of the end of the corresponding request. Said
input
data packet Is1 (32) is received by the REX subsystem (5) and retransmitted as
Is1 (26), over the external wiring, to the LEX (4). Said retransmitted
response
Is1 (26) is not immediately forwarded to the Host PC (1 ), but is stored
within the
memory of the LEX subsystem (4).
The Host PC (1 ) notices that it did not receive a response to its input data
request R1 (20), and retries the transaction by generating a new request R2
(21 ) to the same USB address and end-point. Upon receiving request R2 (21 ),
the LEX subsystem 4 retrieves response Is1 (26) from its memory buffers and
forwards it to the Host PC as response Is1 (22).
Said second request R2 (21 ) is repeated as R2 (27) through the LEX and
forwarded as R2 (33) to the device. The target device generates a second
response Is2 (34) which is retransmitted as Is2 (28) by the REX to the LEX.
Response Is2 (28) is again stored within the memory of the LEX subsystem,
from where it is sent to the host PC (1 ) as response Is2 (24) to a third
request
R3 (23). The process is repeated as necessary with requests R3 (23), R3 (29)
and R3 (35) and responses Is3 (36) and Is3 (30).
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Figure 6 illustrates one embodiment of a Local Expander (LEX) (4)
according to the invention. Said embodiment comprises four major blocks. The
USB Interface (50) enables a Host PC to be connected to the unit using
standard USB cabling. Said USB Interface (50) receives signals in USB format
from said Host PC and delivers USB packets to a LEX Controller (51 ). Said LEX
Controller (51 ) determines what response is necessary to the received packet
and generates the appropriate USB packets. If said LEX Controller (51 )
requires
information to be stored prior to transmission, then said information is
stored in
RAM (53).
Information that must be sent to the Remote Expander (REX) (5) is
forwarded to the UTP interface (52} wherein the information is converted into
digital signals suitable for propagation over copper cabling.
In the reverse direction, digital signals originating from the Remote
Expander (5) are received by UTP interface (52) and forwarded to LEX
Controller (51 ). Said LEX Controller (51 ) saves any necessary information in
RAM (53) and generates the required responses. Information that must be sent
to the Host PC is transferred to the USB Interface (50), from where it may be
forwarded to said Host PC using standard USB hardware and protocols.
Figure 7 illustrates an embodiment of a Remote Expander (REX) (5)
according to the invention. Said embodiment comprises a UTP Interface (60), a
plurality of USB Interfaces (63, 64, 65 and 66), a REX Controller (61 ) and a
RAM (62). Operation of the REX (5) is similar to that of the LEX (4).
Figure 8 provides a sequence diagram showing an isochronous input
transfer according to the invention. The format used in the diagram is
attributable to Jacobson et al. (Ivar Jacobson, Magnus Christerson, Patrick
Jonsson and Gunnar Overgaard, Object-Oriented Software Engineering: A Use
Case Driven Approach, Addison-Wesley, 1992).
In Frame 1, the control logic (100) within the Host PC generates a
request for input data, addressed to a particular USB function. Said request
is
transmitted to the LEX subsystem as an "In Addr" packet. The control logic
(101 )
within the LEX subsystem forwards the In Addr packet to the REX subsystem.
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The control logic (102) within the REX subsystem forwards the In Addr packet
to
the Device.
The control logic (103) within the device assembles the requested
isochronous data and transmits it as a "Data Payload" packet to the REX
subsystem. The control logic (102) within the REX subsystem forwards the Data
Payload packet to the LEX subsystem. The control logic (101 ) in the LEX
subsystem stores the Data Payload packet in its buffer memory.
In Frame 2, the control logic (104) within the Host PC recognizes that it
has not received a response to its previous request for input data. Said
control
logic automatically retries the transaction by generating a further request
addressed to the same USB function as in Frame 1. Said further request is
transmitted to the LEX subsystem as a second In Addr packet. On receipt of the
second In Addr packet, the control logic (105) within the LEX subsystem
recognizes that it has a Data Payload packet stored in memory from the same
function identified by the further In Addr packet. Said control logic (105)
retrieves said stored packet from memory and transmits same to the Host PC.
Said control logic (105) also forwards the new In Addr packet to the REX
subsystem. The control logic (106) within the REX subsystem forwards the
In Addr packet to the Device.
The control logic (107) within the device assembles the requested
isochronous data and transmits it as a Data Payload packet to the REX
subsystem. The control logic (106) within the REX subsystem forwards the Data
Payload packet to the LEX subsystem. The control logic (105) in the LEX
subsystem stores the Data Payload packet in its buffer memory.
The above-described process is repeated for subsequent frames by the
distributed control logic (e.g. 108, 109, 110 & 111 ).
Figure 9 provides a sequence diagram showing an alternative
isochronous input transfer according to the invention. This method differs
from
that described in Figure 8 in the manner in which the first request for an
isochronous input transfer is handled.
In Frame 1, the control logic (100) within the Host PC generates a
request for input data, addressed to a particular USB function. Said request
is
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transmitted to the LEX subsystem as an In Addr packet. The control logic (101
)
within the LEX subsystem forwards the In Addr packet to the REX subsystem as
in Figure 9.
At this point in the sequence, an additional step is introduced. Said
control logic (101 ) within the LEX subsystem generates a synthetic data
packet
and transmits it as a Data Payload packet to the Host PC. Said synthetic data
packet is labelled Data Payload_0 in Figure 9. The remainder of the protocol
handling continues as described in.Figure 8.
The USB specification defines a DATA packet as being composed of a
single byte Packet Identifier (PID), a Data Payload ranging in size from 0
bytes
to 1023 bytes, and a Cyclic Redundancy Check of 2 bytes. Said synthetic data
packet can therefore be constructed by assembling a data packet containing a
zero length payload. Said synthetic data packet can thereby be used to satisfy
the timing requirements of the USB protocol while causing no disturbance to
the
actual information being carried by the protocol.
Figure 10 provides a sequence diagram showing an isochronous out ut
transfer according to the invention.
In Frame 1, the control logic (120) within the Host PC generates a
notification of output data, addressed to a particular USB function. Said
notification is transmitted to the LEX subsystem as an Out Addr packet. The
control logic (121 ) within the LEX subsystem forwards said Out Addr packet to
the REX subsystem. The control logic (122) within the REX subsystem forwards
said Out Addr packet to the Device. The information is received by the control
logic (123) within the device.
The control logic (120) within the Host PC assembles the notified
isochronous data and transmits it as a Data Payload packet to the LEX
subsystem. The control logic (121 ) within the LEX subsystem forwards the Data
Payload packet to the REX subsystem. The control logic (122) in the LEX
subsystem further forwards the Data Payload packet to the device. The
information is received by the control logic (123) within the device.
Figure 11 provides an algorithm for implementing isochronous transfers
at the Local Expander (4). In the embodiment of the invention described in
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Figure 6, said algorithm is implemented in hardware by LEX Controller (51 ).
(It
will be apparent to those skilled in the art that this implementation is not
unique
and that other mechanisms for implementing said algorithm are possible.) In
the
following description, the inbound direction refers to packets travelling from
a
peripheral device and towards a host controller; the outbound direction refers
to
packets travelling from a host controller and towards a peripheral device.
According to the invention, the algorithm of Figure 11 is required to
implement the processing functions represented by processing blocks of Figure
8 (101, 105 & 109), and the processing block (121) of Figure 10.
On initial power-up and after a reset, the system enters Idle state (200)
where it waits for a message (USB packet) to be received. When a packet is
received, the Packet Identifier (PID) field within the packet is examined to
determine what type of packet has been received and what action is required to
process said packet.
If the received packet (201 ) is of type IN, then an identical packet (202) is
transmitted. The system then examines (203) its buffer memory to determine
whether a stored DATA packet from the device addressed by the IN request is
already present in memory. If no such stored DATA packet is present in
memory, the system then returns to the idle state (200). If said stored DATA
packet is present in memory, the system retrieves (204) said stored packet
from
memory and sends (205) said packet to the host as a packet of type DATA. It
then returns to the Idle state (200).
If the received packet (206) is of type DATA and is travelling in the
inbound direction, the system saves (207) said data packet in its buffer
memory.
The system then returns to the Idle state (200).
If the received packet (208) is of type OUT Addr, the system retransmits
the packet (209) packet and returns to the Idle state (200).
If the received packet (210) is of type DATA and is travelling in the
outbound direction, the system retransmits packet (211 ) and returns to the
Idle
state (200).
Figure 12 provides an enhanced algorithm for implementing isochronous
transfers at the Local Expander according to the invention described in Figure
9.
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This algorithm is essentially identical to that described in Figure 11, other
than
in the way in which it handles the situation wherein the Host issues a request
for
data and the LEX does not yet possess said data to return to the host. This
situation may occur on the first request of an isochronous input session.
In Figure 12, if the received packet (201 ) is of type IN, then an identical
packet (202) is transmitted. The system then examines (203) its buffer memory
to determine whether a stored DATA packet from the device addressed by the
IN request is already present in memory. If no such stored DATA packet is
present in memory, the system generates (212) a synthetic DATA packet and
transmits (213) said generated packet to the host within the 16 bit-times
permitted by the USB protocol. If said stored DATA packet is present in
memory,
the system retrieves (204) said stored packet from memory and sends (205)
said packet to the host as a packet of type DATA. It then returns to the Idle
state
(200).
If the received packet (206) is of type DATA and is travelling in the
inbound direction, the system saves (207) said data packet in its buffer
memory.
The Packet Aging Flag associated with the newly saved packet is reset (214)
for
reasons that will be described in the context of Figure 13. The system then
returns to the Idle state (200).
Figure 13 is a logic diagram of a further enhancement to the LEX
Controller enabling vestigial packets to be removed. Said vestigial packets
can
occur during Isochronous Input transfers when the Host PC decides to terminate
a session but the LEX Controller already possesses an inbound data packet
belonging to said session. This situation is illustrated, for example, in
Figure 8
by packet Data Payload 3.
Under the USB Specification, the start of every frame is marked by a
unique Start Of Frame (SOF) signal. The embodiment described in Figure 13
uses this SOF signal to identify and discard packets that are no longer
required.
Processing commences when SOF signal (221 ) is received by the Local
Expander. A search of local buffer memory is commenced by setting the context
to the first buffer (222). The Packet Aging Flag of the packet pointed to by
the
current context is examined (223). If the Packet Aging Flag is already set,
then
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the packet in the buffer is discarded (224). If the Packet Aging Flag is not
set,
then the Packet Aging Flag is set (225) to ensure that the packet will be
discarded in the next frame if it has not been transmitted by then.
The aforesaid process is then repeated by setting the context to the next
buffer (227) and continuing until all buffers have been checked, as detected
by
test (226).
Note that as an additional precaution to prevent the premature discarding
of vestigial packets, the packet Aging Flag is also reset by step (214) of
Figure
12 whenever a data packet is stored in the buffer memory of the LEX
Controller.
Figure 14 provides an algorithm for implementing isochronous transfers
at the Remote Expander. In the embodiment of the invention described in Figure
7, said algorithm is implemented in hardware by REX Controller (61 ). (It will
be
apparent to those skilled in the art that this implementation is not unique
and
that other mechanisms for implementing said algorithm are possible.) In the
following description, the inbound direction refers to packets travelling from
a
peripheral device and towards a host controller; the outbound direction refers
to
packets travelling from a host controller and towards a peripheral device.
According to the invention, the algorithm of Figure 14 is required to
implement the processing functions represented by the processing blocks (102),
(106) & (110) of Figure 8, and the processing block (122) of Figure 10.
On initial power-up and after a reset, the system enters Idle state (230)
where it waits for a message (USB packet) to be received. When a packet is
received, the Packet Identifier (PID) field within the packet is examined to
determine what type of packet has been received and what action is required to
process said packet.
If the received packet (231 ) is of type IN, the system retransmits (232)
said IN packet and returns to the Idle state (230).
If the received packet (233) is of type DATA and is travelling in the
inbound direction, the system retransmits (234) said data packet and returns
to
the Idle state {230).
If the received packet (235) is of type OUT, the system retransmits (236)
said out packet and returns to the Idle state (230).
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If the received packet (237) is of type DATA and is travelling in the
outbound direction, the system retransmits (238) said data packet and returns
to
the Idle state (230).
Figure 15 provides an enhanced algorithm for implementing isochronous
transfers at the Remote Expander. Under certain conditions, there is a risk
that
packets sent from the Remote Expander and arriving at the Local Expander may
not be in the order expected by the Local Expander. The enhancement
described in Figure 15 adds an Address field to packets sent in the inbound
direction so that the Local Expander can be certain of the origin of said
packets.
According to the enhancement, when an IN packet is received (231 ) by
the REX Controller, the Address it contains is stored in memory (239). The IN
packet is then retransmitted as before (232) and the system returns to the
Idle
state (230). When the system receives the next DATA packet travelling in the
inbound direction (233), the system retrieves the Address stored in memory
(240) and includes this information with the DATA packet that it retransmits
(234) to the LEX. The system returns to the Idle state (230).
Having described specific embodiments of the present invention, it will be
understood that modifications thereof may be suggested to those skilled in the
art, and it is intended to cover all such modifications as fall within the
scope of
the appended claims. Additionally, for clarity and unless otherwise stated,
the
word "comprise" and variations of the word such as "comprising" and
"comprises", when used in the description and claims of the present
specification, is not intended to exclude other additives, components,
integers or
steps.
