Note: Descriptions are shown in the official language in which they were submitted.
2~ 35
LOCAL AREA NETWORR 8Y8TEM
Background of the Invention
1. Field of Invention. This invention relates to local
area network systems.
2. Background Information. Local area networks (LANs)
using wired interconnections have the disadvantage that
extensive cabling is required to interconnect the
stations. The provision of such cabling is generally
inconvenient and gives rise to inflexibility if it is
desired to alter the physical locations of the stations.
Wireless transmission links, operating at radio
frequencies, have been used to replace the cabling
interconnections of a LAN. However, certain problems
are associated with the use of a wireless transmission
link, particularly for LANs in an indoor environment.
One problem associated with wireless LANs operated over
radio frequency links is that the radio frequency band
is a scarce resource. In order to economize with this
resource, it is desirable for stations to utilize the
same frequency band.
If the application dictates that a large number
of stations be interconnected, then, in order to avoid
overloading the LAN, it is desirable that groups of
stations be interconnected into separate LANs. Such
LANs can be interconnected by bridges. Such bridges are
well known devices and are the subject of IEEE Standard~
802.lD. However, where a single wireless transmission
channel is utilized, a problem arises if stations of one
LAN have overlapping coverage areas with stations of
another LAN. In this situation, interference may arise
between stations of different LANs, possibly causing the
loss of some transmitted message frames. Although error
recovery procedures are available in some circumstances
to recover lost frames, such procedures are not always
convenient to apply in some practical implementations.
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Summary of the Inventlon
It is an ob~ect of the present lnventlon to provlde
a method and apparatus for transmlttlng messages from a host
computer to a plurallty of termlnals, uslng a wlreless
transmlsslon llnk, whereln hlgh rellablllty ls achleved.
Therefore, accordlng to one aspect of the lnventlon,
there ls provlded a method of transmlttlng message frames form
a host computer to a plurallty of termlnals comprlslng the
steps of: (a) provldlng flrst, second to nth brldges
communlcatlng vla respectlve flrst ports wlth sald host
computer and communlcatlng vla respectlve second ports over a
common wlreless communlcatlon channel wlth respectlve wlreless
local area networks contalnlng respectlve pluralltles of sald
termlnals; (b) transmlttlng sald message frames from sald
host computer to sald flrst, second to nth brldges; (c)
transmlttlng, when sald common wlreless communlcatlon channel
ls clear, selected message frames from sald flrst, second to
nth brldges over sald wlreless communlcatlon channel so that
each selected message frame ls transmltted ln non-overlapplng
sequentlal tlme lntervals by all of sald flrst, second to nth
brldges so as to avold the posslblllty of colllslon at a sald
termlnal of one wlreless local area network wlth a sald
termlnal of an ad~acent wlreless local area network; and (d)
retransmlttlng sald selected message frames from sald flrst,
second to nth brldges over sald wlreless communicatlon channel
so that each selected message frame ls transmltted ln non-
overlapplng sequentlal tlme lntervals by all of sald flrst,
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second to n brldges so as to avold the posslblllty of
colllslon at a sald termlnal of one wlreless local area
network wlth a sald termlnal of an ad~acent wlreless local
area network; sald provldlng step ta) belng effected by; (a-l)
lnterconnectlng sald host computer and sald brldges ln a wlred
local area network; and sald transmlttlng step (b) belng
effected by: (b-l) utlllzlng a control fleld lncluded ln sald
message frames to lndlcate that sald selected message frames
are unnumbered lnformatlon frames and contaln no lndlvldual
destlnatlon address and that sald transmlttlng and
retransmlttlng steps (c) and (d) are effected upon only sald
selected message frames.
A preferred embodlment of the present lnventlon wlll
now be descrlbed by way of example, wlth reference to the
followlng speclflcatlon, clalms and drawlng.
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Brief Description of the Drawing
Fig. 1 is a block diagram of a local area
network system according to this invention.
Fig. 2 is a flowchart illustrating the
operation of bridges included in the local area network
system of Fig. 1.
Fig. 3 is a timing diagram illustrating a
download operation.
Fig. 4 is a diagram illustrating a format of
message frames transmitted in the local area network
system.
Detailed Description of the Invention
Referring now to Fig. 1, there is shown a local
area network (LAN) system 10. The system 10 includes a
wired LAN 12 which is formed by a host computer 14
connected by cabling 16 to a plurality of bridges 18,
referred to individually as bridge 18-1, bridge 18-2,
etc. In the described embodiment six bridges 18 are
utilized, but the total number of bridges 18 may be
varied according to a particular implementation. Also,
optionally connected in the wired LAN 12 are a plurality
of terminals referenced individually as terminal 20-1,
terminal 20-2, and terminal 20-3. The number of
terminals 20 is arbitrary, and in some environments, the
terminals 20 may be omitted altogether, such that only
the host computer 14 and the bridges 18, are connected
into the LAN 12. The host computer 14, the bridges 18,
and the terminals 20 may be implemented by personal
computers (PCs).
The bridges 18 have respective first ports 22
referenced individually as port 22-1, port 22-2, etc.,
and respective second ports 24, referenced individually
as port 24-1, port 24-2, etc...Each bridge 18 connects
via its first port 22 with the LAN 12 and via its second
port 24 with a respective wireless LAN. In the
described embodiment there are six wireless L~s, 100,
26~
200,..., and 600, Each second port 24 communicates with
its associated wireless LAN via an antenna 26,
referenced individually as antenna 26-1, antenna 26-2,
etc.
In the described embodiment each wireless LAN
100, 200,..., and 600 is shown as having three terminals
included therein, although it will be appreciated that
this is merely by way of example. Each wireless LAN
could have any desired number of terminals, and the
number of terminals in the respective wireless LANs is
not necessarily the same. The wireless LAN 100 is shown
as comprising terminals 102, referenced individually as
terminal 102-1, terminal 102-2, and terminal 102-3. The
respective terminals have antennas 104, referenced
individually as antenna 104-1, antenna 104-2, and
antenna 104-3. Similarly, the wireless LAN 200
comprises terminals 202, referenced individually as
terminal 202-1, terminal 202-2, and terminal 202-3,
having respective antennas 204, referenced individually
as antenna 204-1, antenna 204-2, and antenna 204-3. The
remaining wireless LANs (only LAN 600 is shown) have
analogously identified terminals.
It should be understood that the LAN 12
operates as a conventional Starlan network at a 1
Megabit per second rate, utilizing a conventional
CSMA/CD (Carrier Select Multiple Access with Collision
Detection) medium access protocol. The wireless LANs
100, 200,..., and 600 operate at a 2 Megabits per second
rate utilizing a CSMA/CA (Carrier Select Multiple Access
with Collision Detection) medium access protocol. With
this protocol, if a station (terminal or bridge) in the
network wishes to transmit a data frame, but the
transmission channel is sensed as busy, transmission of
the data frame is deferred, and after the carrier
becomes inactive, a new attempt is made to transmit the
frame after a random backoff time.
2~
The wireless LANs 100, 200,..., and 600 operate
over a common wireless communication channel. The
bridges 18 may have overlapping coverage areas. For
example, the wireless coverage area of the bridge 18-1
may partially overlap the wireless coverage area of the
bridge 18-2. With this arrangement, it will be
appreciated that there is a small risk that message
frames can get lost, despite the use of the CSMA/CA
protocol. Thus, message frames can get lost where
wireless LAN stations (bridge or terminal) transmit
parallel-in-time. Such parallel-in-time transmission can
occur because the CSMA/CA protocol avoids this only when
LAN stations are in the coverage area of each other. By
way of example, assume that terminal 102-3 is in the
coverage range of bridges 18-1 and 18-2, but the bridges
18-1 and 18-2 are outside each other's covering range.
Then, the transmission of a message frame from the
bridge 18-1 to the terminal 102-3 could be corrupted by
a transmission by the bridge 18-2. This is because, in
the environment of a wireless LAN, in order to achieve
reliable reception, a receiving station requires a
certain capture ratio, for example 10 dB, for the
received signal over other signals in the channel, and
the transmission by the interfering bridge 18-2 may
prevent this ratio being achieved for the reception of
the transmission by the bridge 18-1. Also, it will be
appreciated that even within a single one of the
wireless LANs 100, 200,..., and 600, there is a (very
slight) chance of two transmissions commencing
substantially simultaneously and the two transmissions
corrupting each other, despite the use of the CSMA/CA
protocol.
The described system 10 has a particular
application in a retail shop or store, where the
terminals 20, 102, 202,..., and 602 are retail
terminals, and the system operates under the general
control of the host computer 14, although it should be
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understood that other applications are possible. It
will be appreciated that in such a environment, it is
advantageous to use wireless transmissions in order to
avoid the need for cabling. Terminals can very easily
be added to or removed from the system 10.
In the retail system environment, various types
of messages are transmitted in the system 10. Thus, all
the terminals 20, 102, 202,..., and 602 are polled with
"alive" messages at predetermined time intervals, for
example 20 seconds, to determine whether the terminals
are active. Also, when a retail transaction is
effected, numerous message frames are transmitted in
each direction between the terminal effecting the
transaction and the host computer 14, including item
price look-up message frames and transaction initiating
and terminating message frames. Nevertheless, it is
found that in such a retail system the total load factor
of all partially overlapping wireless LANs 100, 200,....
and 600 is very low. Therefore, only a very small
proportion of transmitted frames are lost in the manner
described hereinabove. "Alive" and transaction message
frames that are lost on the "Physical Layer" can be
recovered by known procedures on the "Logical Link
Layer", in accordance with the known IEEE 802.2 standard
using sequence numbering for the message frames.
For the transmission of such "alive" and
transaction messages, each bridge 18 applies the known
spanning tree algorithm, in accordance with IEEE
Standard 802.lD, for example, whereby it is ensured that
message frames from the host computer 14, which include
a destination address for the frame, are retransmitted
by only one bridge 18. For this purpose, each bridge 18
includes a database identifying the terminals which are
"below" that bridge for the purpose of the spanning tree
algorithm. The database acts as a filtering database
for the message frames and is updated by source address
information in the message frames from the wireless
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terminals 102, 202,..., and 602, directed to the host
computer 14.
The terminals 102, 202,..., and 602 are
diskless and will start up using a boot ROM (not shown).
When a terminal is added to the system 10, for example,
a download procedure of a large number of information
frames is required to supply programming information and
set up the terminal in accordance with the particular
application. Initially, at start up, a terminal 102,
202,..., and 602 generates a Load Request Message, which
advises the relevant bridge 18 that the terminal is
below that bridge. In this connection, it should be
understood that each terminal 102, 202,..., and 602 is
assigned to a particular bridge, such assignment being
based on network identification codes (NWID codes at the
Physical Layer), which codes identify the respective
wireless LANs 100, 200,..., and 600.
A download operation from the host computer 14
to the terminals is made initially to start up the
system 10. Also, a download operation can be effected
for one or more of the terminals while the system 10 is
operational, upon request from a terminal, for example,
as by a newly added terminal.
A download operation will now be described by
way of example, with reference to Figs. 2 and 3. A
download operation is commenced by a terminal issuing an
"Initiate Load" message to the host computer 14. Assume
that the terminal 102-1 issues such a message. The host
computer issues a "Response Load" message for the
terminal 102-1 and for any other terminal which also has
issued an "Initiate Load" message within a predetermined
time interval determined by a timer (not shown) in the
host computer 14. Assume that the terminal 202-1 has
also issued an Initiate Load message. Next, the
terminals 102-1 and 202-1 each send a "Load
Configuration" message to the host computer 14. The
download now starts. In the download, a sequence of
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approximately 1000 "Load Record" message frames,
designated "Unnumbered Information" frames, of 550 bytes
each, is sent to the terminals, with a frame separation
time of 200 milliseconds (corresponding to the
MinBlockDelay parameter time mentioned in the IEEE
Standard 802.lE). Finally, an "End of Load" message is
sent by the host computer 14 to the terminals and the
individual terminals respond with a "Status Response"
message.
Referring to Fig. 2, there is shown a flowchart
for the operation of a bridge 18. In a wait state, the
bridge 18 waits for the next frame (box 250). If a
frame is received from the wired LAN 12 (box 252), a
determination is made as to whether the frame is a
download (UI) frame (box 254). If a frame is received
from the relevant wireless LAN 100, 200,..., and 600
(box 256), the normal bridge functionality is effected
(box 258); for example, the frame is transmitted to the
host computer 14, and the bridge 18 returns to the wait
state (box 250). Returning to box 254, if the frame
received from the wired LAN 12 is not a download frame,
as for example, if the frame relates to an "alive"
message or is a transaction frame, then the normal
bridge functionality is effected (box 258); for example,
the frame is transmitted to the relevant destination
terminal over the wireless channel, and the bridge
returns to the wait state (box 250). If in box 254, it
is determined that the frame is a download (UI) frame,
then a time interval of N times 15 milliseconds is
timed, where N is 0 for bridge 18-1, 1 for bridge 18-2,
2 for bridge 18-3, and so forth., After the elapse of
such time interval, the download frame is transmitted by
the bridge over the wireless channel.
Referring briefly to Fig. 3, line A shows the
receipt time 350 by the bridges 18 of a download frame
over the 1 Megabit per second wired LAN 12, such frame
having a duration of 4.5 milliseconds. Referring to
line B, the retransmission by the bridge 18-1 of the
download frame is shown as having a 2.4 millisecond time
352, it being appreciated that transmission over the
wireless channel is at a 2 Megabit per second rate, and
allows for a long preamble time and MAC overhead bits in
the transmitted frame. After a 15 millisecond time
delay, the bridge 18-2 transmits the download frame. As
shown by time durations 356-362 in lines D-G, the
remaining bridges 18-3 to 18-6, transmit the download
frame with respective delays each 15 milliseconds
greater than the delay of the previous bridge. Thus,
the download message frames are transmitted by the
bridges 18 in non-overlapping fashion, avoiding the
possibility of collisions at a terminal which may lie in
the coverage area of more than one bridge 18.
Returning again to Fig. 2, it is seen that
after transmitting the download frame on the wireless
channel (box 258), the bridge waits a further 100
milliseconds (box 260) and then re-sends the download
frame on the wireless channel (box 262). This is shown
by the 2.4 millisecond time duration 370 in Fig. 3, line
B, which represents transmissions by the bridge 18-1.
Similarly, although not shown in Fig. 3, the bridges 18-
2 to 18-6 re-send the download frame with successive 15
millisecond delays, precisely as for the first
transmissions of the download frame shown by time
durations 354-362. When a bridge has re-sent the
download frame for the second time, it returns to the
wait state (box 250, Fig. 2). It should be understood
that it is important that the time spacing (200
milliseconds) between the download frames on the wired
LAN 12 is substantially greater than the total time to
send out a download frame time twice with different time
shifts on the six wireless LANs 100, 200,..., and 600 (2
x 6 x 4.5 milliseconds).
It will be appreciated that the duplication of
the sending of the download frames makes the receipt of
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the information contained in a single download frame
very reliable with regard to risk of a collision of such
a message with an "alive" message or a transaction
message both from a neighboring LAN and from its own
LAN, particularly bearing in mind the low load factor
for alive/transaction messages discussed hereinabove.
Referring to Fig. 4, there is shown a
representation of a message frame 400 showing the
features thereof which are of importance to the subject
invention. The message frame 400 includes a Physical
Layer preamble portion 402 which includes a network
identification (NWID) code field portion 404. The
message frame 400 also includes a MAC (Medium Access
Control Layer) portion 406 which includes a control
field portion (CONTR) 408. The NWID code portion
identifies the particular wireless LAN 100, 200,..., or
600 to which the message frame relates, as discussed
hereinabove. The control field portion 408 identifies
the type of MAC frame, for example, a (numbered)
information frame, a supervisory frame, or an unnumbered
information (UI) frame. It should be understood that
the unnumbered information frames contain no individual
destination address, and that only these selected frames
are subject to the time-shifted and duplicated
transmission described hereinabove.
Summarizing, it is seen that there has been
described a local area network system wherein a host
computer 14 communicates via a wired local area network
12 with a plurality of bridges 18 which communicate via
respective wireless LANs 100, 200,..., and 600 with
terminals 102, 202,..., and 602. The bridges 18
normally transmit message frames to the relevant
destination station, wherein the spanning tree algorithm
determines the message routing. However, certain
selected message frames, determined by the bridges to be
unnumbered information frames belonging to download
messages, are transmitted by the respective bridges 18
in a time-shifted manner, such time-shifted transmission
taking place twice for each such selected download
message frame.
The described embodiment has several
advantages. Thus, a very reliable download session from
a host computer to terminals is achieved for wireless
channel transmission. A further advantage is that
existing application programs for host-terminal system
based on a wired LAN can be used with no significant
changes. In this connection it will be appreciated that
existing application programs for such systems adapted
for a retail environment do not generally implement an
error recovery procedure for download sessions. The
described embodiment avoids the high cost which would be
involved in adapting an existing download procedure to
provide reliable error recovery. Also, the duration of
the download session in the described wireless network
system is substantially the same as the duration of the
corresponding download session in a wired LAN.
