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Claims  |
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We claim:
1. A communications system for interconnecting multiple local area networks
across backbone networks independently and transparently of protocols
above the data link layer and so that the multiple local area networks
appear to the users of stations of all local area networks as one large
single local area network, said system comprising,
a plurality of local area networks with each local area network having at
least one station for sending or receiving communications to or from
another station using data link frames containing at least a destination
address and a source address,
at least one backbone network comprising only simplex broadcast channels
and connecting two or more local area networks and wherein each backbone
network has one and only one transmit simplex channel associated with one
or more receive simplex channels,
a plurality of bridge means with each bridge means interconnecting across
simplex channels a plurality of local area networks to permit one or more
stations in one local area network to communicate with one or more
stations in one or more of the other local area networks,
each said bridge means comprising,
network interface means for interfacing the bridge to related local area
networks and for sending information in frame format between related local
area networks across one or more backbone networks,
bridge management means for defining the association, in a backbone
network, of the single transmit simplex channel and the receive simplex
channels in three possible topologies, i.e. non-rooted, single rooted and
multi rooted,
learning means for learning the location of end user stations relative to
the backbone network so that frames received from the backbone network can
be forwarded or discarded,
forwarding means for forwarding a frame received from one station in one
local area network to a designated station in another local area network
across one or more backbone networks independently and transparently of
protocols above the data link layer,
discarding means associated with each receive simplex channel for
selectively discarding frames,
wherein the transmit simplex channel characteristics are completely
independent of receive simplex channel characteristics and the
characteristics of each individual receive simplex channel are independent
of other receive simplex channel characteristics, and
wherein the backbone network is not required to be related to the local
area network access technique or bandwidth.
2. The invention defined in claim 1 wherein the bridge means include local
processes and the forwarding means are operative to forward a frame
received from
(a) one station in one local area network attached to the bridge or
(b) a process within the same bridge means or
(c) simplex links attached to the bridge means to and from one or more
(1) local area networks attached to the bridge means and
(2) simplex channels attached to the bridge means and finally to a
designated station in another local area network or
(3) to a local process in another bridge means.
3. The invention defined in claim 1 wherein there is a unique network
interface means operatively associated with each input and output simplex
channel connected to the bridge means.
4. The invention defined in claim 2 wherein the bridge management means
include data store and index means for associating the transmit simplex
channel with one or more receive simplex channels to form a non-rooted,
rooted or multirooted network.
5. The invention defined in claim 4 wherein the data store and index means
include a forwarding data store which contains entries created from frames
received with a unique source address value and a local variable which
identifies the receive simplex channel of the frame.
6. The invention defined in claim 5 wherein the data store and index means
are effective, for each entry created in the forwarding data store, to
insert the address value also into a source network cache such that a
frame received from the network with a destination address value already
in the cache is discarded quickly without accessing the forwarding data
store.
7. The invention defined in claim 5 wherein the forwarding means for
locating a matching forwarding data store entry use the last 12-16 bits of
a 48 bit destination address.
8. The invention defined in claim 5 wherein the discarding means discard a
frame from an receive simplex channel based upon the recognition that the
frame's receive simplex channel and the simplex channel identified in the
matching forwarding data store entry are part of the same network so that
discarding that frame thereby avoids unnecessary propagation.
9. The invention defined in claim 5 wherein the forwarding means for
forwarding a frame to a transmit simplex channel are based on associating
the receive simplex channel identified in the matching forwarding data
store entry to the transmit simplex channel of the network to which the
frame is to be forwarded.
10. The invention defined in claim 4 including encapsulation,
decapsulation, and discarding means to give rooted and multi-rooted
networks the appearance of being non-rooted such that the retransmitted
frames are appropriately filtered.
11. The invention defined in claim 10 wherein the bridge management means
include reconfiguration means for permitting reconfiguration, through
communication with a reconfiguration process in the bridge means, of part
of a network from a non-rooted to a rooted topology configuration and vice
versa.
12. The invention defined in claim 11 wherein the dynamic reconfiguration
means are constructed for dynamically reconfiguring the topology from a
non-rooted to a rooted configuration and vice versa in response to a
signal indicating the desirability of such reconfiguration.
13. The invention defined in claim 2 including configurable discarding
means for allowing networks to be sheltered from frames with specific
single destination addresses or multicast destination addresses such that
those frames from remote local area networks are not forwarded onto
specific simplex channels or input from specific input simplex channels to
thereby preserve locality.
14. The invention defined in claim 2 including configurable discarding
means for allowing networks to be sheltered from all but frames with
specific destinations addresses.
15. The invention defined in claim 2 wherein the forwarding means execute
in an interrupt level mode and including local processes that are within
the bridge means and that execute in a non-interrupt mode such that the
bridge management means of the bridge means cannot interfere with the
forwarding means.
16. The invention defined in claim 4 wherein the data store and index means
include a forwarding data store for receiving a frame and finding a
matching forwarding data store entry for the single destination address,
the forwarding data store also contains a source network ID,
the source network ID is not a network in the sense of being a rooted
network or a non-rooted network,
the source network ID identifies a receive simplex channel,
the transmit network ID identifies a transmit simplex channel,
the source network ID is used as an index into a receive data store,
the index locates a receive data store entry in the receive data store,
the entry so located defines the receive simplex channel,
one of the values in the located entry is a transmit network ID,
the transmit network ID is used as an index into the transmit data store,
said one value further locates an entry in the transmit data store,
the entry so found in the transmit data store defines the tranmit simplex
channel associated with the forwarding data store entry that the frame was
received on, and
wherein when the bridge means receive a frame from the receive simplex
channel, the bridge means get with the frame from the network interface
means the received network ID associated with that receive simplex channel
and
wherein the bridge means go through the same train of logic to find the
transmit simplex channel associated with that network.
17. The invention defined in claim 1 wherein the forwarding means are
effective to forward frames based on a determination of two questions:
(1) What simplex channel is the destination on? and
(2) What simplex channel did the frame come from?
18. A method of interconnecting in a communications system multiple local
area networks across backbone networks independently and transparently of
protocols above the date link layer and so that the multiple local area
networks appear to the users of stations of all local area networks as one
large single local area network, each of the local area networks having at
least one station for sending or receiving communications to or from
another station using data link frames containing at least a destination
address and a source address, said method comprising,
interconnecting local area networks through a plurality of bridges with
each bridge having at least one associated backbone network which
comprises only simplex broadcast channels and which has one and only one
transmit simplex channel associated with one or more receive simplex
channels,
defining in each bridge the transmit simplex channel and receive simplex
channels of an associated backbone network in three possible topologies,
i.e. non-rooted, single rooted and multi rooted, and
learning the location of end user stations relative to each backbone
network so that frames received from the backbone network can be forwarded
or discarded,
forwarding through a bridge a frame received from one station in one local
area network to a designated station in another local area network across
one or more backbone networks,
selectively discarding frames received on the receive simplex channels,
employing transmit simplex channel characteristics which are completely
independent of receive simplex channel characteristics and employing
characteristics of each individual receive simplex channel which are
completely independent of other receive simplex channel characteristics,
and
forwarding frames across a backbone network in a way which is not required
to be related to the local area network access technique or bandwidth.
19. A bridge for a communications system of the kind in which there are a
plurality of local area networks with each local area network having at
least one station for sending or receiving communications to or from
another station using data link frames containing at least a destination
address and a source address and of the kind in which multiple local area
networks are interconnected across backbone networks independently and
transparently of protocols above the data link layer and so that the
multiple local area networks appear to the users of stations of all local
area networks as one large single local area network, said bridge means
comprising,
network interface means for interfacing the bridge to related local area
networks and for sending information in frame format between related local
area networks across one or more backbone networks and wherein each
backbone network comprises only simplex broadcast channels and has one and
only one transmit simplex channel associated with one or more receive
simplex channels,
bridge management means for defining the association, in a backbone
network, of the single transmit simplex channel and the receive simplex
channels in three possible topologies, i.e. non-rooted, single rooted and
multi rooted,
learning means for learning the location of end user stations relative to
the backbone network so that frames received from the backbone network can
be forwarded or discarded,
forwarding means for forwarding a frame received from one station in one
local area network to a designated station in another local area network
across one or more backbone networks independently and transparently of
protocols above the date link layer,
discarding means associated with each receive simplex channel for
selectively discarding frames,
wherein the transmit simplex channel characteristics are completely
independent of receive simplex channel characteristics and the
characteristics of each individual receive simplex channel are independent
of other receive simplex channel characteristics, and
wherein the backbone network is not required to be related to the local
area network access technique or bandwidth. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to methods and apparatus for bridging together local
area networks.
This invention relates particularly to a communication system for
interconnecting multiple local area networks across broadcast simplex
channels independently and transparently of protocols above the data link
layer so that the system appears to a user at a station in a local area
network as one large single network.
Ethernet networks and/or 802.3 Local Area Networks (LAN's) are being
installed in conjunction with a wide variety of office automation and data
communication products. The LAN's are used to interconnect a number of
products which use various network architectures (e.g., TCP/IP, XNS,
DECnet, etc.). As additional LAN's are installed in other locations the
need to link together the remote LAN's is often initially ignored. Then,
when interconnect options for interconnecting the remote LAN's are
investigated, it often becomes apparent that the simple, multipurpose data
highway environment (that exists within a building or a single LAN) has
disappeared.
Connecting a number of remote LAN's can present problems in software and
can also present problems in complex mult-vendor compatibility. The
architecture for interconnection can also become an issue. Redundant
configurations for different internet protocols may be required, and some
to the LAN stations may not support an internet implementation.
SUMMARY OF THE PRESENT INVENTION
It is a primary object of the present invention to interconnect multiple
Local Area Networks by a communications system which avoids problems
presented by prior art techniques.
It is a specific object of the present invention to connect more than two
Local Area Networks across simplex channels through a bridge and to
provide communication between stations.
It is a related object to communicate with one or more stations and one or
more remote Local Area Networks independently and transparently of
protocols above the data link layer so that the system appears to a user
at a station in a Local Area Network as one large single network.
In accordance with the present invention a plurality of Local Area Networks
are connected together by multiple bridges. Each Local Area Network has at
least one station for sending or receiving communications to or from
another station using data link frames containing at least a destination
address and a source address. The bridge interconnects the Local Area
Networks across simplex channels and permits one or more stations in one
Local Area Network to communicate with one or more stations in one or more
of the other Local Area Networks independently and transparently of
protocols above the data link layer.
The bridge is constructed to permit more than two Local Area Networks to be
interconnected across simplex channels through the bridge.
In the present invention there are four basic novel principles involved in
the operation of the system.
First, a simplex channel is associated with one and only one network.
Secondly, at each bridge a network has one and only one output simplex
channel and one or more input simplex channels.
Thirdly, from the standpoint of the bridge, all networks and LAN's can be
defined in terms of simplex channels.
Fourthly, a bridge is capable of bridging between more than two networks
and LAN's.
Communication system apparatus and methods which incorporate the structures
and techniques described above and which are effective to function as
described above constitute further, specific objects of this invention.
Other and further objects of the present invention will be apparent from
the following description and claims and are illustrated in the
accompanying drawings which, by way of illustration, show preferred
embodiments of the present invention and the principles thereof and what
are now considered to be the best modes contemplated for applying these
principles. Other embodiments of the invention embodying the same or
equivalent principles may be used and structural changes may be made as
desired by those skilled in the art without departing from the present
invention and the purview of the appended claims.
BRIEF DESCRIPTION OF THE DRAWING VIEWS
FIG. 1 is a diagram illustrating a taxonomy for describing Local Area
Network (LAN) interconnection.
FIG. 2 is a view of four Ethernet networks bridged together across a
satellite network in accordance with one embodiment of the present
invention.
FIG. 2A is a view which corresponds to FIG. 2 but which shows the actual
simplex channel configuration for a four node network.
FIG. 3 illustrates how the FIG. 2 configuration can be expanded using a
terrestrial line.
FIG. 4 illustrates how two bridges can be interconnected in accordance with
the present invention using either a broadcast medium or a point to point
medium (e.g., a terrestrial data link).
FIG. 5 is a comparison view showing the use of simplex channels including a
single broadcast simplex channel for star communications contrasted with
the use of multiple point to point duplex links for star communications.
FIG. 5A is a view like FIG. 5 showing a star topology. FIG. 5A is a four
node network and illustrates the simplex channel used for star
configuration.
FIG. 5B illustrates another topology. In FIG. 5B a four node network is
connected in what is referred to as multistar topology. FIG. 5B
illustrates the simplex channels required to support that topology.
FIG. 6 shows how simplex channels are used in accordance with the present
invention to support a fully connected topology. FIG. 6 is a view like
FIG. 2 but emphasizing and illustrating the simplex channels.
FIG. 7 illustrates a configuration containing two star topologies connected
to a LAN in the central site.
FIG. 8 is a view of an expanded FIG. 7 configuration. FIG. 8 shows a
communication system constructed in accordance with the present invention
and embodying a fully connected network. FIG. 8 illustrates how star
configurations are connected through a Local Area Network, and how a
number of those locations can be connected by a fully connected network.
FIG. 8 illustrates the actual configuration (as distinct from the user
perspective). The user perspective is illustrated in FIG. 9.
FIG. 9 is a diagrammatic view showing the user perspective of a
communication system incorporating the present invention. As illustrated
in FIG. 9 the overall configuration is viewed by all LAN stations as
containing a single LAN.
FIG. 10 illustrates the primary role of the bridge of the present
invention.
FIG. 11 illustrates secondary roles of the bridge of the present invention.
FIG. 12 is a view of a bridge constructed in accordance with one embodiment
of the present invention. FIG. 12 shows major component parts of the
bridge. Subsequent figures of the drawings show further details of these
component parts.
FIG. 13 illustrates features of the forwarding function of the bridge.
FIG. 14 illustrates features of the management functions of the bridge.
FIG. 15 shows the format of a forwarding data store incorporated in the
bridge of the present invention.
FIG. 16 shows details of a multicast array data store entry structure as
used in the present invention.
FIG. 17 illustrates the logical structure of a local or backbone network
control component of the bridge shown in FIG. 12.
FIG. 18 illustrates how a frame is encapsulated in certain operations of
the bridge illustrated in FIG. 12.
FIG. 19 illustrates how an encapsulated frame is decapsulated in the bridge
illustrated in FIG. 12.
FIG. 20 is a pictorial view of a communications system for interconnecting
Local Area Networks in accordance with one embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the text of this description the following references will be referred
to by the abbreviations in the brackets as indicated.
[DEC84] Digital Equipment Corporation, Network and Communications Catalog,
Summer, 1984
[DIX82] Digital, Intel, and Xerox, The Ethernet: A Local Area Network Data
Link Layer and Physical Layer Specifications, Version 2.0, November, 1982
[Hawe84] Bill Hawe, Alan Kirby, and Bob Stewart, "Local Area Network
Connection", Telecommunications, April, 1984
[IEEa83] IEEE Project 802 Local Area Network Standards, "IEEE Standard
802.3 CSMA/CO Access Method and Physical Layer Specifications", Approved
Standard, July, 1983
[IEEb83] IEEE Project 802 Local Area Network Standards, "Draft IEEE
Standard 802.4 Token Bus Access Method and Physical Layer Specifications",
Working Draft E, July, 1983
[IEEc83] IEEE Project 802 Local Area Network Standards, "Draft IEEE
Standard 802.5 Token Ring Access Method and Physical Layer
Specifications", Working Draft, July 1983
[ISO33] ISO-3309, "HDLC, Frame Structure", available from Computer and
Business Equipment Manufacturers Association, 1828 L St., N.W. Washington,
DC, 20036
[Orns75] Severo M. Ornstein and David C. Walden, "The Evolution of a High
Performance Modular Packet Switch", 1975 Internat. Conf. on Comm., San
Francisco, CA, June, 1975
[Stew84] Bob Stewart, Bill Hawe, and Alan Kirby, "Local Area Network
Interconnection", Telecommunications, Mar. 1984
Currently, Ethernet networks and/or 802.3 Local Area Networks(LAN's) are
being installed in conjunction with a wide variety of office automation
and data communication products. Once installed, many of these LAN's
become the data highway for interconnecting multiple products which
utilize various network architectures (e.g. TCP/IP, XNS, DECnet, etc.).
After one successful LAN installation, many organizations repeat the
installation in multiple other locations. In many cases the need to link
remote LAN's together is initially ignored. Later as organization begin to
investigate their interconnect options, they discover that the simple
multiple purpose data highway environment that exists within the building
has disappeared.
The traditional Internet LAN interconnection techniques support a single
architecture and consequently a subset of the current or potential LAN
population. Also, since Internet processes in the LAN stations must assist
in the interconnection, costly software upgrades may be required and
complex multi-vendor compatibility problems can occur. Which architecture
to interconnect becomes an issue. Redundant configurations for the
different Internet protocols may be required. Some of LAN stations may not
support an Internet implementation.
In contrast the TransLAN configuration and method of the present invention
provide a simple and elegant LAN interconnect solution that transparently
extends the public data highway paradigm to LAN interconnection. From the
perspective of all LAN stations, the present invention turns on arbitrary
number of Ethernet/802.3 LAN's into a single LAN. Using the ISO Reference
Model, this description briefly describes the relationship of the present
invention to other LAN interconnect devices. Next the simple architecture
and operational characteristics of the present invention are defined. This
is followed by a description of the flexibility of the present invention
relative to satellite, terretrial, and mixed configuration support, as
well as its extensibility to 802.4 andother LAN's.
Relationship of the Present Invention to Other Products
An interconnection system and method constructed and operated in accordance
with the present invention uses a device termed with Vitalink Bridge.
Before discussing how the Vitalink Bridge operates, it is useful to
understand its relationship to other LAN interconnect devices.
FIG. 1 illustrates a taxonomy for describing LAN interconnection [Stew84].
The taxonomy associates a LAN interconnection device with an ISO Reference
Model layer. A device is associated with the layer in which it relays
information from one network to another. The term network in this context
ranges from LAN segments, satellite links, and terrestrial lines in the
lower layers to network architectures (e.g. DECnet to SNA) in the higher
layers.
In this taxonomy it is important to note that the layer performing the
relay does not utilize information from the higher layers. In fact,
differing higher layer protocols can (and do) concurrently utilize the
same lower layer relay. Generally, the higher the relay layer, the more
specialized are the set of products and protocols serviced by the relay.
Also, factors such as overhead and complexity increase the higher the
layer number.
The layer relays of direct interest to the present invention are Repeaters,
Bridges, and Routers, layers 1-3 respectively. The most common of the
three, Repeaters and Routers are briefly described and then compared and
contrasted with Bridges.
REPEATERS: Physical Layer Relays
Repeaters relay physical layer protocol data units (bits) and control
signals (e.g. collision detection). They operate at LAN speed and add only
a very small amount of addition delay (e.g. less than 1 microsecond).
Repeaters are used to extend LAN configurations by connecting LAN channel
segments together directly or across an internal point to point link. In
general, the use of Repeaters in a LAN configuration is transparent to LAN
station protocols.
However, the use of Repeaters as a general mechanism for interconnecting
multiple LAN's is severely limited. The length of a single LAN (including
any internal point to point links) is limited by Physical layer
constraints such as maximum round trip propagation delay budget. This
limits LAN expansion using Repeaters to a few kilometers. The maximum
number of stations that can be effectively serviced by a single LAN is
another limiting factor.
Since Repeaters relay bits, they are unable to selectively filter Data Link
frames. Consequently, LAN expansion is restricted by maximum LAN capacity.
Another consequence of the absence of filtering is that links used by
repeaters to tie together two segments must operate at LAN speed.
An Ethernet Repeater [DIX82] is an example of a Repeater device.
ROUTERS: Network Layer Relays
Routers are the traditional LAN interconnect devices. When using these
devices, LAN stations must be able to distinguish between communication
with a station on the same LAN and a remote LAN. Remote communication
requires LAN stations to transmit/receive Data Link frames to/from a
Router on the same LAN.
The frames contain Internet protocol data units (packets) created by the
LAN stations. Routers utilizes the Internet protocol control information
in the packets and a local configuration topology table to determine how
to relay a packets between the LAN and other networks. (e.g., point to
point data links).
When compared to Repeaters, Routers are not transparent to LAN station
protocols. They only work with LAN stations having a compatible Internet
layer. Also, compared to Repeaters, Routers add significant delays. They
operate as a store and forward packet relay (not a bit relay). Their
internal processing time usually ranges from 5 to 50 milliseconds but more
significant are the internal queue delays and transmission time between
Routers.
Since LAN stations perform the filtering function for a Router (by only
sending it packets destined for a remote LAN), the Router to Router links
do not need to operate at LAN speeds. Typical link speeds range from 9.6
Kbps to 56 Kbps. Also, the maximum number of stations that can be
effectively serviced by a single LAN is no longer a limiting factor.
Stations can be spread amoung multiple interconnected LAN's.
A DECnet Router Server [DEC84] is an example of a Router device.
BRIDGES: Data Link Layer Relays
Bridges interconnect LAN's using the same media as Routers, but operate
totally within the Datak Link layer. LAN's connected together by Bridges
logically appear to the LAN stations as a single LAN.
LAN stations simply address Data Link frames to other stations as if they
were on the same LAN. Broadcast and Multicast destination frames are
handled properly. They are received by the addressed group of stations
regardless of location. LAN stations do not address frames to Bridges as
they must with Routers.
The Frame Check Sum value created by the source system is delivered to the
destination station. Bridged LAN's have the same level of protection
against corrupted data as is present on a single LAN. With Routers, the
original FCS is removed by the first Router and recreated by the Last.
Like Routers, Bridges store and forward frames. This means, that unlike
Repeaters, they are able to selectively filter and discard frames
addressed to local stations. Bridges keep local traffic on one LAN from
interfering with local traffic on the other LAN's. As a result, Bridge to
Bridge links can operate at less than LAN speeds. In fact, in almost all
configurations the same link speeds used to interconnect Routers can be
used to interconnect Bridges.
Also, as with Routers, the maximum number of stations that can be
effectively serviced by a single LAN is no longer a limiting factor. The
stations can be spread amoung multiple bridged LAN's [Hawe84]. In contrast
with Routers, since Bridges relay and filter for all LAN stations, they
provide the more general solution for a congested LAN.
Since Bridges operate at a lower layer than Routers, they have less
processing overhead and are capable of processing and relaying frames at
higher rates (thousands of frames/second). Consequently, Bridges are
capable of effectively utilizing high bandwidth links (1-10 megabits/sec)
between LAN's.
When bridging remote LAN's together with a link operating at LAN speed or
two local LAN's together directly, Bridges add a very small amount of
additional delay (at most a few milliseconds). In contrast, when utilizing
lower speed links, Bridges like Routers add significant delays due to
transmission time. However, for the same configuration, the delay
associated with a Bridge should be less than with a Router. This is due to
the reduced processing overhead within a Bridge.
While conceptually a Data Link Bridge is not a new idea, recently the
potential for these devices has greatly increased. Specifically, Digital
Equipment Corporation was the first to recognize this new potential
[Stew84]. The use of 48 bit global addressing in Ethernet and the 802
LAN's for the first time places a unique world wide identifier in the Data
Link layer. Also, Bridges are processing and memory intensive devices that
are able to exploit medium to high speed broadcast and point to point
technologies. Significant cost reductions and technical advancements are
occurring in all of these areas.
Operational Characteristics
To describe the operational characteristics of the present invention it is
useful to first illustrate and discuss one configuration of the present
invention. FIG. 2 illustrates four Ethernet [DIX82] and/or 802.3 [IEEa83]
LAN's bridged together across a satellite backbone network.
The backbone is operating in a fully connected broadcast mode such that any
frame transmitted by one Vitalink Bridge (VB1, VB2, VB3 or VB4) is
received by all other Bridges. Each Vitalink Bridge can be configured to
Transmit at the same or a different rate.
A fully connected Vitalink satellite network is very similar to an Ethernet
or 802.3 LAN. Both are a broadcast transmission media, support a
promiscuous (receive all frames) reception mode, and have a very low bit
error rate.
Both Ethernet and 802.3 utilize an unacknowledged datagram protocol.
Likewise, the Vitalink Bridges utilize an unacknowledged datagram protocol
across satellite backbone. The forwarded Ethernet/802.3 frames are simply
enveloped inside the HDLC frame structure [ISO33]. In order to allow for
concurrent support of Ethernet and 802.3 stations, the Vitalink Bridges
support the 48 bit 802.3 Address Field.
A single Vitalink Bridge can concurrently relay between 2-9 different
networks. For clarity and brevity, the following discussion configures
each Vitalink Bridge with only two networks. This allows a simplified
operational model to be utilized.
Listen.sub.-- Only Mode
When Vitalink Bridge 1 in FIG. 2 is powered on it enters into LISTEN.sub.--
ONLY mode. It remains in LISTEN.sub.-- ONLY mode for 10-60 seconds. VB1
operates in Promiscuous mode relative to LAN I and the satellite backbone.
As a result it receives all frames being transmitted by LAN stations A-C
or Vitalink Bridges 2-4. No frames are relayed by VB1 during LISTEN.sub.--
ONLY mode.
During LISTEN.sub.-- ONLY mode, VB1 automatically creates a local data base
(termed the Forwarding data store). A Forwarding data store entry is
created from each frame received with a unique Source Address value. The
entry contains the address and a local variable which identifies the
source of the frame (LAN I or satellite backbone).A Vitalink Bridge can
support a Forwarding data store of up to 8000 entries.
The following assumptions are made about the current activity within the
FIG. 2 configuration. Stations (A,B), (M,N), (Q,R), and (X,Y) are only
communicating locally on LAN I, II, III, and IV respectively. Stations
(N,S), and (R,Z) are communicating with each other across the satellite
backbone. Station C is turned off. As a result, the initial VB1 Forwarding
data store (in summary form) contains the following entries.
Entry 1--address=A, source=LAN.sub.-- I
Entry 2--address=B, source=LAN.sub.-- I
Entry 3--address=N, source=SATELLITE.sub.-- BACKBONE
Entry 4--address=S, source=SATELLITE.sub.-- BACKBONE
Entry 5--address=R, source=SATELLITE.sub.-- BACKBONE
Entry 6--address=Z, source=SATELLITE.sub.-- BACKBONE
The entry source values of LAN.sub.-- I or SATELLITE.sub.-- BACKBONE are a
locally assigned value. They are not globally administered or used as a
global identifier between Vitalink Bridges in any manner.
Forwarding Mode
After the LISTEN.sub.-- ONLY time period, the Vitalink Bridge enters
FORWARDING mode. In FORWARDING mode the maintenance of the Forwarding data
store based on Source Address continues in the background as defined
above. Determining whether to filter (discard) or relay frames becomes the
major foreground activity.
Relaying and Filtering Rules
When a single destination frame is received, a hash is created from the
Ethernet/802.3 destination address. The hash is used to locate a matching
Forwarding data store entry (in under 40 microseconds). If the matching
entry's source value identifies the frames source network, the frame is
discarded. Otherwise, the frame is relayed to the identified network. If
no matching entry is located, the frame is relayed to all networks other
than the source.
Since multicast or broadcast address values never appear as Source
Addresses, Forwarding data store entries are not automatically created. As
a result, multicast and broadcast frames are relayed like single
destination frames with no matching entries. However, this can be changed
by configuring broadcast and multicast entries into the Vitalink Bridges.
When this is done, multicast and broacast destination frames are
selectively filtered in the same manner as single destinaion frames.
Upon entering FORWARDING mode, VB1 in FIG. 2 begins relaying and filtering
frames in the following manner.
(1) Frames received from LAN I destined for A or B are not relayed to the
satellite network. (i.e., frames local to LAN I are filtered)
(2) Frames received from the satellite network destined for N, S, R, or Z
are not relayed to LAN I. Frames destined for M, N, Q, X, and Y are not
received on the satellite network because they are filtered locally by the
associated Bridge. These stations are not communicating with remote LAN
stations.
(3) Frames received from LAN I destined for L-Z are relayed to the
satellite network.
(4) Frames received from the satellite network destined for A or B are
relayed to LAN I.
Maintaining the Forwarding Data Store In FORWARDING mode Vitalink Bridges
learn the location of new LAN stations very quickly. For example, assume
that when station C is initialized, it generates an initial multicast
frame containing a "Hello" or "Help" message. This is normal behavior for
many just initialized LAN stations. VB1 relays the frame from LAN I to the
satellite backbone and creates the following Forwarding data store entry:
Entry--address=C, source=LAN.sub.-- I
VB2-4 receive the "Hello or Help" frame on the satellite backbone and relay
the frame to LAN's II-IV respectively. In addition, they each create the
following Forwarding data store entry:
Entry--address=C, source=SATELLITE.sub.-- BACKBONE
As a result, the "Hello" or "Help" message is received by all addressed LAN
stations. Also, all Vitalink Bridges learn the relative location of
station C and are able to appropriately filter and relay frames destined
to it.
If a Vitalink Bridge does not receive a frame containing a particular
destination or source address value for about 15 minutes, the associated
Forwarding data store entry is considered stale. Stale entries are
automatically deleted. If station A in FIG. 2 moves to LAN II, the
Vitalink Bridges will forget A's association with LAN I independent of any
action by station A.
If station A in less than 15 minutes moves and generates, for example, a
"Hello" or "Help" multicast frame on LAN II, the VB1 and VB2 entries
change as follows:
VB1 Entry--address=A, source=SATELLITE.sub.-- BACKBONE (was LAN I)
VB2 Entry--address=A, source=LAN.sub.-- II (was SATELLITE BACKBONE)
The source value in the VB3 and VB4 entries remains equal to SATELLITE
BACKBONE. Relative to VB3 and VB4, station A did not change position.
Experience has shown that the "no matching entry" case for single
destination frames is rare. When it does occur, it usually occurs for one
frame and NEVER results in a Vitalink Bridge forwarding error. The frames
always reach the addressed destination.
Expanding the Configuration
Expanding a TransLAN configuration of the present invention is extremely
easy. For example, the FIG. 2 configuration can be expanded as illustrated
in FIG. 3. The addition of VB5 and VB6 results in VB1-4 learning about
more stations. For example, if station D generates a single destination
frame to station Z, the following following entries are created:
VB6 Entry--address=D, source=LAN.sub.-- V
VB5 Entry--address=D, source=TERRESTRIAL.sub.-- LINK
VB4 Entry--address=D, source=LAN.sub.-- IV
Since VB4 does not relay the frame to the Satellite Backbone (the VB4 Entry
for station Z has a source value of LAN IV), VB1-3 do not create entries.
Subsequently, if D generates a single destination frame to station A, VB4
will relay the frame and VB1-3 will then create the following entries:
VB3 Entry--address=D, source=SATELLITE.sub.-- BACKBONE
VB2 Entry--address=D, source=SATELLITE.sub.-- BACKBONE
VB1 Entry--address=D, source=SATELLITE.sub.-- BACKBONE
If station E initializes and generates a "Hello" multicast frame, VB1-6
create the following entries:
VB6 Entry--address=E, source=LAN.sub.-- V
VB5 Entry--address=E, source=TERRESTRIAL LINK
VB4 Entry--address=E, source=LAN.sub.-- IV
VB3 Entry--address=E, source=SATELLITE.sub.-- BACKBONE
VB2 Entry--address=E, source=SATELLITE.sub.-- BACKBONE
VB1 Entry--address=E, source=SATELLITE.sub.-- BACKBONE
The Vitalink Bridges automatically adapt to the new configuration. The
addition of VB5 and VB6, a terrestrial link, and LAN V requires no
configuration changes to existing Bridges. The new and existing Bridges
simply learn the relative location of new stations.
Supported Topologies
The configuration illustrated above indicates that the Vitalink Bridge
supports interfaces to both a broadcast satellite network and a point to
point data link. The present invention is also capable of supporting other
point to point and broadcast media such as terrestrial microwave.
Both broadcast and point to point interconnect media are supported by the
system and method of the present invention in a number of ways.
Dual Bridge Topologies
Two Vitalink Bridges can be interconnected using either a broadcast medium
or a point to point medium (e.g., terrestrial data link). A broadcast and
point to point dual end point configuration is illustrated in FIG. 4. In
both configurations, VB1 and VB2 are connected to a LAN.
When utilizing a broadcast medium, VB1 or VB2 relay frames destined to
remote LAN stations onto a simplex broadcast channel. They each receive
the other Bridges transmit channel. When utilizing the point to point
medium, VB1 and VB2 each transmit on one side of the duplex data link and
receive from the other.
Relative to both the broadcast and point to point configurations, frames
transmitted by one Bridge are almost always relayed and not filtered by
the other Bridge. This occurs because each Bridge normally filters frames
received from its LAN that are destined for local stations. As a result,
only frames destined for stations on the other LAN are transmitted.
Typically, a point to point medium (terrestrial line), provides the same
transmit rate in both directions. In contrast, the concept of broadcast
simplex channels encourages the use of different transmit rates to cost
effectively accommodate asymmetric data transmission requirements. For
example, if most of the traffic is LAN I stations transferring files to
LAN II stations, the present invention allows the transmit rate of the VB
attached to LAN I to be much higher.
Star Topology
The system and method of the present invention can interconnect more than
two Vitalink Bridges using a star topology. The medium used to
interconnect the star can be broadcast or point to point. See FIG. 5. In
both cases, the present invention automatically relays and filters frames
as appropriate. Support of the broadcast and point to point medium is
summarized below using the configurations illustrated in FIG. 3. In both
of the configurations, VB1 through VBN are each connected to LAN.
Broadcast Star Topology
In broadcast star topology each Vitalink Bridge has a simplex transmit
channel. VB1's simplex channel is received by all remote VB's. Each remote
VB's transmit channel is only received by VB1. This allows numerous remote
LAN stations to statistically share a high speed VB1 transmit channel. The
VB2-N transmit channels can be low speed in comparison.
In configuration | | |
