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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of computer networking,
specifically to the field of hub-based communications networks. More
specifically, the present invention relates to methods and apparatus
providing for a multiport router in which an asynchronous transfer mode
(ATM) switch is utilized as a backplane such as may be implemented in a
local area network (LAN) hub.
2. Description of the Related Art
The present invention relates to the field of ATM and similar networking
system. Such systems are characterized by use of high-speed switches which
act to switch message cells of a fixed size and format through the
network. Below is provided a general description of ATM networks. The
present invention further relates to the field of networked communications
systems employing centralized concentrators or hubs which allow
interconnection of devices in what is sometimes termed a star
configuration. Further, the present invention relates to use of a router
for routing information packets between devices interconnected by
centralized concentrator.
ATM Networks
The preferred embodiment of the present invention is implemented utilizing
an asynchronous transfer mode (ATM) switch as a router in a centralized
concentrator. Such ATM switches are well-known in the art and, in fact,
are described in various references. One such reference is Handel, R. and
Huber, M. N., Integrated Broadband Networks, an Introduction to ATM-based
Networks, published by Addison-Wesley Publishing Company, 1991 and
hereinafter referred to as the Handel et al. reference. Another such
reference is de Prycher, M., Asynchronous Transfer Mode solution for
broadband ISDN, published by Ellis Horwood Limited, West Sussex, England,
1991.
In ATM switches information is communicated in fixed-size cells which
comprise a well-defined and size-limited header area and user information
area. ATM switches may utilize a variety of switching architectures
including, for example, a backplane bus architecture (such as has been
described by Hughes LAN Systems, Inc. and announced under the trademark
Enterprise Hub.TM.), a matrix switching architecture (such as has been
described with reference to both Handel et al. and de Prycher) as
preferred by the present invention or other architectures, as will be
mentioned briefly below. It is noted that the preferred embodiment of the
present invention utilizes a matrix switching architecture for its ATM
switch; however, it is thought that many of the teachings of the present
invention have equal application the various other architectures mentioned
herein.
The matrix switching architecture provides for switching of cells through a
switch fabric which is designed to act upon information in the header area
in order to provide for routing of cells in the network. The switch fabric
is normally implemented in hardware, for example using large-scale
integrated circuits, in order to provide for high-speed switching of cells
in the network.
Standards have been adopted for ATM networks, for example, by the
International Telegraph and Telephone Consultative Committee (CCITT). The
CCITT standards require a header area comprising a fixed set of fields and
being of a fixed size and a payload area, also referred to as a user
information area, and also of a fixed size but allowing user-defined
information fields. The CCITT standards define the header to be of a very
limited size to keep at a minimum overhead associated with each cell
ATM Cell Format
In an ATM network, all information to be transferred is packed into
fixed-size slots which are commonly referred to as cells. Of course, such
fixed-size slots may be referred to with other terminology, for example,
packets. In one standard ATM format (CCITT Recommendation I.150, "B-ISDN
ATM Functional Characteristics", Geneva, 1991), the format is generally
shown in FIG. 1(a) and includes a 5-byte (also called octet) header field
101 and a 48-byte information field 102. The information field is defined
by the standard to be available to the user and the header field is
defined by the standard to carry information pertaining to ATM
functionality, in particular, information for identification of the cells
by means of a label. See, Handel et al., at pages 14-17.
The standardized format for the header field 101 is better shown in FIG.
1(b) and 1(c) and is described in greater detail with reference to Handel
et al., at pages 84-91. The header field 101 will be discussed in greater
detail below; however, it is worthwhile mentioning here that the header
field 101 comprises two fields: (1) a virtual channel identifier (VCI) and
(2) a virtual path identifier (VPI). The VPI field is defined as an
eight-bit field in one format (see FIG. 1(b)) and as a twelve-bit field in
another format (see FIG. 1(c)) and is defined to be used for routing of
the cell. The VCI field is also used for routing in the defined format and
is defined as a sixteen-bit field.
The de Prycher reference further describes the format of the ATM cell, for
example at pages 55-124 and, especially at pages 106-108.
ATM Switching
Two primary tasks are generally accomplished by an ATM switch: (1)
translation of VPI/VCI information and (2) transport of cells from the
input port to the correct output port. The functions of an ATM switch are
more fully described in Handel et al. at pages 113-136.
A switch is typically constructed of a plurality of switching elements
which act together to transport a cell from the input of the switch to the
correct output. Various types of switching elements are well-known such as
the already-mentioned matrix switching elements and bus-type switching
elements. In addition, an ATM switch may utilize central memory switching
elements, and ring-type switching elements. Each of these are discussed in
greater detail in the Handel et al. reference and each carries out the
above-mentioned two primary tasks.
Translation of the VPI/VCI information is important because in a standard
ATM network the contents of these fields only has local meaning (i.e., the
same data would be interpreted differently by each switch). Thus, the
VPI/VCI information is translated by each switch and changed prior to the
cell being output from the switch. This translation is accomplished
through use of translation tables which are loaded into the switch fabric,
generally under control of a switch controller.
Importantly, it is the switch fabric, as controlled by the translation
tables, which provides for making routing decisions within the switch. The
translation tables may updated from time-to-time in order to provide for
new virtual paths/virtual circuits or to remove existing ones (this
process is sometimes referred to as call set-up and tear-down). Thus, a
VPI/VCI is supplied in the cell header at the input of the switch and the
VPI/VCI is translated by the switch fabric and the cell is routed to the
appropriate output port. However, generally, the device generating the
cell has no knowledge of the specific output port on which switch will
send the cell. Rather, this routing decision is made by the switch based
on the then current translation tables.
As will be seen, the present invention provides for an ATM switch having
preconfigured VPI/VCIs and allows for selection of an appropriate output
path by the device generating the cell (i.e., the device generating the
cell provides for the routing decision rather than the ATM switch.)
Network Concentrators
Network concentrators or hubs are well-known in the art. Two well-known
examples of network concentrators are the SynOptics' LattisNet System
2000.TM. and LattisNet System 3000.TM. concentrators. The concentrators
are better described in "LattisNet.RTM. Product Overview, A comprehensive
Description of the Lattisnet Product Family" (the "LattisNet Product
Brochure"), available from SynOptics Communications, Inc. of Santa Clara,
Calif., the assignee of the present invention.
Network concentrators typically comprise a number modules, each module
having a plurality of ports to which local area network segments may be
connected. A local area network segment may support one or more devices
such as data terminals, computers, file servers, printers, etc. The
various modules are interconnected through a backplane bus or the like in
the concentrator module. Thus, a device attached to module 1 may
communicate an information packet to the devices attached to the other
modules by providing communicating the packet onto the local area network
segment connected with module 1 and, module 1, when it receives the
packet, communicating the packet onto the backplane bus. The other modules
may then receive the packet from the backplane bus and communicate the
packet to the devices coupled with network segments connected to the other
modules. The various modules may support, for example, Ethernet (or other
CSMA/CD protocol), Token Ring and/or FDDI networks.
Objects of the Invention
It is an object of the present invention to utilize an ATM switch as a
routing backplane in a network concentrator to provide for relatively fast
routing of information packets between LAN segments coup led to the
network concentrator.
This and other objects of the present invention will be understood with
reference to the detailed description of the preferred embodiment and the
accompanying drawings.
SUMMARY OF THE INVENTION
The present invention relates to methods and apparatus providing for a
multiport router in which an asynchronous transfer mode (ATM) switch is
utilized as a multi-port, non-blocking backplane. An exemplary application
of the present invention is provided in which the multiport router is
utilized in a concentrator (also referred to as a hub) in a local area
network (LAN).
In particular, the present invention provides for a multiport router for
routing of information packets, such as for example Ethernet or CSMA/CD
packets, through a network where the multiport router provides for front
end modules which segment the information packets into ATM cells. The
front end modules provide, in the cell header, routing information (such
as by providing destination port information) and then the cell is
provided at an input to a ATM switch.
The switch fabric of the ATM switch switches the cell based on the routing
information provided by the front end module to an output port of the
switch. In the described system, the ATM switch is preconfigured to
provide a fully connected topology of front-end module to front-end module
connections.
In an embodiment of the invention, the multiport router is utilized as a
backplane in a local area network concentrator (also referred to as a
hub).
These and other aspects of the present invention will be discussed in
greater detail with reference to the detailed description and the
accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a), (b) and (c) are diagrams illustrating the format of an ATM cell
as may be utilized in the present invention.
FIG. 2(a) is an illustration of a concentrator as may be utilized by the
present invention.
FIGS. 2(b) and 2(c) are front side view and back side view illustrations of
an alternative concentrators as may be utilized by the present invention.
FIG. 3(a) is a diagram illustrating components of a concentrator as may be
utilized in the present invention including connectors 301-304.
FIG. 3(b) is a diagram illustrating components of a concentrator of the
present invention and illustrating permanent virtual circuits 331-336
interconnecting the host modules 201-204.
FIG. 3(c) is a diagram illustrating dedicated virtual circuits
interconnecting ports in a concentrator as may be utilized by the present
invention.
FIGS. 4(a) and 4(b) illustrate an ATM-to-LAN front end module as may be
utilized by the present invention.
For ease of reference, it might be pointed out that reference numerals in
all of the accompanying drawings typically are in the form "drawing
number" followed by two digits, xx; for example, reference numerals on
FIG. 1 may be numbered 1xx; on FIG. 3, reference numerals may be numbered
3xx. In certain cases, a reference numeral may be introduced on one
drawing and the same reference numeral may be utilized on other drawings
to refer to the same item.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
What is described herein is a method and apparatus for utilizing an
Asynchronous Transfer Mode (ATM) switch as a router backplane in a hub for
a local area network. In the following description, numerous specific
details are set forth in order to provide a thorough understanding of the
present invention. It will be obvious, however, to one skilled in the an
that the present invention may be practiced without these specific
details. In other instances, well-known circuits, structures and
techniques have not been shown in detail in order not to unnecessarily
obscure the present invention.
OVERVIEW OF A COMMUNICATIONS NETWORK SUCH AS MAY UTILIZE THE PRESENT
INVENTION
The present invention relates to methods and apparatus utilizing an ATM
switch as a routing backplane in a hub for a local area network. The use
of network hubs (also referred to as network concentrators) is well-known
in the art. However, a brief overview of network hubs will also be
provided to facilitate an understanding of the present invention. Also,
ATM switches have been well-described in the art; however, a general
overview will be provided below for reference in an understanding of the
present invention.
Network Hubs
Generally, network hub or concentrator acts to concentrate wiring for a
communications network in a central location such as a facilities
telephone wiring closet. The hub comprises a cabinet having multiple ports
where each port supports one local area network segment. Each local area
network may support multiple devices which may communicate over the local
area network. In many such hubs, individual modules are plugged into the
cabinet and each module comprises multiple ports (e.g., 16 ports per
module is common in the current state of the art.) The modules are
interconnected with each other over a backplane bus or the like so that
network message packets from a device connected to a LAN segment coupled
with a port on module 1 may be communicated to a device connected to a LAN
segment coupled with a port on module 2 over the bus.
Network hubs as may be utilized by the present invention are described in
greater detail with reference to FIGS. 2(a)-(c) and will be discussed in
greater detail below.
Examples of prior an network hubs include the SynOpties' LattisNet System
2000.TM. and LattisNet System 3000.TM. concentrators.
ATM Switches
Generally, a network ATM switches are more fully described in, for example,
de Prycher and with reference to standards adopted by the International
Telephone and Telegraph Consultative Committee (CCITT). Briefly, it should
be understood that ATM is a telecommunication technique or transfer mode
implemented to facilitate a broadband network such as the Broadband
Integrated Services Digital Network (B-ISDN) in which cells of data are
asynchronously transferred between two switching devices without the need
to synchronize the clocks of the sender and the receiver packet.
Of course, alternatives to ATM switches have been proposed which employ
principles similar to the principles employed by ATM switches. In general,
when the term "ATM switch or the like" or the term "ATM Switch" is used
herein, such term may be thought of as covering switches having the basic
characteristic of packet switching switch with minimal functionality in
the network. More specifically, an ATM switch may be thought of as a
circuit-oriented, low-overhead switch providing virtual channels which
have no flow control or error recovery wherein communication in the
virtual channels is accomplished with fixed-size (and relatively short)
cells. The virtual channels provide the basis for both switching and for
multiplexed transmission. Another important feature of an ATM switch is
the possibility to group several virtual channels into one so-called
virtual path. ATM switches are perhaps better generally defined at Handel
et al., pp. 14-17.
ATM has been the official name adopted by the CCITT for such a network.
Asynchronous Time Division (ATD) and Fast Packet Switching are terms which
have been employed to describe similar network transfer modes. These
alternative networks are discussed in de Prycker at pages 55-56.
The present invention proposes utilizing an ATM switch as a routing
backplane or packet switching engine in a more traditional data
communications network such as a network employing the well-known CSMA/CD,
token ring standards and/or FDDI standards. (It is noted that, for
purposes of this specification, the terms CSMA/CD and Ethernet may be used
interchangeably and each of these types of systems, e.g., CSMA/CD, token
ring and FDDI, are referred to generally as local area network or LAN
systems.)
General Overview of ATM switching as applied to the present invention
This specification will describe in greater detail a network hub
implementing an ATM switch in accordance with the teachings of the present
invention. First, however, it is worthwhile to provide an overview of ATM
switching.
In an ATM switch, the information is actually transmitted through the
switch in fixed-length through virtual paths/virtual channels which are
set up to facilitate such communications. The virtual paths may comprise a
plurality of virtual channels. The use of virtual channels/virtual paths
allows a large number of connections to be supported on a single physical
communications link. In the art, Virtual Path/Virtual channels are
generally thought to be allocated during set-up of a communication
transmission between two devices (e.g., between two clients) and "tom
down" after the communication has completed. For example, in an ATM
network implemented to support telephone communications, virtual channels
may be set up along the communications link between the caller and the
called party at the time the call is placed and then tom down when the
telephone conversation ends. The process of setting up and tearing down a
virtual path and/or virtual channel generally involves updating
translation tables stored in the switch fabric of each switch involved
with each virtual path/virtual channel link of the virtual path or virtual
channel.
As will be seen, in the present invention, permanent virtual circuits
(PVCs) are established which create a fully connected topology of LAN
module to LAN module virtual connections. These PVCs actually am comprised
of ATM virtual paths. In addition, the dedicated virtual channels
interconnecting any two of the hub ports on the various LAN modules are
provided. These dedicated virtual channels am implemented as ATM virtual
channels.
In the present invention, a device generating cells to be switched by the
ATM switch can determine the output port on which it is desired to have
the cell transmitted and select an appropriate PVC and dedicated virtual
channel on which to input the cell in order to provide for selection of
the desired output. In other words, the device (external to the ATM
switch) effectively provides for the routing decisions of cell in the ATM
switch because them are a known set of PVCs and dedicated virtual channels
which provide for the above-mentioned fully connected topology.
Thus, as one important aspect of the present invention, it is desired to
provide for a fully connected ATM switch allowing for fast switching of
cells with an external "routing entity" providing for routing of the
cells. The external muting entity forwards cells through the switch by,
for example, putting the destination output port number into the VPI
field. The ATM switch fabric is then preconfigured to route the cell to
the specified output port. In the described embodiment, the switch fabric
is further configured to translate the cell header to indicate in the VPI
field the input port number, e.g., the port number on which the cell
entered the ATM switch. This feature of the present invention allows the
destination to perform source discrimination on received cells.
Presently, one application of the present invention is use of the invention
as a multi-port router in which the switch is used as a multi-channel,
non-blocking backplane interconnecting multiple router interface modules
in parallel. This implementation of the present invention will be
discussed in greater detail below in order to provide a better
understanding of the present invention and one specific embodiment of the
invention.
It is noted that a typical switch 205 in the preferred system comprises 16
input/output ports allowing connection of 16 LAN modules over high speed
connectors although for sake of simplicity, in FIG. 3(a) for example, only
four LAN modules 201-204 and 4 high speed connectors 301-304 are
illustrated as being connected to the switch 205. It will be obvious to
one of ordinary skill that the total number of ports supported by a switch
may vary from implementation to implementation and such variance should
not be considered a departure from the present invention.
In addition, each switch comprises switch fabric 205. Switch fabric is well
described with reference to both the Handel et al. and the de Prycher et
al. references.
The ATM cell as defined by the CCITT
It may be worthwhile to briefly describe the basic cell structure of an ATM
cell as defined by the CCITT and as used by the present invention. Such a
cell structure is illustrated by FIG. 1(a) and includes a fixed-size
header area 101 and a fixed-size information field or payload area 102.
The header area 101 is defined to include 5 8-bit bytes while the
information field 102 is defined to include 48 8-bit bytes for a total of
53 8-bit bytes per cell. The information field 102 is available for user
information while the header field is well-defined by the CCITT standard
and includes necessary overhead data. In fact, two header definitions are
set forth by the CCITT standard and these header definitions are described
in connection with FIG. 1(b) and FIG. 1(c). The first header definition is
used at the B-ISDN user-network interface and the second header definition
is used at the B-ISDN network-node interface. The two formats only differ
in the first byte.
FIG. 1(b) illustrates an ATM cell header for a B-ISDN user-network
interface. Field 111 is a 4-bit field used for generic flow control (GFC)
which assists in control of traffic flow from ATM connections at the
user-network interface. ATM networks do not provide for flow control of
the type which is implemented in some packet networks and ATM networks
have no facility to store cells over a long period of time. Therefore,
inside an ATM network there is no need for generic flow control. Thus, in
the header definition of FIG. 1(c), there is not GFC field and the virtual
path identifier field 112 is expanded to use the bits made available by
elimination of the GFC field 111.
The virtual path identifier (VPI) comprises either 8-bits, in the case of
user-network interface headers, or 12-bits, in the case of node-network
interface headers. As illustrated in FIGS. 1(b) and 1(c), the 8- and
12-bits respectively are broken down in a first field 112 of either 4- or
8-bits in the first byte of the header and the high order 4-bits in a
second field 113 of the second byte of the header. The VPI field
identifies a virtual path for routing the cell through the network.
The virtual channel identifier (VCI) comprises 16-bits broken down in three
fields, a first field 114 being the low order 4-bits in the second byte of
the header, a second field 115 being the 8-bit third byte of the header,
and a third field 116 being the high order 4-bits in the fourth byte of
the field. The VCI identifies the virtual channel for routing of the cell.
Certain values have been defined by the CCITT standard.
Bits 2-4 of the fourth byte of the header comprise the payload type (PTI)
field 117 which indicates whether the cell contains user or network
management related information.
Bit 1 of the fourth byte is the cell loss priority (CLP) field 119. If the
value of the field is 1, the cell is subject to discard, depending on
network conditions. If the value of the field is 0, the cell has high
priority and, therefore, sufficient network resources have to be allocated
to it.
Finally, the header error control field 120 takes the entire fifth byte of
the header. It contains the header error control sequence to be processed
by the physical layer of the network and is specified in CCITT
Recommendation I.432.
As can be appreciated, a header functionality has been kept to a minimum by
the standard in order to provide for fast processing in the network. The
main functions of the header are identification of the virtual connection
and certain maintenance functions. By keeping these functions to a
minimum, header processing in the ATM nodes is simple and can be done at
very high speeds.
OVERVIEW OF THE HUB OF AN EMBODIMENT OF THE PRESENT INVENTION
Turning now to FIG. 2(a), one embodiment the present invention is
described. A network hub 200 is shown which comprises four LAN modules
201-204 which each are illustrated as having four ports (211A-D, 221A-D,
231A-D and 241A-D corresponding to modules 201-204, respectively). The
particular choice of illustrating a hub having four module with four ports
each has been made for convenience of illustrating the invention. In fact,
embodiments of the present invention are envisioned as being capable of
supporting a variable number of modules and each module is expected to
have 16 ports. Each of the ports is capable of supporting a LAN segment
such as segments 241-244 (again, for ease of illustration only one segment
is illustrated for each module).
In addition, the present invention provides for an ATM switch 205 in the
hub 200. Although the ATM switch 205 operates substantially as has been
described for above in connection with the overview of ATM switches, the
ATM switch of the present invention provides for permanent virtual
connections (PVCs) which interconnect each of the various modules 201-204
and for dedicated channels interconnecting any two given ports (211A-D,
221A-D, 231A-D and 241A-D). The ATM switch 205 further may provide one or
more ports for connecting the ATM switch 205 to other hubs over a high
speed ATM trunks (e.g., trunk 251) in order to make up a larger network.
In addition to allow larger networks, this technique allows for native ATM
hosts (e.g., servers) to be connected to the network.
It is noted that certain local area network technologies operate at what
will be termed herein relatively low speeds (i.e., Ethernet at 10 Mb/s,
token ring at 16 Mb/s) and provide for sharing the available bandwidth
between the various devices attached to the network. Other known local
area network technologies operate at other speeds (e.g., FDDI at 100 Mb/s)
but still provide for sharing of the available bandwidth. An ATM switch
operates at what will be termed herein relatively higher speeds (currently
on the order of 155 Mb/s; however this is expected to increase) and,
importantly, the full bandwidth of the switch is generally thought of as
being available to all devices attached to the network. Thus, FIG. 2(a)
illustrates that the area of relatively high speed clam transmission as
darkened area 231. This area comprises both high speed connectors used to
interconnect the LAN modules 201-204 to the ATM switch module 205 and the
ATM switch 205 itself.
Generally, the present invention works by a device such as device 221
transmitting a message packet over its LAN segment 241 to port 211A of
module 201. Assume that the message packet was addressed to device 224.
LAN module 224 will then forward the message packet to a port of ATM
switch 205. Either the ATM switch module 205 or the LAN module 224
comprises a LAN-to-ATM from end interface module which will be described
in greater detail with reference to FIGS. 4(a) and 4(b). The from end
interface module acts to segment the message packet into ATM cells. (As
has been discussed the ATM are of fixed length and provide for a 48-byte
information field 102. It is into this information field 102 that the
relatively longer message packet is segmented.) In addition, the front end
interface module provides the appropriate routing information (i.e.,
VCI/VPI) in each cell header 101. The process of segmentation and
providing appropriate routing information will be discussed in greater
detail below with reference to FIGS. 4(a).
As the message packet is segmented, the cells are transmitted to the ATM
switch 205 where the cells are routed to an output port as designated by
the cell's VCI/VPI. As will be appreciated the output port corresponds to
the port associated with the module to which the destination device is
attached (i.e., in the illustrative example, device 224 is attached to
module 204). The cell is then switched (e.g., demultiplexed) to a buffer
where it is reassembled, along with the other cells from the message
packet which have been similarly created by the segmentation process and
transmitted over the switch, to again form the message packet and the
message packet is transmitted on the appropriate LAN segment to the
destination device, i.e., over LAN segment 244 to device 224.
FIGS. 2(b) and 2(c) illustrate an alternative hub 200 configuration in
which the ATM switch 205 resides at the bottom of the switch cabinet. FIG.
2(b) illustrates a front view of the alternative hub and FIG. 2(c)
illustrates a rear view. The LAN modules (201-204) are still coupled with
the ATM switch module over high speed connectors (illustrated as
231A-231C); however, it will be appreciated that the connectors 231A-231C
in this configuration may be of a relatively shorter and relatively equal
length when compared with the hub configuration of FIG. 2(a).
FIG. 3(a) is useful for illustrating the connectors 301-304 which connect
modules 201-204 with the ATM switch 205. FIG. 3(a) also illustrates the
LAN-to-ATM front-end modules 311-314 of the present invention. As
illustrated, these front-end modules 311-314 reside in the LAN modules
201-204. However, as has been mentioned, alternative designs may provide
for the front end-modules to reside in as part of the switch module 205.
The front-end modules will be described in greater detail with reference to
FIGS. 4(a) and 4(b). The connectors 301-304, in the described embodiment,
are provided as 8-bit connectors operating at, for example, 20-40 MHz.
Each connector 301-304 provides for direct connection of the front-end
modules 311-314, respectively, to a full-duplex ATM port on the switch
module 205.
PERMANENT VIRTUAL CIRCUITS/DEDICATED VIRTUAL CHANNELS
Turning now to FIG. 3(b), the concept of permanent virtual circuits as
utilized by the present invention is described in greater detail. FIG.
3(b) illustrates a network as may be implemented by the present invention
including permanent virtual circuits (PVCs) 331-336 providing for a fully
connected topology of module to module virtual connections. Each of the
PVCs 331-336, in fact, represents a pair of uni-directional connections
between two modules although for ease of illustration the connections are
illustrated as a single line. For example, PVC 331 provides a full-duplex
connection through switch 205 between module 201 and 203; PVC 332 provides
a connection between module 201 and 204; and PVC 333 provides a connection
between module 201 and 202.
Therefore, in the example described above, the message packet originated by
device 221 with an intended destination of device 224 is routed through
module 201 over PVC 332 through switch 205 to module 204 and then out port
241A to device 224.
As was mentioned, the PVCs 331-336 are implemented as ATM virtual paths. In
the described system, the PVCs 331-336 are permanent in the sense that the
ATM switch 205 provides for switch translation tables which allow
switching of ATM cells from an input port to an output port based on VPI
and VCI information. As was mentioned above, in many ATM switches, these
switch translation tables are dynamic with entries being added and deleted
as virtual paths and virtual channels are set up and tom down during
transmission of messages in the network. However, in the described
embodiment, the VPI entries establishing the PVCs 331-336 are not deleted
(and, therefore, the virtual path/PVC is permanent) during operation of
the network. This prevents delays in retransmission of cells which may
otherwise be caused by set-up/tear-down time for virtual paths. As the
number of virtual paths is limited (one module-to-module connection), it
is not felt the overhead associated with maintaining the permanent switch
table entries out weighs the benefits of avoiding set-up and tear-down
overhead.
In the described embodiment, N*(N+1) permanent virtual circuits are
configured on for the ATM switch where N is the number of LAN modules in
the concentrator. In particular, there are N permanent virtual circuits
provided for each LAN module allowing for a fully connected topology
between a LAN module and all destination modules (including itself) as
shown in Table I (i.e., N.sup.2 PVCs) and there is one permanent virtual
connection provided to allow for broadcast connections (i.e., N additional
PVCs, thus providing for a total of N*(N+1)). The use of the broadcast PVC
may be useful, for example, to broadcast source address information for
storage in address lookup tables as will be discussed in greater detail
below in connection with FIG. 4(a).
Of course, in an alternative embodiment, use of virtual circuits, while not
permanent, may be employed without departure from the spirit and scope of
aspects of the present invention.
Table I illustrates a switch translation table for translating VPI's in the
network described by FIG. 3(b). It is noted that input port 1 corresponds
to the port of the switch to which module 201 is connected; input port 2
corresponds to the port of the switch to which module 202 is connected
etc. Also, for sake of simplifying the illustration, PVCs connecting a
module with itself are not illustrated. However, in the present invention
it is possible that a device 221 may transmit a message packet to a
destination device which is also coupled with module 201. It is noted that
it is possible for a device to transmit a message packet to a destination
device | | |
