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Description  |
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TECHNICAL FIELD
This invention relates to providing multimedia services using a distributed
server model.
BACKGROUND OF THE INVENTION
The current embedded base of data networks are based on IEEE 802 Local Area
Networks, i.e., so-called "Legacy LANs". These Legacy LANs are so-called
"connectionless" networks, because network entities exchange packets
without establishment of a layer-2 connection. Many existing and emerging
applications are designed to run primarily on Legacy LANs. These
applications reside on top of so-called "layer-2" and "layer-3" protocols
such as Medium Access (MAC) and Internet Protocol (IP), respectively. As
is well known in the art, the layers referred to are those of the
international standards organization (ISO) seven layer networking
reference model.
A classical method for interconnecting different LANs is so-called
"bridging and routing". Inter-LAN communication according to this
technique is achieved through the use of an external router. A deficiency
of this method is that, since it is based on a broadcast principle, and
thus mimics shared-medium operations, all data packets are broadcast to
all ATM destinations, thereby flooding the network with broadcast traffic.
Another deficiency is that the broadcast nature of the technique virtually
requires a mesh network between all bridges and ATM hosts within a LAN,
and all inter-LAN traffic must pass through the router.
The ATM Forum has developed a better solution based on bridging called LAN
Emulation (LANE), to interconnect Legacy LANs and native ATM hosts. LANE
relies on a LAN Emulation Server (LES), which performs ATM-to-MAC address
resolution, i.e., translation, and a Broadcast and Unknown Server (BUS),
which performs data broadcast.
Generally speaking, LAN emulation clients (LECs) are ATM end-stations or
ATM-capable bridges that are directly connected to an ATM network
connecting Legacy LANs "behind" them to the ATM network. An LE.sub.-- ARP
request allows a LEC to request an ATM address corresponding to a MAC
address from a LES. The LE.sub.-- ARP request is defined in the ATM
Forum's LAN Emulation Over ATM Specifications, Version 1.0, which is
incorporated herein by reference and the contents of which are well known
by those skilled in the art. It is noted that a so-called "Proxy LEC"
represents multiple end-point addresses, e.g., the MAC addresses of
several hosts. For a more detailed description of the definitions of LEC
and proxy LEC reference may be made to the above-mentioned ATM Forum's LAN
Emulation Over ATM Specifications, Version 1.0.
Broadcast data packets, such as a so-called "ARP.sub.-- Request", are
forwarded to a BUS, which in turn broadcasts them to all LECs. "ARP.sub.--
Requests" are defined in Bell Communications Research (Bellcore) request
for comments (RFC) 826, which is incorporated herein by reference. Also,
data packets are sent to a BUS until a direct ATM connection is
established to the target address within the Emulated LAN.
The communications between two Emulated LANs (ELANs) is done via an
external router. Either the router has the ATM address of the destination
LEC, or it requests the address from the LES serving the destination LEC.
The router then builds an ATM connection to the destination LEC, and send
thereto the data packet. Thus, disadvantageously, all inter-LAN packets
must pass through the router, which may become a communications
bottleneck, due to low throughput of conventional routers.
A third method builds upon LANE, but incorporates the routing function as
well as bridging function into a so called multi-layer LAN switch.
Fundamentally, there are three major functions associated with routing: 1)
routing, i.e., determination of the layer-3 address of the next-hop-router
along the path to the target address, 2) address resolution or
translation, i.e., determination of a router's ATM address corresponding
to its layer-3 address, and 3) data forwarding, i.e., relaying data packet
from one port of the router to another port. A traditional router performs
functions (1) and (3) while function (2) is required because an ATM
connection must be established between adjacent ATM router hops or to the
target ATM address. A multi-layer switch performs only function (3), i.e.,
data forwarding.
A route server is used to store the next-hop router's layer-3 address, and
an address resolution protocol (ARP) server is used to resolve, i.e.,
translate, layer-3 addresses to ATM addresses. Sometimes these functions
are merged into one server, a so-called "Route/ARP Server". With a
multi-layer LAN switch, the intra-LAN communication is performed just as
in LANE using the local LES and BUS of each ELAN. However, inter-LAN
communication is different.
For example, if a host wants to talk to a host in a different ELAN, then
the LEC for that host acts as a router, obtains either the a)
destination's, or b) the next-hop-router's IP address from the route
server, and obtains the corresponding ATM address from an ARP server. It
then establishes an ATM connection directly to the destination LEC serving
the host and sends the data packet.
This method is more efficient than using an external router since the
external router hop is eliminated. Further efficiency is obtained by each
multi-layer switch performing fast data forwarding both for layer-2 and 3
packets, while complicated route determination and address resolution
functions are logically removed from the switches. Although the Route
Server decouples the routing from data forwarding, it only can serve a few
logical subnets, and is thus not suited to cover large number of logical
subnets. As the number of subnets governed by a single route server
increases the efficiencies provided with this approach greatly diminish,
since several router hops become practically unavoidable.
Thus, the existing LANE and Route/ARP Server methods work effectively only
for small scale local networks.
The forgoing problems with interconnecting ELANs are overcome, as described
in U.S. patent application Ser. No. 08/402,235, filed Mar. 10, 1995 (Chang
1-1-1), which is incorporated herein by reference, by employing a
so-called "ELAN contronect network" which is a separate network for
interconnecting the servers of the sub-ELANs, where a sub-ELAN is a part
of an ELAN having its own LES and BUS that may also be configured as a
stand-alone ELAN. Each of the sub-ELANs is connected to the ELAN
contronect network via a point-to-point connection-oriented connection,
and the ELAN contronect network is configured to present itself to each of
the servers of the sub-ELANs as clients thereof.
In one such system, the ELAN contronect network includes an address server
and a broadcast/route server interconnected over a high-speed backbone.
Also connected to the high-speed backbone is at least one LAN hub, which
connects to at least one server of at least one of the ELANs.
The address server contains an address data base for performing address
resolution, i.e., translation, between the at least two addresses of each
ELAN end-point in response to requests for such translations. The
broadcast/route server receives data packets for broadcast to a different
sub-ELAN from which the packets originated and broadcasts the received
data packets to at least one other sub-ELAN. The broadcast/route server
also recognizes that a packet which is a broadcast packet at layer-2 is
actually a request for an address resolution at layer-3. If so, the
broadcast/route server collaborates with the address server to perform the
necessary address resolution and insures that a response is sent only to
the client originating the request.
Optionally, the ELAN contronect network also includes a configuration
server and one or more multimedia servers, i.e., a multimedia server farm.
SUMMARY OF THE INVENTION
We have recognized that, although it is possible to provide multimedia
services using a multimedia server farm as part of the ELAN contronect
network, or by embedding multimedia servers within an ELAN as LECs
therein, such multimedia servers a) become bottlenecks during peak usage
of multimedia services, b) strains the bandwidth of the ELAN contronect
backbone, and c) creates a single point of failure for the multimedia
service. Moreover, the capability of providing a multimedia service within
an ELAN is fixed by the capacity of the servers that are a part of that
ELAN. While it is possible for a LEC of an ELAN to obtain a multimedia
service from another ELAN, doing so requires going through multiple router
hops, which diminishes the performance of the services as well as creating
issues of security.
Therefore, in accordance with the principles of the invention, a pool of
multimedia servers, which may be geographically diverse, are available to
be allocated to serve one of at least two ELANs served by an ELAN
contronect network. These multimedia servers are a) connected to, b)
dynamically allocated to sub-ELANs, and c) managed by, a multimedia server
manager, which is included as part of the ELAN contronect network. In
particular, each of the multimedia servers is capable of having its
addresses dynamically reconfigured in response to commands from the
multimedia server manager to match that of an ELAN to which it is being
made available, so that the multimedia server becomes a part of that ELAN.
The multimedia server manager obtains a layer-3 address from address
server 427 of the ELAN contronect network which it then assigns to the
multimedia server so that the multimedia server appears to be a part of
the ELAN to which it is assigned to serve. The multimedia server manager
also supplies to the multimedia server the LES and BUS address of a
sub-ELAN of the ELAN to which it is assigned to serve. The multimedia
server can then join with the LES and BUS as a LEC in that sub-ELAN.
All requests for multimedia service by a LEC are initially directed to the
multimedia server manager. The multimedia server manager, in turn, returns
to the LEC at least the layer-3 address of the multimedia server that will
provide the requested service. Thereafter, LECs in the sub-ELAN can
connect to the multimedia server specified by the multimedia server
manager directly, while LECs within the ELAN, but not the same sub-ELAN of
the multimedia server, establish connections to the multimedia server
using the ELAN contronect network. Advantageously, a LEC of the ELAN need
not go through a router or a route server to access to multimedia server.
Also advantageously, additional multimedia servers can be allocated from
the pool to serve an ELAN in the event of increased demand within the ELAN
or failure of any previously assigned multimedia server.
Each multimedia server has two states, 1) an availability state and 2) a
service state. The availability state can take one of two values, a)
"available", which indicates the server is working properly, and b)
"unavailable", which indicates that the server is down or otherwise
inaccessible. The service state of each multimedia server can take one of
three values, 1) "idle", indicating that there are no LECs being served by
the multimedia server, 2) "active", indicating that there is at least one
LEC being served by the multimedia server but that there remains in the
multimedia server additional capacity to serve other LECs, and 3) "busy",
indicating that a predetermined service capacity threshold has been
reached. In one embodiment of the invention, the busy value indicates that
the multimedia server cannot serve any additional LECs.
In a particular embodiment of the invention, the state of each multimedia
server is monitored by the multimedia server manager. The multimedia
server manager only allocates a server when the multimedia server's
availability state is available and its service state is idle. Moreover,
the multimedia server manager will only allocate a new multimedia server
to an ELAN when the already allocated multimedia server for a particular
service becomes unavailable or busy.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 shows an exemplary embodiment of the invention in which an ELAN
contronect network is arranged to interconnect several sub-ELANs in
accordance with the principles of the invention;
FIGS. 2, 3, 4, 5, and 6, when connected together, show an exemplary
procedure for processing packets in the system of FIG. 1, in accordance
with the principles of the invention;
FIG. 7 is a block diagram view of an exemplary embodiment of generic server
1001, which may be used as the servers shown in FIG. 1.
DETAILED DESCRIPTION
FIG. 1 shows an exemplary embodiment of the invention in which ELAN
contronect network 401 is arranged for interconnecting the servers of
sub-ELANs 207, 219, 227 and 237 in accordance with the principles of the
invention. For clarity of exposition, only sub-ELANs 227 and 237 are shown
in detail. Sub-ELAN 227 includes bridge and legacy LAN 179, ATM
end-stations 131, ATM switch 175, and associated servers LES 305 and BUS
307. Sub-ELAN 237 includes bridge and legacy LAN 173, ATM end-station 121,
ATM switch 193, and associated servers LES 301 and BUS 303. ELAN
contronect network 401 includes a) address server 427, b) broadcast/route
server 415, c) optional configuration server 419, d) multimedia server
manager 429, and e) LAN hubs 431, which includes LAN hub 431-1 through
431-N, connected via high speed backbone 425.
Each of sub-ELANs 207, 219, 227 and 237 is connected to ELAN contronect
network 401 via a point-to-point connection-oriented connection through
asynchronous transfer mode (ATM) wide area network 131. ELAN contronect
network 401 is configured to present both a) address server 427 and b)
broadcast/route server 415, as LECs, and more particularly, as proxy LECs,
to each of servers 301, 303, 305, and 307 of sub-ELANs 227 and 237, as
well as to the servers of sub-ELANs 207 and 219 (not shown). Address
server 427 and broadcast/route server 415 are "proxy" in that they
represent all other end-station addresses not within each respective
sub-ELAN.
Address server 427 performs address resolution, i.e., address translation,
between various network end-point layer-3 and layer-2 addresses. As is
well known in the art, each network end-point, i.e., any unit connected to
send and receive data packets, may have more than one address, i.e., an
identifier, by which it is known. Thus, a first end-point may know a
particular address for a second end-point but not the second end-point's
other corresponding address which may be required in order to exchange
data packets with the second end-point. The information necessary to
perform such address translation is stored in an address table. Table 1 is
an exemplary address table. Table 1 assumes that there are N legacy
end-stations in the legacy LAN. If address server 427 does not contain
within itself the necessary information to perform a required address
translation, i.e., the necessary entries are not present in its address
table, address server 427 is able to engage the necessary processes in
order to obtain and store within itself the necessary information.
Thereafter address server 427 will be able to directly perform the
required address translation.
TABLE 1
______________________________________
ATM address Layer-2 address
Layer-3 address
______________________________________
ATM-173 MAC-173-01 IP-173-01
ATM-173 MAC-173-02 EP-173-02
. . .
. . .
. . .
ATM-173 MAC-173-ON IP-173-ON
ATM-121 MAC-121 IP-121
______________________________________
Broadcast/route server 415 performs several functions relating to
communicating to more than one end-point at a time. In particular
broadcast/route server 415 receives a data packet from a sub-ELAN of a
particular ELAN and sends it to one or more sub-ELANs in the same or a
different ELAN. Broadcast/route server 415 also recognizes that a data
packet which is indicated to be a broadcast packet at layer-2 actually
contains a layer-3 address resolution request. When such data packets are
recognized, broadcast/route server 415 communicates with address server
427 to obtain the requested address. Broadcast/route server 415 then
transmits the translated address directly to the end-point which
originated the data packet. Advantageously, doing so eliminates the need
for additional transmissions of the data packet.
An optional function of broadcast/route server 415 is to act as a router
for packets addressed to it by a router/ARP server and requesting routing
of the packets. Typically such requests are performed at layer-3, and
require a packet to be sent to an ELAN other than the ELAN of the
packet-originating end-point. Advantageously, since broadcast/route server
415 is aware of all the layer-3 addresses of those end points that
subscribe to service by ELAN contronect network 401, packets can be routed
to the ultimate destination in no more than two hops, i.e., via the
route/ARP server of the sub-ELAN from which the packet originated and then
via broadcast/route server 415.
Optional configuration server 419 operates to group one or more sub-ELANs
into an ELAN regardless of the locations of the sub-ELANs. Configuration
server 419 can serve more than one ELAN. For example, configuration server
419 has configured sub-ELAN 207 and sub-ELAN 237 into ELAN 301. Likewise,
configuration server 419 has configured sub-ELAN 227 and sub-ELAN 219
together into ELAN 371.
Configuration server 419 operates by communicating with address server 427
and broadcast/route server 415 to establish connections between the
servers of the sub-ELANs to be joined into a single ELAN while ensuring
the disjointedness of the distinct ELANs. One method for achieving this
result is to group together at least one set of the addresses
corresponding to servers serving the sub-ELANs of an ELAN in a so called
"configuration table". For example, the addresses employed may be the ATM
addresses. Packets originating from an address within a particular group
of sub-ELANs may only be transmitted to end-points having addresses within
the same group.
By using the same type of arrangement, in accordance with the principles of
the invention, configuration server 419 also controls access by the
various ELANs and sub-ELANs to services provided under the management of
multimedia server manager 429. This will be further described hereinbelow.
The multimedia services available to an end-point depends on the services
to which the ELAN to which the end-point belongs has subscribed. Exemplary
services provided under the control of multimedia server manager 429
include a) multipoint video teleconferencing, b) multipoint whiteboarding,
c) video-on-demand, and d) delayed playback of retrieved files. In order
to obtain such services, an end-point requests a connection to multimedia
server manager 429 as an end-point itself, with the request specifying the
particular service required.
Multimedia server manager 429 stores service definitions, including program
code and program content, for the multimedia services provided by all
multimedia servers 451. Program content includes, for example, video, text
and audio files, and optionally, links to servers from which such files
can be retrieved. Multimedia server manager 429 can also send program code
and program content to multimedia servers 451 to ensure that these servers
have the most current version of the multimedia services requested by the
LECs.
Similarly, multimedia servers 451 contain stored program code and program
content for the particular services they are capable of delivering at any
particular time. The multimedia servers 451 also contain means of
receiving updated program code and program content from the multimedia
server manager 429. It can also exchange multimedia data with the LECs
during a multimedia session.
Advantageously, such an architecture provides for a single point of control
and maintenance for each multimedia service while enabling the service to
be provided from several geographically diverse locations. Thus, for
example, a single copy of a book, newspaper, magazine or movie may be
loaded into multimedia server manager 429 and access to it by the various
ELANs served by ELAN contronect network 401 is made available by the ones
of multimedia servers 451 serving those ELANS.
LAN hubs 431 provide conversion from ATM to the protocol of high-speed
backbone 425. For each LES, BUS, and route/ARP server in each of the
sub-ELANs there is a single ATM connection to one of LAN hubs 431.
Multimedia servers 451, which include multimedia servers 451-1 to 451-N,
are each connected to one of the ATM switches. Each multimedia server has
two states, 1) an availability state and 2) a service state. The
availability state can take one of two values, a) "available", which
indicates the server is working properly, and b) "unavailable", which
indicates that the server is down or otherwise inaccessible. The service
state of each multimedia server can take one of three values, 1) "idle",
indicating that there are no LECs being served by the multimedia server,
2) "active", indicating that there is at least one LEC being served by the
multimedia server but that there remains in the multimedia server
additional capacity to serve other LECs, and 3) "busy", indicating that a
predetermined service capacity threshold has been reached. In one
embodiment of the invention, the busy value indicates that the multimedia
server cannot serve any additional LECs.
Multimedia server manager 429 of ELAN contronect network 401 dynamically
allocates multimedia servers 451 so that multimedia servers 451 can offer
multimedia services requested by LECs. Multimedia server manager 429 is
connected with a permanent or switched ATM connection to each of
multimedia servers 451. For example, multimedia server 451-N connects to
the multimedia server 429 via ATM connection 453, which passes through ATM
switches 483 and 481 of wide-area network (WAN) 131. ATM connection 453
terminates on LAN hub 431, which converts between ATM and the format of
high-speed backbone 425, so that packets can be exchanged between
multimedia server manager 429 and multimedia server 451-N in ATM wide-area
network (WAN) 131.
Each LEC served by ELAN contronect network 401 is able to communicate with
multimedia server manager 429 by establishing an ATM connection to it. The
layer-3 and ATM addresses of multimedia server manager 429 is known by
each LEC. The addresses of multimedia server manager 429 can be provided
to each LEC via a multimedia application software.
Multimedia server manager 429 interacts with a LEC when a LEC requests a
multimedia service from multimedia server manager 429, e.g., by connecting
to it and choosing the requested service from a multimedia service menu
that resides on multimedia server manager 429. By choosing a multimedia
service, the LEC is also transparently requesting from multimedia server
manager 429 the address of one of multimedia servers 451 that will be
providing the requested multimedia service. Application software provided
in the LEC requests the multimedia service and the address of the serving
multimedia server.
The LEC may request the address of one of multimedia servers 451 at any of
the following points: A) The LEC can request an address each time it
accesses the multimedia server manager 429 for a multimedia service,
although the LEC may already have an address of a multimedia server in its
cache; B) The LEC may request an address only those times that it accesses
the multimedia server manager 429 for a multimedia service when there is
not any address of a multimedia server in its cache; and C) The LEC may
request an address only those times that it accesses multimedia server
manager 429 for a multimedia service when the multimedia server for which
it has an address in its cache is not capable of serving the LEC. In
response to any requests, multimedia server manager 429 responds to the
LEC with the layer-3 address of one of multimedia servers 451 that can
provide the multimedia service which is selected by the LEC.
Multimedia server manager 429 can respond to the LEC with the address of an
active one of multimedia servers 451, i.e., one of multimedia servers 451
which is already serving other LECs in the ELAN of the requesting LEC, if
such a server is available. Multimedia server manager 429 can respond to
the LEC with the address of a newly joined idle one of multimedia servers
451 whose presence was not known by the LEC, i.e., a server that has not
been serving any clients in the ELAN of the LEC and joined with one of the
LES and BUS of the ELAN of the LEC after the last multimedia session of
the LEC. If there are no active or idle ones of multimedia servers 451 for
serving the LEC, multimedia server manager 429 responds to the LEC with
the address of a currently busy server. If there are multiple ones of
multimedia servers 451 available to serve the LEC, the multimedia server
manager 429 responds with the layer-3 address of one of them to ensure
that one or more of these servers can be freed up quickly, so that they
can serve other sub-ELANs.
Multimedia server manager 429 monitors the state changes of each of
multimedia servers 451. If one of multimedia servers 451 determines that
it can not serve any additional LECs, i.e., its service state has changed
from active to busy, it transmits a message to this effect to multimedia
server manager 429. Upon receipt of this message, multimedia server 429
allocates another idle or active one of multimedia servers 451 to serve
the sub-ELANs that are being served by the busy one multimedia servers
451, in accordance with an aspect of the invention. An active one of
multimedia servers 451 can be selected only if it is already serving other
LECs in the same ELAN of the busy one of multimedia servers 451.
Otherwise, multimedia server manager 429 selects an idle server, which may
not already be part of the ELAN of the LEC, to serve in place of the busy
one of multimedia servers 451.
Multimedia server manager 429 receives state-change messages from
multimedia servers 451 as follows: A) if a timer has expired, multimedia
server manager 429 requests the state value of one of multimedia servers
451 (polling approach); B) if a LEC requests the address of it's one of
multimedia servers 451, multimedia server manager 429 requests the most
current state value from that multimedia server; and C) one of multimedia
servers 451 autonomously reports a state value change to multimedia server
manager 429.
Multimedia server manager 429 maintains a database of all multimedia
servers 451. For each of multimedia servers 451, the database contains, at
least a) the most current service and availability states, along with
associated timers, b) the sub-ELAN of the LES and BUS with which the
multimedia server has joined, and c) the ELAN that the multimedia server
serves. Table 2 is an exemplary server database.
TABLE 2
__________________________________________________________________________
Availability
Service Sub-ELAN
ELAN
Server
State Timer
State
Timer
Joined
Served
__________________________________________________________________________
451-1
Available
80 sec.
Idle 10 mins.
227 371
. . . . . . .
. . . . . . .
. . . . . . .
451-N
Available
5 mins.
Active
5 mins.
237 301
__________________________________________________________________________
The timer field of Table 2 allows multimedia server manager 429 to age a
value of the state of a multimedia server, so as to know how current is
the state value.
Multimedia server manager 429 collaborates with address server 427 to
determine an unused layer-3 address for a multimedia server when it
determines to reassign an idle one of multimedia servers 451 to serve a
different sub-ELAN than it last served, in accordance with an aspect of
the invention. This new layer-3 address should be selected such that the
multimedia server is on the same ELAN which it will serve. Optionally,
multimedia server manager 429 collaborates with address server 427 to
determine the ATM and other layer-2 addresses for a multimedia server that
is being reassigned.
Multimedia server manager 429 also collaborates with configuration server
419 to determine the LES and BUS with which the multimedia server 451-N
should join. When multimedia server manager 429 determines to allocate a
new idle server, e.g., multimedia server 451-1, to sub-ELAN 237, it
requests idle server 451-1 to unregister from it's current LES and BUS,
e.g., LES 305 and BUS 307, by disconnecting itself from these servers.
Once idle server 451-1 disconnects from LES 305 and BUS 307, multimedia
server manager 429 sends a new layer-3 address to multimedia server 451-1
that corresponds to the subnet supported by ELAN 301. Multimedia server
manager 429 also sends the ATM addresses of the LES and BUS, e.g., LES 301
and BUS 303, of the new sub-ELAN that idle server 451-1 will serve. Upon
receipt of their addresses, multimedia server 451-1-joins with LES 301 and
BUS 303.
Multimedia server manager 429 also maintains a database of multimedia
services, and synchronizes the databases of all multimedia servers 451
with it's database, or, optionally, with each other's database. Also
optionally, multimedia server manager 429 maintains with the database the
authorization of sub-ELANs for specific multimedia services.
Each multimedia server 451 provides one or more multimedia services to a
LEC within the sub-ELAN, or ELAN, to which it has been allocated. As noted
above, exemplary services include a) multipoint video teleconferencing, b)
multipoint whiteboarding, c) video-on-demand, and d) delayed playback of
retrieved files. In order to obtain such services, a LEC a) connects to
multimedia server manager 429, b) selects the desired multimedia service,
c) requests the layer-3 address of a multimedia server which will provide
the service, and d) connects to the multimedia server to obtain the
service.
For example, multimedia server 451-N connects to BUS 303 via ATM connection
461 which passes through ATM switch 483 of ATM wide-area network (WAN) 131
and local ATM switch 193. Multimedia server 451-N also connects to LES 301
via ATM connection 459, which also passes through ATM switch 483 and the
local ATM switch 193. When so configured, multimedia server 451-N serves
all LECs in sub-ELAN 237 by using LES 301 and BUS 303, and it also serves
all LECs in ELAN 301, other than the ones in sub-ELAN 237, using address
server 427 and broadcast and unknown server 415 of ELAN contronect network
401.
Multimedia server 451-N | | |
