Active star-configured local area network

A data transmission network is disclosed which includes a plurality of nodes each adapted to send a packet of data. A controller, or hub, is connected to each of the plurality of nodes for immediately relaying a packet from one of the nodes to a desired destination when it is the only packet received by the controller means. However, when a plurality of packets are simultaneously received by the controller from plural nodes, the controller immediately relays a selected packet, and temporarily stores the other packets for subsequent transmission.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a data transmission system. More 
specifically, the present invention relates to a method and apparatus for 
efficiently controlling the exchange of data packets within an active 
star-configured local area network. 
In local area networks, there are increasing needs for the transmission of 
multiple information such as voice and video images along with data 
traffic. To cope effectively with the associated need of integrating 
information, large capacity (high speed) is required. This requirement can 
be satisfied by the construction of local area networks. 
Local area network applications of, for example, optical fiber systems have 
generated a considerable interest in recent years, this being due to the 
unique characteristics of the optical fiber such as wide bandwidth, low 
loss, electromagnetic immunity and the potential for supporting new 
services on installed fibers for future expansions. One of the popular 
topologies in fiber-optic local area networks (FOLAN's) is the star 
topology which can be either passive or active in design. 
An active star-configured FOLAN has the following features: (1) all the 
network nodes (e.g., access controllers) are interconnected via a 
controller, or hub, with each node being connected to the hub by two 
optical fiber-links--one for transmitting to the hub and the other for 
receiving from the hub; (2) interconnection is point-to-point, which of 
course makes it suitable for fiber-optic based implementation; (3) data 
loss due to numerous connectors (prevalent in bus-configured FOLAN's) can 
be avoided, thus a reasonably large number of nodes can be supported; (4) 
simple cost-effective optical-electronic devices such as light emitting 
diodes (LED's) and PIN diodes can be used at the network nodes thus 
implying less circuit design complexity and lower cost and (5) the hub 
contains some intelligence which means the network access protocol becomes 
simplified. 
The star-configured FOLAN is analogous to a system consisting of a single 
resource which is shared by a number of independent users and the main 
problem is how to effectively share the hub (i.e., the resource) among the 
nodes in the star network. To achieve effective hub sharing, the network 
nodes and the hub must adopt a set of rules known as protocols for 
governing data transmission. 
The first study of protocols for active star-configured FOLAN's was made by 
Schmidt et al. and was called "Fibernet II". "Fibernet II" is based on the 
carrier sense multiple access with collision detection (CSMA-CD) protocol. 
Collisions occur when two nodes attempt to transmit data signals to a 
shared central controller simultaneously. In such an event, all of the 
data being transmitted becomes garbled (i.e., undecipherable) and must be 
re-transmitted after randomly selected delays. A well-known problem 
inherent in the CSMA-CD is the network instability under heavy traffic due 
to a large number of collisions. Hence, the "Fibernet II" is not very 
efficient. 
The second protocol developed for active star-configured FOLAN's assumes 
the controller is intelligent so that collisions are avoided. One such 
example of an active star-configured FOLAN which includes such an 
intelligent controller is disclosed in U.S. Pat. No. 4,570,162 to Boulton 
et al. In this patent, a data transmission network is disclosed which 
includes a number of access controllers (i.e., nodes) connected to a 
central hub. Each access controller is connected to a hub by both a 
transmission link and a reception link. The hub includes a selection means 
connected to the hub end of each access controller transmission link. The 
hub also includes a broadcast means connected to the reception link of 
each access controller. 
In accordance with the operation disclosed in the Boulton et al. patent, 
the selection side of the hub receives a single packet of data from a 
selected access controller, and passes it through a link to the broadcast 
means. The broadcast means then transmits that particular packet to all of 
the access controllers connected to the hub. Each access controller 
monitors the packets which it receives from the hub in order to retain 
packets intended for it, and in order to determine whether the packet it 
previously transmitted was the packet selected by the hub for 
transmission. If the access controller determines that its packet was not 
the one selected for transmission by the hub, it will again try to have 
the hub broadcast its packet by re-transmitting the packet to the hub. 
Because the data packet of only a single access controller is selected for 
transmission, a data transmission network as discussed with respect to the 
aforementioned patent avoids the potential occurrence of destructive 
collision. Although such a system thus provides a collision-avoidance 
protocol which yields an improved network performance over the "Fibernet 
II", it is still possible to have unbounded packet delays, especially 
under heavy traffic. An unbounded packet delay occurs when there is large, 
potentially unlimited delay in the transmission of a particular packet of 
data from one location in a network to another location in the network. 
These large delays can occur for two reasons: (1) when two or more network 
access controllers simultaneously contend for transmission during a free 
state of the hub, only one of the contending network access controllers is 
allowed to successfully transmit and the other contending network access 
controllers must retry after a timeout interval; thus, it is possible for 
a network access controller to be involved in a contention at every 
try/retry and as it may continually not be the one selected to 
successfully transmit, an unbounded delay for its packet will result; (2) 
when a network access controller attempts to transmit during a busy state 
of the hub (i.e., when the hub is already in the process of transmitting 
other data packets), the network access controller will back off and retry 
at a later time; thus, a network access controller may always attempt to 
transmit/retransmit at a time when the hub is busy such that an unbounded 
delay for the network access controller's packet could again result. 
Thus, a need exists in the prior art for a more efficient active 
star-configured local area network. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a new 
collision-avoidance multiple access protocol which will overcome the 
unbounded delay of the previously developed collision-avoidance protocol. 
Such a protocol is realized by the following network system operation: 
(1) when the hub is in the free state and two or more nodes simultaneously 
forward their packets to the hub (i.e., more than one node contends for 
service by the hub), the hub will begin to store the packets in a buffer, 
and then use a prescribed ordering sequence to transmit one packet from 
each of the contending nodes, this being realizable because the controller 
is intelligent; 
(2) when the hub is in a busy state and a node forwards its packet to the 
hub, the packet is simply stored in the hub buffer to await transmission 
on, for example, a first-come first-serve basis. 
Thus, in accordance with the present invention, a data transmission network 
is disclosed which includes a plurality of nodes each adapted to send a 
packet of data. A controller means, or hub, is connected to each of the 
plurality of nodes for immediately relaying a packet from one of the nodes 
to a desired destination when it is the only packet received by the 
controller means. However, when a plurality of packets are simultaneously 
received by the controller means from plural nodes, the controller means 
immediately relays a selected packet, and temporarily stores the other 
packets for subsequent transmission. 
In accordance with a further embodiment of the present invention, the 
controller means includes a buffer for temporarily storing the data 
packets, and operates in three basic states: 
(a) a first state in which the buffer does not contain any 
previously-stored packets and in which only one packet is received by the 
controller means such that the controller means will virtually immediately 
relay the packet toward its respective destination; 
(b) a second state in which the buffer does not contain any 
previously-stored packets and in which a plurality of packets are received 
at substantially the same time from more than one node, such that the 
controller means will virtually immediately relay a selected one of the 
packets towards its respective destination and will temporarily store the 
other packets in the buffer to be retrieved and relayed in a prescribed, 
ordered sequence after the selected packet has been relayed; and 
(c) a third state in which the buffer does contain at least one 
previously-stored packet and in which a plurality of packets are received 
at substantially the same time from more than one node, such that the 
controller means will temporarily store at least some of the incoming 
packets in the buffer along with the at least one previously-stored packet 
and relay the packets in a predetermined order at a time after the 
controller means has completed its current transmission. 
The selected packet may be chosen on a first-in first-out basis or, in the 
case where a random access buffer is used, on the basis of some 
predetermined level of priority. In addition, a packet may be broadcast 
universally, or may be selectively broadcast to only a single node or 
group of nodes. 
Other features and advantages of the present invention include, but are not 
limited to: 
a data transmission network which is truly collision-free, and in which a 
node never has to re-transmit a packet to a hub, such that additional 
propagation delays due to re-transmissions can be completely avoided and 
data throughput can be increased; 
a data transmission network which exhibits a reasonable degree of service 
fairness among contending nodes; 
a collision-free data transmission network which has a bounded access delay 
time regardless of how busy the network becomes, such that data packets 
(e.g. voice packets) may be transmitted with a high confidence level; 
a data transmission network which combines the features of multiple buffers 
in a hub and true physical full-duplex operation, so as to permit a node 
to have a packet in transit to its own hub or elsewhere in the network at 
the same time it is receiving an entirely different packet from another 
node; 
a data transmission network which does not depend strongly on the network 
propagation delay such that there is no restriction on packet size; 
a data transmission network having a selective broadcast capability such 
that a node may transmit a packet directly to one other node or to all the 
nodes accessing a specific hub without any of the remaining network nodes 
being aware of the transmission; 
a data transmission network having a local selective broadcast capability 
whereby a node may send a packet to another node hub such that the packet 
is only transmitted within that particular hub, thus lowering packet 
transmission delay and increasing overall network efficiency; 
a data transmission network in which there is a greater degree of overall 
network fairness in allocating bandwidth since packets addressed to local 
or close-by destinations do not have to climb to the top of a network 
tree, competing at every hub for service; 
a data transmission network in which a priority packet service scheme can 
be utilized such that the highest priority packet will immediately go to 
the head of the queue at every hub it visits; and in which if there are 
numerous high priority packets, the hub will ensure that the lower level 
packets receive a certain minimum level of service so as to avoid 
unbounded delays; and 
a data transmission network in which a priority scheme that includes 
pre-emptive or nonpre-emptive status designations can be utilized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be discussed as it relates to a multiple 
access protocol for active star-configured fiber-optic local area 
networks, (FOLAN's). The main features of the protocol include 
collision-avoidance, no packet retransmissions, a reasonable degree of 
fairness among the nodes contending for transmission, a bounded access 
delay, priority selection of packets and selective broadcasting. A typical 
active star-configured FOLAN consists of a finite number of nodes, each of 
which comprises an intelligent hub. For example, as shown in FIG. 1, the 
typical prior art active star-configured FOLAN includes a number of nodes 
2, labelled l-n, which are connected to a particular hub 4. Such a network 
and its general operation are generally known in the art, as described in 
the aforementioned U.S. Pat. No. 4,570,162, to Boulton et al, which is 
hereby incorporated by reference. Each node in the network is connected to 
its hub by two high speed optical fiber links, 10 and 12, which have 
full-duplex capability. 
FIG. 2 shows a partial schematic representation of one embodiment of the 
inventive controller means, or hub which is used in the network of the 
present invention. In one preferred embodiment of the present invention a 
plurality of hubs are employed to form a network. 
Referring to FIG. 2, the hub 14 is shown to include a selection and 
ordering means 16 and a broadcast means 18. In accordance with the present 
invention, the selection and ordering means 16 further includes a buffer 
means 20, whose storage capacity is equal to the number of nodes on that 
hub. The hub includes a clock recovery means 5 and a CPU 7 to control 
operation of the hub. A route directory 9 is also included, and has stored 
therein, information regarding the path a particular packet of data must 
follow through a data network in order to be properly transmitted from a 
particular source node to one or more destinations. 
A priority level, destination address and broadcast mode element 11 
provides priority information contained in data packets received by the 
hub from various nodes via optical receivers 3 to the CPU 7. After 
determining which incoming packet is to be next transmitted, the CPU will 
include a self identification signature in the data packet via element 13 
and then issue routing instructions to the broadcast means via a broadcast 
mode selector 15. As can be seen in FIG. 2, the broadcast means includes a 
plurality of switches 17 which are used to route the data packets to their 
respective destinations. 
The buffer means 20 included in the hub can be of a known first-in, 
first-out (FIFO) type or of a random access type. The buffer means 20 is 
used to temporarily store data information which is received substantially 
simultaneously over a plurality of data link inputs to the hub. The phrase 
"substantially simultaneously" or simply "simultaneously" is used herein 
to mean approximately within the time it takes to transmit one bit. 
Because data which is transmitted simultaneously over a plurality of data 
link inputs to the hub is stored, collisions of data transmissions are 
avoided. That is, the hub will receive and store most of the data, and 
then transmit it in turn. Thus, the collision-free protocol of the present 
invention can completely avoid the additional propaqation delays of prior 
art systems which were incurred by the need to re-transmit data 
information which had been previously rejected by the hubs located all the 
way up a network tree to the central controller. 
More specifically, because the hub 14 shown in FIG. 2 is an intelligent 
controller, it can arrange the transmission of contending inputs due to 
simultaneous multi-arrivals at the hub, in a prescribed order. For 
example, data which is simultaneously transmitted to a hub from various 
nodes linked to the hub can be ordered in the buffer by address. Thus, a 
node with a higher address may be transmitted by the hub before data from 
a node having a lower address when two or more nodes contend for 
transmission simultaneously. 
An example of an active star-configured FOLAN which includes a plurality of 
hubs 14 labelled 1-M, is shown in FIG. 3. As can be seen in FIG. 3, in 
addition to providing data links between the hubs 14, data links are also 
provided between a particular hub and a plurality of local nodes 
associated only with that hub. For example, the hub 14 which is labelled 
#1 in FIG. 3 is not only linked directly to the hubs labelled #2, #4, #5 
and #6, but is also linked to the local nodes 2 which are labelled l-n. 
A node can be in either an idle state or in an active (transmit/wait) 
state. A node is in the active-transmit state if it forwards a packet to 
the hub during the hub free state and in the active-wait state if it 
forwards a packet to the hub during the hub busy state. A node is in the 
idle state if it does not have a packet to forward to the hub. 
FIG. 4 shows a partial schematic representation of the switching 
capabilities of each of the hubs 14 shown in FIG. 3. As exemplified by the 
schematic in FIG. 4, any data received by the hub 14 either locally from a 
node associated only with that hub or from another hub, can be directed to 
any or all of the output transmission links of the hub. Thus, such a 
configuration permits the hub to selectively control the receivers in the 
network which will receive a particular transmission from the hub. 
The nodes transmit and receive data which has been placed into a packet 
field format. The overall packet format could, for example, correspond to 
a standardized Fibre Distributed Data Interface (FDDI) Type Field Format. 
Such a packet is shown in FIG. 5. In FIG. 5, each box or field in the 
packet corresponds to a particular portion of data which is transmitted 
when a transmission is initiated. Transmission of a complete packet thus 
requires a set number of time slots. 
It should be noted that because the network of the present invention is not 
affected by propagation delays associated with retransmissions, there is 
virtually no restriction on the number of bytes contained in a packet. 
Thus, a network constructed in accordance with the teachings of the 
present invention could handle mixed traffic, consisting of short and long 
messages, equally well. 
The first five fields in the FIG. 5 data packet, beginning from the left, 
correspond to standard FDDI field format information. That is, the first 
field "PA" corresponds to an idle preamble field. The data contained in 
this field alerts the receiving hub that there is an incoming packet. For 
example, because a hub may go into an idle state if it is inactive for a 
period of time, the idle preamble field contains information which will 
reactivate the hub and synchronize its operation with the hub or node 
sending the packet. Thus, the use of such a format would provide the same 
method of point-to-point (node-hub-node) clock synchronization as proposed 
for the FDDI. Accordingly, overall synchronism of the entire network is 
not required in a preferred embodiment of the invention, but in other 
embodiments it may be preferred to transmit idle signals at continuous 
intervals in order to retain synchronization of idle nodes at all times. 
The second through the fifth fields also correspond to the standard FDDI 
field format. That is, "SD" corresponds to starting delimiter data; "FC" 
corresponds to the frame control (i.e., indicates to the hub whether the 
information being transmitted is, for example, data as opposed to some 
type of internal control message); "DA" corresponds to the destination 
address; and, "SA" corresponds to the source address. The tenth field in 
the packet corresponds to the data to be transmitted, while the final 
field in the packet, "FCS", corresponds to frame check sequence. The 
information in this field is used to indicate whether a frame of data in a 
packet has been received without error at the destination address. 
The sixth through the ninth fields in the packet field format of FIG. 5 
provide specific information used by the hub in the local area network of 
an embodiment of the present invention. The sixth field provides an 
indication to the receiving hub of the priority level of the packet. For 
example, bits corresponding to a particular code, represented in FIG. 5 as 
a Roman number II, might correspond to the highest level priority, while 
bits represented as, for example, a Roman numeral I might correspond to a 
lower level priority. A receiving hub, upon detecting that a data packet 
corresponds to the highest priority would let this packet proceed to the 
top of the buffer queue so that it could be served immediately upon 
completion of a current operation. 
If there are numerous high priority packets, the hub may ensure that the 
lower level packets receive a certain minimum level of service in a number 
of ways. For example, every high level packet could be followed by a low 
level packet or, a timer could be associated with each of the input 
buffers such that if the hub delay exceeds a specified value then a lower 
priority packet would be transmitted regardless of the existence of higher 
priority packets which have been stored more recently. 
The seventh field in the FIG. 5 packet, if present in the particular data 
packet being transmitted, provides an indication to the hub of whether the 
packet has a pre-emptive (P) or nonpre-emptive (N) status. If a packet is 
afforded a preemptive status, it will cause the hub to discontinue service 
to the packet it is processing so that the packet with pre-emptive status 
will be immediately served and the pre-empted packet retransmitted as soon 
as the pre-empting packet has been transmitted. In a preferred embodiment 
of the invention however, packets would normally be designated with 
nonpre-emptive status. Accordingly, even the highest priority packet would 
not be serviced until the hub had completed its current transmission. 
The eighth field in the packet generally identifies to the hub the type of 
data transmission which is to be effected by the hub with respect to the 
packet. For example, a particular packet which originates from a node can 
be retransmitted by a hub universally (U), selectively (S) or globally 
(G). 
Referring to the network depicted in FIG. 3, if a data packet received from 
the node #5 which is linked to the hub #3 is to be universally 
transmitted, the eighth field in the packet of FIG. 5 would contain data 
indicative of this. Accordingly, the hub #3 would transmit the data packet 
to all nodes in the network via a particular hub which has been 
arbitrarily designated as a master hub for transmitting all universal 
transmissions in the network. For example, in FIG. 3 the hub #1 would be 
the logical choice as the master hub since it is located at the center of 
the network. 
If the eighth field of the data packet indicates that a selective broadcast 
of the data packet is to be performed, the hub would only transmit the 
packet to a particular, addressed node. Such a feature enhances network 
security by avoiding altogether the presentation of a packet to a node or 
hub which is not intended to receive the packet. Thus, for example, if the 
node #5 in the hub #3 transmitted a packet destined solely for the node #1 
of the hub #1, then the packet would contain the address of the latter 
node as the destination and would indicate to the hub that a selective 
broadcast is to be performed. Where a local selective broadcast is to be 
performed between two nodes which are connected to the same hub, the 
selective transmission will again be indicated to the hub. In such a case, 
the transmission can be handled completely by the particular hub involved. 
Thus, unlike prior art systems in which packets always had to be 
re-transmitted through a hub designated as a central controller, the 
present system can operate more efficiently by performing such a 
transmission locally. 
The provision of a buffer memory in each hub reduces concern that a data 
packet directed to a hub which is currently performing a local selective 
broadcast will be involved in a collision. Therefore, during such a local 
transmission, other transmissions within the FIG. 3 network can be 
performed, such that the network efficiency can be further enhanced. 
Furthermore, with a selective broadcast in accordance with the present 
invention, there is a greater degree of overall network fairness in 
allocating bandwidth, since packets addressed to a local or close-by 
destination do not have to climb to the top of the network tree competing 
for service at every hub along the way. 
The eighth field in the FIG. 5 packet can, as mentioned above, also 
indicate to the hub that a local global broadcast is to be performed 
(i.e., bits corresponding to "G" included in the eighth field). In such a 
case, a packet received by a hub from a particular node associated with a 
particular hub will be transmitted to all other nodes which are connected 
to that same hub. For example, the first node connected to the hub #3 in 
FIG. 3 can use this packet indicator to inform the hub that the packet is 
to be transmitted to the second through the fifth node of the hub #3. The 
source node #1 will also receive a copy of the packet transmitted by the 
hub to ensure proper transmission. 
The ninth field in the data packet of FIG. 5, which is labelled "1 to M" 
provides an indication of the hub which is currently serving the packet. 
The letter M corresponds to the number of hubs in the network. During, for 
example, a universal or non-local selective broadcast in the FIG. 3 
network, a packet would be transmitted through more than one hub. 
Accordingly, as the packet proceeds through the network, this field would 
keep track of the hub which is currently servicing the packet. 
Turning now to FIGS. 7A and 7B, examples of packet information for 
transmitting data from one location to another in, for example, the FIG. 3 
network are shown. The packets depicted in FIGS. 7A and 7B have been 
abbreviated in the interest of clarity. 
In FIG. 7A, representating a fifth buffer the first example corresponds to 
a universal broadcast from the node #5 of the hub #3 to all of the nodes 
in the FIG. 3 network. The first packet shown in this example indicates to 
the source hub #3 that the packet will be transmitted to the hub #1 which 
has, for purposes of this example, arbitrarily been designated as a master 
or central hub to perform all universal broadcasts. However, it should be 
noted that although all universal broadcasts can be performed by a single 
designated hub, any hub in FIG. 3 could, in an alternative embodiment, 
perform such a broadcast since all of these hubs are similarly structured. 
Returning to the first example in FIG. 7A, the first field from the left in 
the first packet shown indicates that hub #1 will perform a universal 
broadcast, as indicated by the third field. The zero in the second field 
confirms that all nodes directly connected to the central hub are to 
receive the data packet. The fourth field corresponds to the current hub 
serving the packet (i.e., field 9 in FIG. 5), and the last two fields 
correspond to the source hub and node, respectively. 
After the packet has been received by the hub #1, the hub #1 transmits the 
second packet shown in the first example of FIG. 7A to complete the 
universal broadcast. As can be seen, the zeroes in the first two fields of 
this second packet indicate that universal transmission to all hubs and 
nodes, respectively, is desired. The fourth field has been changed to 
indicate that it is the hub #1 which is now servicing the packet. 
Examples 2 and 3 of FIG. 7A correspond to selective broadcasts as discussed 
previously. The fields in these examples correspond to those discussed 
with respect to the first example. A bit sequence designated "S" has been 
included in the third field to indicate that a selective broadcast is to 
be performed. In order to perform the transmission in example 2, the 
packet must pass through the hub #2 (see FIG. 3), and thus, the first two 
packets are required. The third packet in this example corresponds to a 
confirmation packet which provides an indication to the source node that 
the transmission was completed. 
The fourth and fifth examples in FIG. 7A correspond to local global and 
local selective broadcasts, respectively. As mentioned previously, bits 
designated by the letter "G" in the third field of example 4 indicate to 
the hub that the packet is to be transmitted by the hub to all of the 
other nodes linked to the hub. The "S" in the third field of the example 
5, in combination with a "2" in the second field and an identical source 
and destination hub (the first and fifth fields) indicates to the hub that 
a local selective broadcast is to be performed. 
The examples shown in FIG. 7B correspond to an embodiment where a Random 
Access Buffer is used. Although for the most part, the information in the 
fields 3 to 8 of these examples corresponds to that of FIG. 7A, two 
additional fields have been included. The two additional fields correspond 
to information regarding priority level and nonpre-emptive status, 
respectively, as discussed with respect to FIG. 5. 
In operation, the inclusion of a buffer in each of the hubs permits 
increased flexibility of a star-configured network, using a more 
sophisticated protocol to ensure system efficiency. Accordingly, the 
network in a preferred embodiment of the invention uses the following 
protocol: 
(i) An arriving packet at an idle hub is transmitted virtually immediately 
without any carrier sensing. That is, when a packet arrives at an idle 
hub, the hub immediately begins to place the packet in the buffer. While 
the first few fields of the packet (which include the destination of the 
packet) are being stored, the hub will rapidly determine the destination 
address of the packet. If the packet is destined for a node associated 
with another hub, the hub currently serving the packet will, after 
consulting a route directory, immediately begin to transmit the packet 
towards that other hub, and will not delay the packet even though a copy 
is retained until the packet is delivered. Thus, precious network time is 
not wasted storing the total packet in the buffer of an idle hub which is 
not the destination hub for the packet. All packets are transmitted only 
once by a node (no retransmissions are necessary), because no collision 
can occur, and each packet is guaranteed of successful delivery at its 
destination after it has been transmitted. 
(ii) Each node receives all of the packets addressed to it by the other 
nodes. That is, packets which are not addressed to a particular node will 
not even be received by that node. 
(iii) Each node also receives a copy of its own packet from the hub as a 
method of providing a first-order form of acknowledgement that the packet 
has been received at its destination. 
(iv) A copy of the transmitted packet by a node is retained in its buffer 
and is only deleted after the packet has been received at its destination. 
(v) The hub protocol further depends upon the state of the buffer, which is 
either empty (hub is idle, or free) or nonempty (hub is busy). The 
protocol is described as follows (assuming only a single hub network). 
(a) When only one node forwards a packet to the hub while the hub is idle, 
the selection and ordering component immediately passes the packet via the 
link to the broadcast section where it is broadcast to the particular 
network nodes. Because the hub can identify the packet destination from 
the first few fields it receives, time is not wasted storing the total 
packet in the hub's buffer before it is transmitted. "Immediately" or 
"virtually immediately" herein are meant to include circumstances where 
the hub examines the first few fields of the packet before beginning 
transmission even though each packet is stored there is no delay because 
it is reading out as it reads in. 
(b) When two or more nodes contend for transmission simultaneously while 
the hub is idle, the selection and ordering component selects one of the 
contenders to begin transmission virtually immediately using a prescribed 
ordering sequence as discussed previously. Again, because the destination 
addresses of the packets are received first, the hub can quickly determine 
which packet has the highest priority and begin transmission of that 
packet before it is completely stored in the buffer. 
(c) When one or more nodes contend for transmission while the hub is busy, 
the packets from these nodes wait in the hub buffer until the packet which 
is being transmitted by the hub is completed and only after this will the 
transmission of the contending packets begin in accordance with the 
prescribed ordering sequence. In addition, the hub will ensure that no 
packet placed in the buffer will be subject to an unbounded delay. This 
can be achieved by, for example, providing that the hub will monitor the 
time at which a packet is stored in the buffer. Any low priority packet 
which is precluded from transmission for a preset period of time would be 
reprioritized within the buffer to ensure its transmission. Other equally 
effective methods of preventing unbounded delay, within the buffer would 
be readily recognizable to one of ordinary skill in the art as mentioned 
previously. 
The use of a buffer to store packets as a method of providing a truly 
collision free network is novel with this invention. The use of such 
buffers to store data temporarily surprisingly does not entail any undue 
delays in overall system performance. In order to demonstrate the 
advantage of using buffers in the hub controllers as disclosed in the 
present invention, a mathematical analysis with the FIFO Buffer Model is 
now provided. 
Using an analytic model, the throughput and delay characteristics of an 
embodiment of the inventive protocol for active star-configured FOLAN'S 
can be ascertained. The analysis of the protocol is based on an embedded 
discrete-time queuing process obtained by examining the system only at the 
beginning of the node transmission times. Mathematical expressions for the 
throughput and the mean packet delay are derived. The throughput is 
obtained from the transmission cycle analysis, where expressions are 
derived for the mean lengths of the transmission cycle components. The 
mean packet delay consists of the mean waiting time of a packet and the 
packet transmission time. Based on the modified mode of operation, the hub 
is modeled by an M/D/1 queue with batch arrivals, where the batch size is 
equal to the number of nodes contending for transmission simultaneously. 
Results from queuing theory will be used to calculate the mean waiting 
time of a packet. The effects of the system population, packet size and 
transmission rate on the throughput and delay characteristics will also be 
considered. 
The following assumptions have been made in the formulation of the analytic 
model: 
(A.sub.1): The hub controller time is slotted. The duration of a slot, 
.DELTA., is equal to the maximum propagation delay (in seconds) between 
any two nodes in the network and is identical for all nodes in the 
network. .DELTA. is taken to be the time unit. 
(A.sub.2): All nodes are statistically identical. 
(A.sub.3): All nodes are synchronized so that the contention for 
transmission occurs only at the time slot boundaries. 
(A.sub.4): Each node has a buffer space of one unit. 
(A.sub.5): Packets arrive at a node according to the Bernoulli process with 
parameter .sigma. which is the probability of one arrival at the end of a 
slot. 
(A.sub.6): The system is lightly loaded and .sigma. is taken to be very 
much less than unity, that is, .sigma.&lt;.sigma.&lt;&lt;1. 
(A.sub.7): Packet lengths are constant and equal to T slots, where T is an 
integral multiple of slot time. The node transmission time is equal to T 
slots and consists of the packet transmission time plus an additional slot 
which accounts for the propagation delay due to the buffer, hence T'=T+1 
slots. 
Based on the above assumptions, a discrete-time queuing model can be 
constructed for the system by observing the state of the system at the 
beginning of each slot. However, the analysis of such a model is 
formidable. Hence, an approximate model shall be formulated by examining 
the system only at the beginning of each node transmission time 
(represented by the numbered dots in FIG. 8). In order to do so, the 
following additional assumption is made on the packet generation by each 
idle node in a node transmission time. 
(A.sub.8): An idle node generates one packet with probability v=.sigma.T' 
at the end of a node transmission time. 
Since .sigma.&lt;&lt;1 and typically 1/.sigma.&gt;&gt;T', then the system state changes 
very slowly in a node transmission time. Consequently, assumption 
(A.sub.8) is approximately valid and the performance of the model based on 
adopting assumption (A.sub.8) does not significantly differ from the 
performance of the model without making the assumption. 
The throughput (controller utilization), S.sub.H, is defined as the 
fraction of time the controller is busy during a transmission cycle (FIG. 
8) and the expression for S.sub.H is given by 
##EQU1## 
where E[x] is the mathematical expectation of x. 
The mathematical relationship for the throughput is obtained from the 
transmission cycle length analysis which is presented as follows. FIG. 8 
depicts a timing diagram of the transmission cycle for the controller and 
it consists of two types of transmission subcycles. Transmission subcycle 
Type I consists of a free period which is followed by a busy period Type I 
that is generated by all the nodes contending for transmission 
simultaneously when the controller is free. Transmission subcycle Type II 
consists of only the busy period Type II which is generated by all the 
nodes contending for transmission when the controller is busy. Under the 
assumption that the system is lightly loaded, almost all the transmission 
subcycles are transmission subcycle Type I which are independent and 
identically distributed. A transmission cycle therefore consists of a 
number of contiguous transmission subcycles Type I and one transmission 
subcycle Type II. The mean lengths of the free period, the busy period 
Type I and the busy period Type II, which are the transmission cycle 
components will now be evaluated. 
(i) Mean Length of the Free Period: From FIG. 8, the mean length of the 
free period is geometrically distributed with parameter .theta., where 
.theta. is defined as the probability that no node contends for 
transmission at the beginning of a node transmission time. The probability 
that the length of a free period, I, consists of i node transmission times 
is given by 
EQU Pr [I=i]=.theta..sup.i-l (1-.theta.), i=1,2,3, . . . (2) 
so that the mean length of the free period, I, (in node transmission times) 
is expressed by 
##EQU2## 
where .theta. is given by (1-v).sup.M. 
(ii) Mean Length of the Busy Period Type I: Let N be the random variable (N 
.epsilon. {1,2,3, . . . ,M}) denoting the number of nodes contending for 
transmission simultaneously when the controller is free (batch size of the 
transmission subcycle Type I). Then the length of an arbitrary busy period 
Type I, B.sub.1, is defined by 
EQU B.sub.I =N T' (4) 
so that the mean length of the busy period Type I (in slots) is given by 
EQU B.sub.I =N T' (5) 
where N, the mean number of nodes contending for transmission 
simultaneously when the controller is free is obtained as: 
##EQU3## 
Equation (6) can be obtained as follows. Let N be the random variable 
defining the number of nodes which have generated a busy period Type I. 
Clearly, N&gt;0. By applying the conditional expectation concept in 
probability theory, we can write 
EQU E[N]=E[N.vertline.N&gt;0]Pr[N&gt;0]+E[N.vertline.N=0]Pr[N=0] (6.1) 
where E[x] is the mathematical expectation of x. It is easy to see that 
E[N.vertline.N=0]=0. By defining N=E[N.vertline.N&gt;0] and then solving for 
N in (6.1), we obtain 
##EQU4## 
Pr[N&gt;0] is the probability that at least one node has generated an arrival 
at the end of a node transmission time and will thus contend for 
transmission at the beginning of the next transmission epoch. If .theta. 
is defined as the probability that none of the idle nodes generates an 
arrival, Pr[N&gt;0] is then given by (1-.theta.). Substituting the expression 
for Pr[N&gt;0] into (6.2) and then using the definition of mathematical 
expectation for discrete random variables, we have 
##EQU5## 
Since each node independently generates a packet according to the 
Bernoulli process, the total number of nodes in the active-transmit state 
is binomially distributed. Equation (6.3) becomes 
##EQU6## 
After simplifying we obtain 
##EQU7## 
which proves Equation (6). 
(iii) Mean Length of the Busy Period Type II: The length of the busy period 
Type II depends on the total number of nodes in the active-wait state at 
the end of a busy period Type I. From the unity buffer size assumption, 
this is equivalent to the number of idle nodes each of which has generated 
one packet during the current busy period Type I. At this point it is 
convenient to define the following random variable. Let 
X=random variable defining the total number of nodes in the active-wait 
state at the end of a busy period Type I 
Furthermore, recall that 
N=random variable defining the number of nodes which have generated the 
current busy period Type I, that is, number of nodes in the 
active-transmit state at the beginning of the current busy period Type I 
Noting that X depends on N, we can write [9] 
EQU E[X]=E[E[X.vertline.N]] (7) 
which can be written in expanded form as: 
##EQU8## 
In proving equation (8), an important property of conditional expectation 
is that for all random variables X and N 
EQU E[X]=E[E[X.vertline.N]] (8.1) 
Since N is a discrete random variable, then Equation (8.1) states that 
##EQU9## 
where E[X.vertline.N=k] is the conditional expected number of nodes that 
are in the active-wait state. X is also a discrete random variable and we 
can write E[X.vertline.N=k] as 
##EQU10## 
where Pr[X=j.vertline.N=k] is the conditional probability that j nodes are 
in the active-wait state at the end of a busy period Type I given that k 
nodes are in the active-transmit state at the beginning of a busy period 
Type I. Since k.gtoreq.1 in (8.2), Pr[N=k] must be normalized by the 
probability that at least one node is in the active-transmit state at the 
beginning of a busy period Type I(=Pr[N.gtoreq.1]). Similarly, since 
j.congruent.1 in (8.3), Pr[X=j.vertline.N=k] must be normalized by the 
probability that at least one node is in the active-wait state at the end 
of a busy period Type I (=Pr[X.gtoreq.1.vertline.N=k]). By introducing 
these normalization probabilities into (8.2) and (8.3), Equation (8.1) 
becomes 
##EQU11## 
which proves (8). 
From the assumption that a silent node cannot generate a new packet during 
the current transmission subcycle, we have E[X]=0 for k=M. Hence equation 
(8) reduces to 
##EQU12## 
.alpha. is the probability that none of the idle nodes generate a new 
packet at the end of a node transmission time and is given by 
(1-v).sup.(M-k). 
The conditional probability Pr[X=j.vertline.N=k] in (9) is given by 
##EQU13## 
wherein .beta..sub.j.sup.i is the probability of having j arrivals in 
"exactly" i node transmission times. Note that .beta. can be interpreted 
as the probability of having at least one arrival in a specified number of 
node transmission times. Examples of the possible combinations of the 
number of arrivals in a given number of node transmission times are 
provided as follows: 
.beta..sub.j.sup.i is defined as the probability of having j(&gt;1) arrivals 
in exactly i(.gtoreq.1) node transmission times. 
For j=1, i=1: {one arrival in exactly one node transmission time} 
##EQU14## 
For j=2, i=1: {two arrivals in exactly one node transmission time} 
##EQU15## 
For j=2, i=2: {two arrivals in exactly two node transmission times} 
##EQU16## 
which is interpreted as the probability of generating one packet in each 
of the two node transmission times. 
For j=3, i=1: {three arrivals in exactly one node transmission time} 
##EQU17## 
For j=3, i=2: {three arrivals in exactly two node transmission times} 
##EQU18## 
Equation (11.3b) is the probability of generating three packets in exactly 
two node transmission times, that is, two packets are generated in one 
node transmission time by two idle nodes and one packet is generated in 
the second node transmission time by one idle node. 
For j=3, i=3: {three arrivals in exactly three node transmission times} 
##EQU19## 
which is the probability of generating three packets in exactly three node 
transmission times, that is, one packet is generated in each of the three 
node transmission times. Similar expressions can be written for higher 
values of j and i. 
As can be seen from the above discussion, the mean length of the busy 
period Type II, B.sub.II, (in slots) is therefore given by 
EQU B.sub.II =E[X].multidot.T' (12) 
The expected length of a transmission cycle, E[T.sub.c ], (in slots) can 
then be written as 
EQU E[T.sub.c ]=C.sub.I (I T'+B.sub.I)+B.sub.II (13) 
where C.sub.1 is the mean number of contiguous transmission subcycles Type 
I before a transmission subcycle Type II. C.sub.I is given by: 
##EQU20## 
where p is the probability that at least one node out of the (M-N) idle 
nodes (in an arbitrary transmission subcycle Type I) are in the 
active-wait state. p is expressed as 
EQU p=1-[(1-v).sup.(M-N) ].sup.N (15) 
Equation (14) is obtained as follows. Let C.sub.I denote the number of 
contiguous transmissions subcycles Type I before a transmission subcycle 
Type II. Clearly, the transmission subcycle Type II cannot occur without 
the occurrence of at least one transmission subcycle Type I. Hence C.sub.I 
is a discrete random variable and assumes positive integers, that is, 
CI=1,2,3, . . . . Now, let p denote the probability that at least one node 
out of the (M-N) idle nodes (in an arbitrary transmission subcycle Type I) 
are in the active-wait state at the end of N node transmission times. 
Clearly, 
EQU p=1-q (15.1) 
where q is the probability that none of the (M-N) idle nodes is in the 
active-wait state at the end of N node transmission times. q is given by 
EQU q=[(1-v).sup.(M-N) ].sup.N (15.2) 
where (1-v).sup.(M-N) is the probability that none of the (M-N) idle nodes 
is in the active-wait state at the end of one node transmission time. From 
the above, the occurrence of a transmission subcycle Type II after several 
transmission subcycles Type I can be modeled by a geometric distribution 
and the distribution for C.sub.1 is given by 
EQU Pr[C.sub.1 =i]=q.sup.i-l p, i=1,2,3, . . . (15.3) 
from which the mean number of contiguous transmission subcycles Type I 
before a transmission subcycle Type II, C.sub.1, is found to be 
##EQU21## 
From the above analysis and (1), the throughput can then be expressed as 
##EQU22## 
Based on the system operation and the constant node transmission time, the 
controller is modeled by an M/D/1 queue with batch arrivals, where the 
batch size is equal to the number of nodes contending for transmission 
simultaneously. The average packet delay, D, is defined as the average 
time interval from the instant a packet is generated to the instant it is 
received at its destination. The average packet delay consists of two 
components: the mean waiting time, D.sub.W, and the node transmission 
time, T'. The mean waiting time in an M/D/1 queue with batch arrivals is 
given by [10] 
##EQU23## 
where E[B] is the mean number of nodes in a batch initiating an arbitrary 
transmission subcycle and .sigma..sub.B.sup.2 is the variance of the 
number of nodes in a batch initiating an arbitrary transmission subcycle. 
By assuming that the batch size of nodes contending for transmission 
simultaneously in the last transmission subcycle Type I (of an arbitrary 
transmission cycle) is unity, the expressions for E[B] and 
.sigma..sub.B.sup.2 are derived respectively as 
##EQU24## 
where X' is the total number of nodes in the active-wait state at the end 
of a busy period Type I under the assumption that the batch size of nodes 
contending for transmission simultaneously in the last transmission 
subcycle Type I is unity. 
To derive the expressions for E[B] and .sigma..sub.B.sup.2, successive 
transmission cycles in FIG. 8 can be considered as an alternating renewal 
process which describes the controller as being in either of two states: 
State I (represented by the block of contiguous number of transmission 
subcycles Type I) and State II (represented by the transmission subcycle 
Type II). Then, from renewal theory, the long run proportion of time that 
the controller is in State I is given by 
##EQU25## 
where E[T.sub.I ] is the mean time that the controller is in State I and 
E[T.sub.II ] is the mean time that the controller is in State II. 
Similarly, the long run proportion of time that the controller is in State 
II is expressed by 
##EQU26## 
E[T.sub.1 ] is related to E[.psi..sub.I ], the mean number of batches 
initiating the busy period Type I. Similarly, E[T.sub.II ] is related to 
E[.psi..sub.II ], the mean number of batches initiating the busy period 
Type II. (19.1) and (19.2) can then be rewritten as 
##EQU27## 
Clearly, E[.psi..sub.I ]=C.sub.I, the mean number of transmission 
subcycles Type I before a transmission subcycle Type II. However, 
E[.psi..sub.II ] is not easily determined. For .sigma.&lt;&lt;1, it is 
reasonable to assume that the batch size of nodes contending for 
transmission simultaneously in the last transmission subcycle Type I (in 
an arbitrary transmission cycle) is unite, then under this assumption the 
mean batch size is unity and E[.psi..sub.II ]=E[X'], the mean number of 
packets which are in the active-wait state at the end of the last 
transmission subcycle Type I of an arbitrary transmission cycle. 
The mean batch size initiating an arbitrary busy period can then be written 
as 
##EQU28## 
which proves (18). 
From the definition of variance, .sigma..sub.B.sup.2 is expressed by 
##EQU29## 
which proves (19). 
C.sub.I is given by (14) and the expression for E[X'] is obtained from (9) 
with X replaced by X' and the conditional probability 
Pr[X'=j.vertline.N=k] redefined to take into consideration the unity batch 
size assumption. Then we have 
##EQU30## 
for j=1,2,3,. . . 
The mean packet delay, D, (in slots) of an arbitrary packet is then given 
by 
EQU D=D.sub.w +T' (21) 
Instead of calculating the throughput {S.sub.H }, the mean branch size (E 
[B]) and the variance of the batch size (.sigma..sub.B.sup.2) using the 
above results which are obtained on the basis of the stated heuristic, we 
observe that these parameters could also be obtained from a strict 
modeling of the hub by an M/D/1 queue with batch arrivals where the batch 
size has a binomial distribution with parameters m (the number of nodes in 
the network) and v (the packet generation made by a node). From the busy 
node idle period analysis of an M/D/1 queue with batch arrivals, it can be 
shown that the hub utilization (throughout) is given by 
EQU S.sub.H =V.sub.B E[B] (22) 
which must be less than unity for stability v.sub.E is the mean batch 
arrival rate (in batches per node transmission time) and for a binomially 
distributed batch size E [B] and .sigma..sub.B.sup.2 are given 
respectively by 
EQU E[B]=Mv (23) 
EQU and 
EQU .sigma..sub.B.sup.2 =Mv(1-v) (24) 
The numerical and simulation results presented below are based on a system 
with the following chosen parameters: maximum node-controller-node 
distance is 4 km and this gives a maximum propagation delay of 20 .mu.sec 
(1 slot). Two packet sizes are considered, a packet size, L, of 250 bytes 
and L=2500 bytes. Note that the 250 byte-packet and the 2500 byte-packet 
correspond to 2 and 20 slots respectively for a transmission rate, B, of 
50 Mbits/sec on each optical fiber link. Similarly, the chosen packet 
lengths correspond to 1 and 10 slots for a transmission rate of 100 
Mbits/sec. Two system populations--a 4-node system and a 10-node system 
are considered and for each system the system throughput and the mean 
packet delay are determined using the analytic expressions derived in the 
previous section. In addition, the accuracy of the analytic results is 
assessed by discrete-event simulations. 
FIG. 9 and FIG. 10 depict the throughput-input traffic characteristics for 
the 4-node system and the 10-node system respectively. In the range of 
packet arrival rates considered, there is a linear relationship between 
the throughput and the packet arrival rate. Note the excellent agreement 
between the analytic and simulation results at low values of packet 
arrival rate and the slight deviation at high values of packet arrival 
rate. The agreement obtained validates the heuristic that was employed in 
the throughput analysis. Three points are noteworthy from FIG. 9 and FIG. 
10. First, for a fixed transmission rate, an increase in packet length (in 
bytes or in slots) implies an increase in system throughput. Second, if 
the packet length is fixed, an increase in transmission rate does not 
necessarily imply an increase in system throughput. To achieve a high 
system throughput at a high transmission rate, the packet size must be 
increased accordingly. The third and final point is concerned with the 
relationship between the increase in packet length system throughput. From 
the specifications marked on the curves in FIG. 9 and FIG. 10, the packet 
length are found to be 1, 2, 10 and 20. Thus, it is seen that the system 
throughput improves as the packet length (in slots) increases. In FIG. 11, 
the throughput-input traffic characteristics for the two system sizes 
considered are compared and it is observed that the 10-node system 
exhibits higher throughput than the 4-node system for the range of packet 
arrival rates considered. 
The mean packet delay (in slots) versus input traffic per node (packet 
arrival rate) are shown in FIG. 12 and FIG. 13 for the 4-node system and 
the 10-node system respectively. It is observed from these figures that 
the mean packet delay is constant (equal to the node transmission time) at 
low values of the packet arrival rate but increases at high packet arrival 
rate values. Note the very good agreement between the analytic and 
simulation results in the low traffic region but the agreement differs 
with increasing input traffic. The following additional observations are 
evident from FIG. 12 and FIG. 13. For a fixed transmission rate and at a 
given packet arrival rate, an increase in packet size corresponds to an 
increase in the mean packet delay because of the increase in the 
transmission time of a packet. From the discussion given above on FIG. 9 
and FIG. 10 under identical condition of fixed transmission rate, the 
system throughput increases with an increase in packet size. Thus, a 
trade-off exists between the throughput and the mean packet delay. The 
question is how large the packet size can be (given that the transmission 
rate is fixed) so as to have high system throughput and simultaneously 
prevent intolerable mean packet delay. It is further observed from FIG. 12 
and FIG. 13 that if the packet size is fixed, an increase in the 
transmission rate implies smaller mean packet delay because packet 
transmission time decreases with higher transmission rate. This result is 
desirable but an undesirable effect (throughput degradation) has already 
been noted above from FIG. 9 and FIG. 10 under the same condition of fixed 
packet size and increasing transmission rate. We again see the existence 
of a trade-off between the throughput and mean packet delay. In this case, 
we are concerned with how high the transmission rate can be so as to 
reduce the mean packet delay and at the same time prevent system 
throughput degradation. From the specifications on the curves in FIG. 12 
and FIG. 13, it is further observed that the mean packet delay increases 
with increasing packet length in slots. In FIG. 14, it is seen that for a 
constant transmission rate, the mean packet delay is the same for the 
4-node and 10-node systems at low values of packet arrival rate. However, 
at high packet arrival rates, the packets transmitted on the 10-node 
system experience larger average delay than those transmitted on the 
4-node system and this implies increasing packet queuing delay. 
It is appropriate at this point to explain why there is less agreement 
between the analytic and simulation results in the high packet arrival 
rate region of FIG. 12 to FIG. 14. Recall that in the formulation of the 
analytic model, it was assumed that the system is lightly loaded 
(Assumption A.sub.6). A consequence of this assumption is that most of the 
transmission subcycles in a transmission cycle are subcycle Type I which 
are independent and identically distributed. The occurrence of a 
transmission subcycle Type II after several transmission subcycles Type I 
is then modeled by a geometric distribution. When the system is heavily 
loaded, most of the transmission subcycles in a transmission cycle are 
subcycle Type II. The main problem in this scenario is that the contiguous 
subcycles Type II are not independent. The dependence arises from the fact 
that the number of nodes contending for transmission at the beginning of a 
transmission subcycle Type II is a function of the number of nodes which 
initiated transmission at the beginning of the preceding transmission 
subcycle Type II. Hence, the occurrence of a transmission subcycle Type I 
after several transmission subcycles Type II cannot be modeled by a 
geometric distribution. In FIG. 9 to FIG. 14, we intentionally extend the 
plots to the heavy traffic region so as to investigate how this region can 
be approximated by the independence assumption. The results reveal the 
validity of the analytic approach which is exemplified by the very good 
agreement between the analytic and simulation results especially at low 
values of packet arrival rate. However, in the region of high packet 
arrival rates, the analytic results are more pessimistic. 
FIG. 15 shows a comparison between the mean packet waiting time calculated 
using equations (16), (18) and (19) in equation (17) and that computed 
using equations (22) to (24) in equation (17). It is shown that the 
difference between the simulation and analytic results in FIG. 12 to FIG. 
14 are not attributed to the heuristic employed in the derivation of 
equations (16), (18) and (19). 
In FIG. 16 we compare throughput-delay characteristics of an embodiment of 
inventive protocol with those of the known HUBNET and the CSMA-CD which 
are the previously proposed protocols for active star-configured FOLAN's. 
The nonpersistent variant of the CSMA-CD protocol which incorporates the 
delayed first transmission procedure is considered. In the delayed first 
transmission procedure, a new packet and a backlogged packet (a packet 
which has sensed the controller busy or a packet that has been involved in 
collision(s) with other packet(s)) will sense the controller with the same 
sensing probability, .tau.. The choice of the delayed first transmission 
procedure implies that a new packet incurs a geometrically distributed 
delay with a mean of 1/.tau. slots and also the retransmission delay of a 
backlogged packet is geometrically distributed with the same mean of 
slots. FIG. 16 shows the improvement in throughput-delay performance of 
the collision-avoidance protocols (HUBNET and STARMAP) over the 
collision-prone protocol (CSMA-CD). In particular, for the chosen set of 
input parameters, the embodiment of the inventive protocol displays an 
improved performance over the HUBNET protocol especially at high values of 
throughput. 
Thus, a throughput-delay analysis of active star-configured fiber-optic 
local area networks incorporating an embodiment of the inventive protocol 
has been presented. The analysis was based on an embedded discrete-time 
queuing process and the expressions derived for the system throughput and 
the mean packet delay. Numerical results were obtained using the derived 
analytic results and the validity of the analytic results has been 
confirmed by computer simulations. The numerical examples considered 
reveal the trade-off between the system throughput and the mean packet 
delay. The numerical examples also provide valuable insights on the 
performance of active star-configured fiber-optic local area networks. 
The novelty of the disclosed embodiments of the inventive protocol arises 
from particular features which are not present in the other protocols so 
far developed for active star-configured FOLAN's. As discussed above, the 
features include, but are not limited to, the following: 
(i) There are no packet retransmissions as all packets are transmitted only 
once. This feature leads to a significant improvement in network 
performance especially under heavy traffic. 
(ii) Given that the hub traffic intensity is less than unity, the packet 
access delay (time interval from the instant a packet is generated to when 
the packet is broadcast by the central hub) is bounded and the upper bound 
is dependent upon the total number of nodes and hubs connected to that 
hub. 
(iii) A reasonable degree of fairness is exercised--though in the case of 
simultaneous multiarrivals at the hub, it is possible that one node may 
always have its packet transmitted before the packets of other contending 
nodes and this may be thought to imply unfairness. However, there is still 
a reasonable degree of fairness because each contender is guaranteed a 
time frame within which its packet will be transmitted. This is not so 
with the existing collision-avoidance protocol which arbitrarily selects 
one of the contenders and ignores the rest. 
(iv) A network having selective, local global and local selective broadcast 
capabilities can be employed. 
(v) A priority packet service scheme can be utilized to ensure efficient 
handling of data within the network. 
(vi) The intelligence at the hub can be put to a greater use to improve 
network performance. For example, if the receive section of a node is 
faulty and cannot receive a packet, the hub can transmit a fault-alert 
packet to the other nodes, hence none of the other nodes will send packets 
to the faulty node. 
It will be appreciated by those of ordinary skill in the art that the 
present invention can be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. For 
example, although the invention has been discussed above as it might 
advantageously be embodied in a fiber optic network, it will be apparent 
to one of ordinary skill in the art that it could also be embodied in an 
electrical system. The presently disclosed embodiments are therefore to be 
considered in all respects illustrative and not restrictive. The scope of 
the invention is indicated by the appended claims rather than the 
foregoing description, and all changes that come within the meaning and 
range or equivalents thereof are intended to be embraced therein.