A TYPICAL local a k a network (LAN) today has the following characteristics:

Transmits bits serially rather than in parallel. Employs clock rates of 1-20 mill ion bls. Is relatively noise free compared to analog voice communications lines: error rates of 1 bit in 1 billion are typical, while the comparable error rate for analog voice might be 1 bit in 10000. Switches packets or frames of bits (messages consisting of one'or more packets) rather than, as in conventional circuit switching, holding bandwidth for the duration of a communications session.

. .

Has a geographic extent of 1-10 km at most. . .

Can have a wide variety of devices attached (for example, sensors, thermostats, security alarms, process control devices, low-speed data terminals, voice, facsimile, computer high-speed I/O, or video) offering multiple services.

We focus here mainly on bus LAN's where the trans- mission medium is used for both control and data transmission. This is a topic that has been treated in a great many papers over the past decade. 'Our intent is to briefly survey and summarize this work. A bus LAN is a 'type of feedback communications channel, where all stations can sense only the channel state and their own state and use this information to arbitrate transmission medium access via distributed access methods, such as with Ethernet'. There is no central repository of state information: all the devices together control access to the network, as opposed to widespread central access methods, such as those found in private branch exchanges (PBX's) where a single controller arbitrates contention. In the interest of brevity, we will drop the discussion of PBX systems and ring LAN's. (These topics are included in the bibliography at the end of the paper.)

A key design consideration in LAN's is that for typical applications with packet lengths of 500-10000 bits; at most; 1 packet will be in transit at any time, in contrast to long-haul terrestrial or space satellite networks where more than 1 packet (perhaps 10-100 packets) may be in transit at any time. The LAN is a serially reusable resource; since the transmission medium is used for control as welt as data transmission, the design intent is to make the control arbitration time small compared to the data transmission time.

Figure.1 shows an illustrative hardware block diagram of a data communications system, consisting of two com- puters-each with a secondary storage device (moving head disk), a group of terminals, and,a shared printer. All devices are interconnected to each ottier via an LAN; a network interface unit handles protocol conversion from each specific type of device to .the LAN protocol. The LAN is a component in building a .distributed communications system. The, local area network allows

files to be shared among the two computer systems, the printer to be shared among the users; and ' . any terminal to access any computer.

'Ethernet is a trademark of the Xerox Corporation. Portions of this material have appeared previously in Data

Communications. vol. 12, no. 1, pp. 107-120, 1983; and in Computer. vol. 16, no. 5, pp. 72-76, 1983.

Since a common multiplexed channel is employed by an LAN, port congestion for physical ports to each computer system can be reduced by setting up multiple logical channels or virtual circuits through one physical port, interconnecting the terminals, computers, and printers.

Model

Logically, any LAN appears to be a set of stations connected to a common hub or pincushion, as shown in .Fig. 2. This can considerably simplify routing, naming, and addressing issues compared with larger, . more complex networks. Here we confine our attention to bus or broadcast medium LAN's, where N stations are geographically separated, with transmission attempts by any one station received by all stations, as shown in Fig. 3. Each station has internal state information, and can monitor the state of the bus. Based on internal state information plus bus state information, each station moves from state to state. There is no central repository of state information: access to the bus is controlled by this distributed arbitration policy.

The bus can transmit C b l s . Some bits are used for controlling bus access, while others are used for station data transmission. The offered load is measured in b/s. Station K generates messages at a rate AK, and each station has a

mean number of bits per message: The offered load of the bus is the sum of the product of the message generation rate and the mean number of bits per message for each station.

N bus offered load = (message rate)K (mean number of

K= 1 bits/message)K

In order that no messages are lost, the offered load must be less than the bus transmission rate:

N offered load = (message rate)K (bits/message)K

K= 1 < c b l s

Granted that the offered load is less than the bus trans- mission rate, the next question is message delay: What mix of message rates leads to what message delay statistics? Message delay is defined as the time from a message's arrival at a station until the end of successful transmission. An average delay might be of interest in some applications such as office automation, while a maximum delay might be of interest in other applications such as robotic manufac- turing. Associated with delay is the concept of latency: At any given instant of time, what is the maximum duration of a time interval until a given station can next begin successful transmission? For some applications, such as in discrete process control and manufacturing, i t is mandatory .that the latency be finite, while for others, such as office automation, this is much less important. Our intent is to make each of these concepts.more precise in the rest of this section, and then turn to an illustrative example quantifying different performance criteria.

Offered Load

Offered load is specified by two ingredients: First is the mean message arrival rate, AK, K = I , . . . , N messages per second; second is the time to transmit a message. We assume that the stations can achieve synchronization over a time interval called a slot, denoted Ts, so that a slot can be measured as CTs bit transmission times, and we call this number of bits a packet. The number of packets per mes- sage is assumed to be a. random variable denoted by PK, K = 1,. . . , N; with the mean denoted by E (PKj. The total offered load is given'by

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N offered load.= AK E(PK)CT~ < C

K= 1

which we assume from this point on to be less than the total bus transmission rate. Stations update their state informa- .tion at the very.end of a time slot. Although messages arrive at arbitrary points in time, transmission attempts only begin at the start of a time slot.

Transmission Medium Access Arbitration

What are desirable properties of a given policy for arbi-

Delay, measured from when a packet is initially ready to be transmitted until it is successfully transmitted, should be acceptable under light load. The transmission medium should be efficiently utilized under heavy load. For a given workload, delay should be insensitive to how the workload is generated among the stations, since in practice this will not be known in fact with any precision.

trating transmission medium access?

Capacity is defined as the largest total workload (message arrival rate mix) in which delay goals are all .met.

Bus States-What are the states'of the bus?

Idle-No transceiver is active, and all transceivers recognize this within a slot time. This can be done implicit1y;via having a clock time out at each station, or explicitly, by one station transmitting a unique bit pattern signifying idle.

Transmit-Exactly one transceiver is actively trans- mitting a packet successfully. Two substates of this state will be needed later on:

Nohfinal packet transmission-Transmitting a packet that is not the last packet in a. network interface unit queue. This might be implemented by setting one particular control bit to zero in such a ' packet. Final packet transmission-Transmitting the final packet in a network interface unit queue. This might be implemented by setting one'particular control bit to one in such a packet.

Collide-Two or more transceivers are s'imultaneously active attempting to transmit, causing interference or a collision between. transmission attempts, and all messages involved .in the collision must be retransmitted later. All stations involved in the collision could broad- cast a 'unique bit pattern that all stations on the network receive, signifying a collision. This means that there is feedback over the channel.

In fact, the state of the bus is more.complicated than what we have described here, because of the spatial separation of stations: one station can be sending, but before the energy reaches a second.stat.ion, the first station thinks the bus is busy in the transmit state and the second station thinks the bus is idle. The approximation adopted here is to go from a disfributed state, specifying the bus state at each point on the bus at each .instant of time, to a global bus state.

Token Passing Transmission Medium Access-This policy polls stations on the bus via distributed control. Control is passed either explicitly via a unique bit pattern called a token from one station to the next, or implicitly via a trailing bit pattern on a successful message or via a timeout. The station with the token is the only one allowed to transmit. In a well-engineered system, collisions are rare events (initiali- zation of the system takes an acceptably short amount of time; loss of a token or multiple tokens being transmitted and so forth are detected and acted upon within an acceptably short time interval), and for purposes of traffic or congestion analysis are ignored from this point on. Figure 4 shows the iilustrative operation of a token-passing or distributed- polling system; stations 0, 1, 4, and 7 have messages to transmit. Station 4 has a two-packet message to transmit, while the other stations have a one-packet message to transmit. In this scenario, station 0 transmits, then passes the token (transmits a unique bit pattern implicit1.y at the end of its packet transmission) to station 1, station 1 transmits and passes control to station 2, and so forth.

Control is passed from station to station as in a.logica1 ring; this allows separation or dedication of transmission capacity to different types of services, by simply assigning a given number of visits per polling cycle to each service. For example, if a work station offered both 9.6-kbls data service and 64-kb1s voice service, the voice port could be visited 8 times (we have rounded upward from the smallest integer greater than 6419.6 to get 8) as often as the data port; that is, we are dedicating.8 times the transmission capacity to voice as to data. ,The latency can. be upper bounded by the time waiting to pass the token to th.e other stations and allow each of them to transmit a given maximum number of packets. Finally, suppose the total workload '(number of packets) is fixed, and we vary the number of devices and the- amount each device transmits. Effectively, we are fixing the fraction of time the transmission medium is busy trans- mitting data-the bus utilization. As the number of stations is increased so that the amount of data per message per station decreases toward zero, the mean waiting time .or delay experienced by.any station in transmitting a message will increase above any fixed threshold, because more and more time will be spent in passing the control token from one station to another rather than in transmitting data. This access method can be quite sensitive to the details of the transmission load. Utilization of the LAN transmission capacity may be low, but delay can be unacceptable.

The transient behavior of distributed-control token passing is very complicated. Thi.s is because a variety of failure modes must be checked (more than one token, no token, and so forth). On the other hand, central control polling is a proven access method, in wide use for over 25 years in applications as diverse as multidrop data communications terminal networks, discrete manufacturing process control, and electronic switching systems handling telephone call processing.. This suggests that the same experience learned there wil l prove valuable in making the relatively unproven distributed token passing access method a very reliable technique.

Carrier-Sense . Collision-Oetection Transmission Medium Ac- cess-Roughly speaking, all stations sense the transmission .medium to see if carrier energy is present; if no carrier is sensed, a station will attempt to seize the transmission medium and transmit its message; if a carrier is sensed, a

AUgUSt 1984-VOl. 22, NO. 8 IEEE Communications Magazine 48

station will defer its attempt to a later time. If two or more stations attempt to seize the transmission medium within a given time interval, a collision is said to occur, and this must be detected by all stations involved and the next attempt deferred to a later time. The mechanis,m for determining the retry time intervals involves spreading retransmission at- tempts out farther and farther in time, in an attempt to more efficiently utilize the'bus while increasing packet delay.

Figure 5 shows the illustrative operation of a carrier-sense collision-detection system; stations 0, 1, 4, and .7 have a message to transmit. Just as in the token-passing polling case, station 4 has a two-packet message, while all other stations have a one-packet message.

Almost immediate media access is possible under light traffic. Given that N stations attempt to transmit in the next time slot, using an unrealizable policy of assuming that each station knows that there are N stations involved in the initial collision, and no more stations attempt to transmit until one station is. successful, it can be shown that there are roughly e - 1 = 1.718 . . . collisions for every successful message transmission. This has not been validated by controlled experimentation on actual networks. On the other hand, virtually nothing can be said about packet delay under load. The access method employs positive feedback: In the event of a collision, no packet is successfully transmitted, and the longer this condition persists, the more likely that new stations will enter the fray, making even more collisions highly likely. In fact, packet delay can be infinite, that is, the access method abandons transmission attemFts if it is

unsuccessful 16 times, and higher-order protocols must be invoked to attempt transmission all over again.

Suppose we fix the.bus utilization, the mean number of packets per unit time that are being successfully transmitted. How sensitive is carrier-sense collision detection to the number of devices generating.messages, and to the load generated by each device? If the total offered load (bits per unit time) is fixed but the number of stations is increased, each station will be sending shorter and shorter packets, there will be more transmission attempts, more and more collisions will take place to successfully transmit one message, and hence the mean packet latency and message waiting' time will increase above any threshold. Utilization is not a complete. measure of the loading on a carrier-sense collision-detection LAN, and more information must be given about the number of stations and what each station is doing in order to say anything concerning delay.

What about mixing voice and data with such an access policy? It is certain'that voice can be transmitted digitally with acceptable quality; this has been done commercially for over two decades. The question is how much voice traffic such an access policy can handle. Again, there are no publicly available and independently'reproducible results to date, but there is a body of evidence that suggests the following scenario is quite plausible. We will assume voice will require 64 kb/s of transmission capacity for the duration of a voice telephone call, say 100 s. With a lO-Mb/s transmission speed, we can have at most the following number of simultaneous voice teleohone'calls:

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maximum number of simultaneous- 10 Mb/s ; 156

For virtually any scenario proposed to date involving digital voice and low-speed data transmission, the total number of bits transmitted will be predominantly voice: check this yourself. People typically make three t o six telephone calls per peak business work hour, lasting two to three minutes each on the average, which generates a lot of voice bits relative to the amount of data bits each person might generate.

If we have much less than this maximum number of simultaneous voice telephone calls in progress, then it may be possible for the data messages to be interleaved with voice packets. If we have much more than this number of simultaneous voice telephone calls in progress, because we have many more voice packets than data packets, the’voice packets will be much more likely to be successfully transmitted, since the nature of the access method gives preference to those stations with the greatest offered load. Voice packets will swamp the transmission capacity, and d ta packets will be locked out from using the LAN.

%uppose we have much less than the maximum number of sihultaneous voice telephone calls in progress, say 100 simultaneous voice telephone calls during a peak business work hour. Rarely, say 1% of the time, wil l there be a surge or fluctuation about this mean value, and,we will have all the available transmission capacity occupied with voice ,packets. There are 3600 seconds in 1 hour, so 1% of the time,

voice telephone calls 64 kb /s - or 36 s out of the hour, the LAN will be completely busy with

voice. As far as data is concerned, the LAN will have failed, that is, there is no transmission capacity for data. Further- more, the duration of these surges of voice will be 10 s here, 5 s there-fairly unpredictable throughout a business hour.

Remember that there are higher-level protocols for flow control with timeouts: these additional control. mechanisms were not designed for handling voice and data (only data), and as far as data is concerned, the LAN has failed at this point. The question is not will this occur but rather how often will this occur. In order to answer, we clearly need to be more specific about the voice and data services, but the phenomenon we have just described must be present: the time scale for voice is tens of seconds, while the time scale for data is tens of milliseconds, and hence a short fluctuation in voice load looks like it lasts for a very long time in the world of data.

At the present time, our understanding of carrier-sense collision-detection access is relatively incomplete compared with that for token-passing access. We understand how one station loads the system and how an infinite number of stations load the system, but we have only a limited idea of any intermediate case (such as two stations), either in terms of simulations or analysis or data (best of all), for one service or a mix of services.

The transient behavior of carrier-sense collision detection is in many ways its’ strength: a wide variety of failures and faults are handled simply and elegantly. The technology is newer than central control polling, but it has been proven

August 1984-VOl. 22, NO. 8 IEEE Communications Magazine 50

over a decade in the laboratory and now in the field:-for what it was intended to do, it is an excellent engineering' design.

Decision Tree Transmission Medium-Access Arbitration-With this access method, a station will successfully transmit its message immediately under light loading, and it will adaptively control the number of stations involved in transmission attempts under heavy loading. The mechanism for determining retry time intervals involves thinning the number of stations that can participate at each round in a controlled manner, in an attempt to have at most one station in a group transmit at any one time.

Figure 6 shows the illustrative operation of a decision tree access policy, where the station 'addresses are used to determine retry priorities for stations involved in message collisions. Station 4 has a two-packet message to'transmit, while stations 0, 1, and 7 each have one-packet messages to transmit.

An illustrative scenario begins at the start of a frame (not to be confused with a packet), when all stations that have a packet to transmit go .active and collide. The retry policy involves a controlled priority arbitration among all stations active at the start of the fram,e, with the station .addresses used to determine or control the retry priorities2 After the first collision, only those stations whose m'ost' significant address bit is a zero can retry, and all other stations are silent: stations 0 and 1 will attempt to,transmit in the next time slot, and stations 4 and 7 wil l defer transmission

2This retry policy is called a topological binary tree sort; the.nodes of the tree are searched depth first, and then left to right. Station retry priorities determine the graph.

attempts. Once more, stations 0 and 1 transmkduring the same time slot, and now all stations whose first two leading, address bits are zero will attempt to transmit in the, next slot, and all other stations will defer. Stations 0 and 1 will attempt to transmit, while stations 4 and:7 will defer transmission attempts. Once more, stations' 0 and 1 transmission attempts collide in the next time slot, and now only those stations with three leading address bits (all zero) will attempt to transmit in the next time slot. Only station 0 will retry, and'stations 1, 4, and 7 wil l defer. In the next time slot, station 0 succeeds in transmitting. Now we allow all stations whose leading address bits are two zeroes followed by a one to transmit in the next time slot: station '1 will transmit then, with stations 4 and 7 deferring. Now all stations with a leading zero and one address bit are allowed to transmit in the next time slot; this would,be stations 2 and 3, neither of which has a message, and. hence the time slot is an idle time slot. All stations whose address begins with a 0 have been allowed to. transmit and have successfully transmitted a message, if they had one at the start of the frame. Now we repeat the process: al l stat,ions with a leading address, bit of one, stations 4 and 7, attempt to transmit in the next timeslot, and their attempts collide. In retrying, all stations with a leading address bit sequence of one followed by zero, which is only station 4, attempt to transmit in the next time slot: station 4 is successful. Finally, all stations with leading address bits of one followed by one, which is only station 7, attempt to transmit in the next time slot: station 7 is successful. Figure 6 is a graphical summary of this process. During a frame, at the start of any time slot other than the first in a frame, there are four states a station can be in: 1) attempting to transmit; 2) already successful in transmit-

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ting its message if it had one at the start of the frame; 3) deferring transmission attempts due to a collision earlier in the frame; or 4) deferring transmission attempts because it was not ready at the start of the frame.

This looks like carrier-sense collision detection under light load, because if only one station has a message, it will transmit it immediately. This looks like token passing or polling under heavy load, because if every station has a message, we will have seven collisions among eight stations, while in token passing the token is passed seven times among eight stations. The key observation here is that we wish to interrogate groups of stations once a collision occurs, with at most one station in each group having a message. By controlling the retry process, and by expanding the state information used to determine which stations retry (which is not done. with conventional carrier-sense collision .detection), operation can be made quite predictable and regular.

Here are highlights of decision tree access:

Transceivers are logically organized into a tree, with the leaves of the tree determining the urgency of message transmission. After a collision, all stations involved successfully transmit before any subsequent arrivals to other stations are transmitted.

If each station can transmit at most one packet at a time, the following results are typical of what is known about decision tree arbitration:

Using station addresses to arbitrate contention, the mean time required to the first successful packet transmission is at most the logarithm of the number of stations (here for eight ready stations we needed three collisions to the first successful transmission) and can be less. For example, if only stations 0 and 4 had one packet to transmit, the time to the first successful packet transmission is one collision. The maximum time required to resolve collisions per transmission during a frame is bounded. The example shows that for eight stations, at most seven collisions occur. The mean number of packets either waiting or in transmission at all stations is linearly proportional to the number of active stations: each station active at the start of each frame is guaranteed of being able to successfully transmit its message within each frame, and there are at most eight stations. In other words, there is roughly one control overhead time slot interval for every one successful message transmission, just as in central control. The variance of the mean number of packets either waiting to be sent or in transmission at all stations grows linearly with the number of stations attempting to transmit at the start of a frame, just as in central control. (This point is quite subtle; the interested reader is referred to the bibliography for further exposition.)

In the example, an additional degree of freedom is available: at what priority level do we attempt to start our retry arbitration? Should we use the most significant address bit of each station, the two most significant address bits of each station, or so forth? One way to quantify our intuition on this is to examine the number of steps required (idle,

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collision, and transmission) as a function of the number of bits used for retry arbitration. This is summarized in Table I.

Hence, for this example, the optimum number of bits to use for starting the retry arbitration would be two. Under light traff.ic, the optimum level to start is zero bits, just as in carrier-sense collision detection. Under heavy traffic, the optimum level to start is three bits, just as in polling. Under light load, no control is needed on the number of retry bits. As load builds, we should start to use more and more station address bits in the priority arbitration retry process, because it is more and more likely that stations are active.

One key feature of decision tree access is its transient behavior: when a station is added or turned off, or fails in a wide variety of ways, the access policy, since it is carrier- sense collision detection, is quite robust to these types 0.f failures, and will recover within a finite time interval (at most one frame transmission time). The key difference between carrier-sense collision-detection access and deci- sion-tree access is the retry policy: Each station using decision tree access must keep track of two parameters in counters, with one counter keeping track of the total tree frame (for synchronization of the start and end of frames), and the other keeping track of the subtree currently involved in collision resolution. In comparison, carrier-sense collision detection requires a random number generator plus a counter, so the complexity of the two approaches on the surface may be comparable (at this level of analysis). It can be shown that decision-tree access enjoys many of the same features of carrier-sense collision-detection access in terms of how station initialization, adding or removing a station, or a variety of failure modes are handled. This would unduly lengthen our treatment, and in a well-designed system these are rare conditions compared to the normal operations we have just dealt with.

How sensitive is this access method if the workload is fixed but the amount of traffic generated by each device varies, as does the number of devices? It can be shown that, provided the total mean arrival rate of packets stays below a threshold, called the capacity, the mean packet delay is finite, independent of the number of devices and the length of packet, .whi le if the total mean arrival rate exceeds this threshold as the number of stations is increased, the mean packet delay exceeds any finite threshold. For this access method, the capacity is given by

1 N €(PI TS 2N- 1 capacity = - -

The term €(PITS is the mean message transmission time. As N - m, this approaches one half the transmission capacity.

TABLE I Number of Bits for Initial Retry Priority

Number Number of Bits for Initial Retry Priority of Steps 0 1 2 3

~ ~ ~ ~ ~~~ ~~ ~ ~~ ~~~~

Idle 1 1 1 4 Collision 4 3 1 0 Transmit 4 4 4 4

Total 9 8 6 8

The mean packet delay cannot be exactly analyzed at the present time, but a great deal of work has yielded very tight upper and lower bounds on packet delay; put differently, we can understand how this access method behaves under any load.

Station Transmission Policy

Once a station gains access to the transmission medium, there are still many possible avenues for transmitting messages. Once a station is allowed to transmit, it can:

Transmit all packets exhaustively, including those that arrived after transmission began, until no packets are waiting to be sent. In other words, the transmission medium can handle the offered packet transmission load, that is, the total mean number of packets waiting to be transmitted or in transmission is finite, provided that the total mean packet arrival rate is less than the maximum rate of transmitting a packet-if and only if

N CAK E(PK) < - On the other hand, packet delay

K= 1 can be unacceptable because one station is exhaustive- ly transmitting, locking out other stations from transmitting. Transmit only those packets present at the start of transmission. This means slamming a gate down to prevent any packets that arrived after transmission began from being transmitted. In other words, the transmission medium can handle the offered packet transmission load, just as in the nongated policy. Transmit only up to a maximum number of packets per access. For example, suppose that all the packets were transmitted by one station, and token-passing trans- mission medium access was used. If a maximum of one packet could be transmitted on each access, then, effectively, the transmitting station has only l /N th of the effective transmission capacity. In other words, the total mean number of packets waiting to be transmitted or in transmission is finite, provided that the total mean packet arrival rate is less than 1lNth the maximum rate of transmitting a packet, that is, if and only if

1 Ts '

N

K= 1 AK EfPK) < N . On the other hand, with token-

passing or decision-tree media access, each station is guaranteed some access within a definite time interval, although the transmission medium cannot be completely utilized. This is not possible with carrier-sense collision- detection media access.

Closing Comment

Transmission medium access delay is only one factor that must be addressed in total communications system per- formance. Figure 7 is a queuing network block diagram of the illustrative system described earlier. Transmit and receive queues hold packets in the computers and network interface units. Delay can be incurred at each of these stages, as well as in the LAN. In fact, many current products incur the bulk of the packet delay in the computers and network interface units, and not in the high-bandwidth LAN. A variety of references on communications-systems engi-

neering are cited at the end of the paper for interested readers.

Comparisons At the present time, it is an open question as to what

packet delay can be tolerated to allow sufficient margin for other factors, such as network interface packetizing and buffering, that have been ignored in this analysis.

Qualitative Comparisons

Our first type of comparison is qualitative. The offered load is fixed at any point less than the bus transmission capacity.

Can there be some loss in offered load for a fixed mean packet delay? In other words, under what conditions, for a fixed mean packet delay, can the bus not handle the offered load? Can packet latency be bounded for a fixed offered load?

The answers to these questions are summarized in Table II for each of the three transmission medium access methods (token passing, carrier-sense collision detection (or CSCD), and decision tree with station address priority arbitration) and station transmission policies (exhaustive transmission per medium access, gated transmission per medium access, and a limited number of packet transmissions per medium access).

Exhaustive or gated station access allows complete utilization of the bus, that is, the offered load can always be carried, but at the expense of unacceptable latency. Limited- service station access bounds latency at the expense of not being able to carry the full offered load. Finally, carrier- sense collision detection offers no control on latency, unlike

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TABLE I1 OUALITATIVE PERFORMANCE COMPARISON SIJMMARV

Medium . Station No Loss with Finite Bounded Access Access Mean Packet Delay? Latency?

Token Token Token CSCD CSCD CSCD Tree Tree Tree

Exhaustive Gated

Limited Service Exhaustive

Gated Limited Service

Exhaustive Gated

Limited Service

Yes Yes No Yes Yes No Yes Yes No

No No Yes No No No No No Yes -

token passing and decision-tree medium access. This table makes evident that decision-tree transmission medium access is in many ways like token passing.

Delay Analysis for Two Stations on a Bus

Here is some insight gained from the special case of a bus with two stations.

Problem Statemenl-The interarrival times between mes- sages at each station are independent, identically distri- buted, exponential random variables with mean interarrival time l / ~ k , K = 1, 2 to station K = 1, 2. The total message arrival rate is the sum of the individual station arrival rates, denoted by A :

A = A 1 + A2

Each message consists of a fixed number of packets, P, with each packet requiring one time slot to be transmitted. The bus is assumed to have a common time clock with period equal to one time slot known io all stations. The access method assumpt ions a re as follows:

Token-passing access-A token requires one time slot to be transmitted from one station to another. Once a station is ready to transmit, it waits for the token, and exhaustively transmits all messages. Once the other station detects the end of transmission of the last packet in the network interface queue, i t can begin transmission of any packets. Carrier-sense collision-detection access-If a station is ready to transmit, it first determines whether the transmission medium is busy with transmission of a nonfinal packet message transmission, and if it is not, it begins to attempt transmission in the next time slot; if i t is busy, it waits until the next time slot and repeats this process. Once a station begins to attempt transmission, i t determines at the end of the time slot whether the attempt was successful or not; if successful, it ex- haustively transmits all messages; if not successful, it increments its collision counter (CC) by one and computes a retry time interval. The retry time interval e'quals the slot time multiplied by a random number that is equally likely to be zero through 2 C C ' - 1 inclusive. Decision-tree access-If a station is ready to transmit, it first determines if the transmission medium is at the end of a frame; if not, it waits until the end of a frame; if so, it begins to attempt transmission in the next time slot.

Once a station begins to attempt transm,ission, it determines at the end of the time slot whether the attempt was successful or not. If successful, it ex- haustively transmits all messages; if not successful, it computes a retry time interval. The retry time interval is based upon the station address: station 0 will retry in the next time slot, station 1 will defer one time slot before attempting to transmit, and begin exhaustively transmitting once station 0 is done exhaustively trans- mitting packets. Once station 1 finishes transmission, station 0 can immediately begin to transmit any ready packets.

Mean Delay Analysis-The mean message delay can be written as the sum of two terms, one due to the access method and one due to the delay for a single, serially reusable resource or queue, labeled intrinsic delay, which is independent of the access method and given by

The random variable P denotes the numberof packets per message. The first term in the intrinsic delay is the mean waiting 'time that would be incurred with zero control overhead, waiting for messages that arrived earlier to be transmitted in order of arrival. The second term in the intrinsic delay is due to message transmission starting at th: beginning of a slot, and since a message can arrive at any point during a slot, on the average one half a slot of mean delay per message is incurred. The final term in the intrinsic delay is the mean message transmission time. The other portion of the mean message delay depends upon the access method and is called the excess delay.

We present numerical calculations in graphical form for two cases: allowing all the traffic to be generated by one of the two stations, the unbalanced case; and allowing the traffic to be equally generated by both stations, the balanced case. Figure 8 shows the total mean packet delay for the case where all messages require one time slot to be transmitted successfully. Figure 8 suggests that there is virtually no difference between message delay for any access method, under balanced or unbalanced loading, once the bus utilization exceeds 50%. Only under light loading is there any difference. Remember that under light loading all access methods must work acceptably, otherwise they would never be used.

The excess delay is shown in Fig. 9 for equal or balanced loading ( A I = ~ 2 ) and totally unbalanced loading (A2 = O), for each of the three access methods. Figure 9 shows that the excess mean delay due to the access method is at most one half of a time slot, for token passing, and drops for token passing as the load increases. For carrier-sense collision-detection, the access method excess mean packet delay grows with load more rapidly than for decision tree, but does not exceed that for token passing. This example is a graphic.case study in analysis: spending a great amount of time determining delays due to transmission media access methods simply may not be fruitful, because the delay due to the access method may be negligible compared to the intrinsic packet delay any sort of rough sizing analysis would uncover.

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Closing Comment-The best available evidence today for

Token passing via a bus has the greatest delay under light load, and under heavy load cannot carry as much traffic as a ring and is sensitive to the bus length (through the propagation time for energy to traverse the bus). Like the ring, its traffic-handling characteristics are sensitive to the precise nature of the workload, only more so. Carrier-sense collision detection offers short delay under light load, while its performance can be unacceptable under heavy load. Its performance is sensitive to the bus length (the shorter the bus the better it performs), to message length (the longer the packet the better it does), and to the precise nature of the workload. Decision-tree arbitration is the least popular and least understood of all access methods. On the other hand, it may enjoy the best traffic-handling characteristics of any bus access method. Its performance is least sensitive to the precise nature of the workload. This access method offers short delay under light load and efficient utilization plus predictable delay under heavy load.

bus LAN’s is:

Numerous caveats and disclaimers are in order: This evidence is currently being examined by those involved in actually building LAN’s. No experimental data is available to independently confirm these claims and plots. It is hoped that others will study this and subject it to independent test. The two-station exact delay analysis strongly suggests that the bus access method may not be as important as other factors, but how does this generalize to .more realistic situations (such as more than two stations, multiple services)?

Furthermore, all of this is confined to bus LAN’s, and has ignored PBX configurations and ring LAN’s, which have their own pluses and minuses but were dropped in the interest of brevity.

In fact, congestion analysis has had little actual impact on LAN engineering. However, it may have a greater impact in the marketplace, where paying customers want to know how to manage, administer, and maintain their systems, as well as how many devices of what type can be interconnected before congestion through the network becomes unac- ceptable. A common theme heard again and again is that it is probably better to deploy these systems and see how people really wish to use them, gathering and analyzing traffic measurements, learning of actual installation, operations, and service problems firsthand, before doing anything else.

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August 1984-VOI. 22, NO. 8 IEEE Communications Magazine

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B. W. Stuck received a doctorate in Electrical Engineering from M.I.T. in 1972. He joined Bell Laboratories in 1972 and worked on a variety of computer and communications network systems, encom- passing network management and administration, ESS overload control, voice and data PBX engineering, loop repair service bureau, and local area networks. He is chairman of the Traffic Handling Characteristics Technical Advisory Group of IEEE Project 802 Local Area Network Standards. In 1984 he joined VIATEL to develop computer systems to support telecommunications operations, for operating telephone companies, regional telephone holding com- panies, OCC’s, and major businesses.

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