Access protocols for an optical-fibre ring network Phil Davies and Faisal Abdul Ghani discuss access protocols for a

fibre-optic network

Optical fibres can be used to provide a high-speed channel with good noise immunity for use in the local area network. The design of a high-speed LAN that uses optical fibre as the communications Channel and operates at line rates of 100 Mbit/s and above is discussed. A comparative study of four ring access protocols, namely, the token-ring, the slotted-ring, the fixed-length register insertion ring and the carrier- sense multiple access ring is presented. Their perfor- mances are compared and an appropriate protocol chosen for implementation.

Keywords: fibre optics, Ioca ~. area network, access protocols, performance

Current local networks (LANs) operate at up to 10 Mbit/s and have been implemented with a variety of transmission media such as twisted wi,e pair, coaxial cable and the radio channel. However, as the LAN becomes increasingly popular and the demand for channel bandwidth increases, the use of optical fibres as the transmission medium of the LAN is likely to increase. The optical fibre offers high bandwidth and good noise immunity and link lengths typically encountered in a I_AN can easily be implemented on optical fibre without the need for a repeater.

The design of a high-speed LAN that uses optical fibre as the communications channel and operates at line rates of 100 Mbit/s and above is considered. As link lengths are likely to be less than 1 km, the upper limit to the channel bandwidth is set by the speed of source devices rather than by fibre bandwidth. The

current upper limit for economically-priced LEDs is around 100 Mbit/s although higher rates are possible with the use of laser diodes.

NETWORK TOPOLOGY

A selection of topologies that might be appropriate 1'2 has been considered and it has been found that the star and the ring networks are most suitable candidates for an optical-fibre I_AN. The ring network is regarded as superior in that a major problem of all optical-fibre networks, namely optical-fibre coupler performance, has been removed because only point-to-point links are required. The optical-fibre ring can be economic in terms of line costs and is attractive in that routing problems are minimized and access protocols can be kept reasonably simple.

The major disadvantage suffered by the ring net- work is in the network reliability, as the failure of one link or node results in the failure of the entire network. Reliability can be improved, however, by using a double loop structure or by the addition of redundant links ~'4.

CONTROL POLICY

Electronics Laboratories, University of Kent at Canterbury, Kent CT2 7NT, UK

0140--3664/83/040185--07503.00 © 1983 Butterworth & Co. (Publishers) Ltd.

In any communications network involving many users, there is a requirement for control of the network. This control operates at various levels, but two particularly important areas can be identified. First, the access of the users must be regulated in some way. This involves the institution of an access protocol that may require the use of a central controller or may allow control to be distributed among the network users. Secondly, the integrity of the network and the overall performance needs to be monitored, and again this may be

vol 6 no 4 august 1983 185

performed by centralized or distr ibuted control. As a general rule, centralized control offers easier imple- mentation of access and reliabil i ty procedures but results in a network that is vulnerable to the failure of the central controller. It is assumed here that distr ibuted control is preferable for an optical-f ibre LAN because it is undesirable that a network should be vulnerable to the failure of any single node.

ACCESS PROTOCOLS FOR OPTICAL-FIBRE RING NETWORKS

Ring networks have evolved access protocols peculiar to the ring topology and four such protocols have been compared. Three of these access protocols have been analysed elsewhere s-8, but no comparative study of all four has been made. In particular, the previous analyses have been l imited to lower bit-rates and with different traffic characteristics. The motivation for performing the analyses was the desire to examine the comparative performance of these protocols at high bit-rates with similar assumptions in each case.

In the analyses, Poisson-generated traffic is assumed and the t ime delay for a packet that is presented to the network is evaluated. As shown in Figure 1, the overall t ime delay, which is called the mean transfer delay,?~, has four components, namely the queueing delay, Tq, the access delay, ta, the service time, t-,, and the propagation delay, tp. Hence

Other parameters used in the analyses are as follows: N is the number of nodes; m is the mean message length in bits; C is the line-rate in bit/s; A is the delay per node in bits; dl is the propagation delay between i and (i + 1)th node in bits; T.d# is the loop propagation delay in bits; D is the total loop delay in bits; X is the mean message generation rate per node in message/s;

Data in

Buffer

Access control I I'

Line

Data out

q

Figure 1. Model used for q_ueueing delay; ta =access tp = propagation delay

protocol analyses; -tq= delay; ~ = service time;

h is the message overheard in bits; n is the number of slots on slotted ring; k is the number of data bits per slot on slotted ring or number of data bits per register in register insertion ring.

The queueing delay and the service t ime are the same for each of the four protocols analysed and the general value for these parameters is therefore cal- culated before considering each protocol in detail.

To find the mean queueing delay, tq, the classical M / M/1 queue is considered. It is well known that the buffer occupancy for such a system is given by p'/(1 - p') where p' = ,Vn/c.

For the ring, each queue is served by an effective line-rate of C/N bit/s as N users are mult iplexed onto a single channel of line-rate C bit/s. In the application under consideration., the buffer occupancy therefore becomes (NXm/C)/(1 -NtVn/C) .

(N2v'n/C) is the ratio of throughput to line capacity which is defined as traffic intensity, p. Hence,

Buffer o c c u p a n c y - 1 - - p

(2)

where (3)

Nt~n p -

C

Thus the queueing delay tq is given by

p -- (m + h) bit (4)

1 - p

The service t ime is the t ime taken to load the packet onto the line and is given by

ts = (m + h) bit (5)

Token ring

A single token system is considered. The token circulates in one direction around the ring and each station may transmit a variable length message when in possession of the token and must pass on the token to the next station on complet ion of transmission. A copy of the packet is taken at the receiving station, and the returned packet is removed from the ring by the original transmitt ing station.

Consider the mean scan t ime 5. This is the average t ime (in bits) for the token to travel completely around the loop and is given by

= ~.di + N A + x (6)

where

x = N X service t ime per node

= N X number of messages generated per scan per node X message size

k~ = N X - - X (m + h )

C

T-x/i+ N A ~= NX

1 - - - (m -I- h) (7) C

186 computer communicat ions

Now, the access delay, ta, is the average t ime that a packet has to wait before the node acquires the token. Therefore

T ~ = - (8) 2

Combining equations (3), (7) and (8) one obtains

1 ~. ,d i4 -NA

2 (m + h ) 1 - p - -

m

(9)

and it is assumed that the mean propagation delay is the t ime required for the packet to travel halfway around the loop, so

1 t-P = 2 (Xd, + N Z~) (10)

Finally, using equations (1), (4), (5), (9) and (10) one can obtain

p 1 ~d i + N A t f - - - - - - ( m + h ) + + ( m + h )

1 - p 2 m + h 1 p

1 m + -- (%d~ + N A)b i t

2 (11)

Slotted ring

In this technique, a fixed number of contiguous slots circulate around the ring. Free slots are fi l led by transmitters which have packets readyto send. Packets are removed by the receiver and the free slot is passed on to the next station.

To simplify the analysis, an integral number of slots are assumed on the r ingand the node separation is also assumed to be an integral number of slots. Slot f i l l ing and emptying is considered under equi l ibr ium con- ditions.

Letn be the number of slots on the ring and letp be the number of full slots. Then for each node, the probabi l i ty that an incoming slot is full isp/n. As there are N stations on the ring, the probabi l i ty that the data in the slot is destined for that station is (p/nN). There are, however, N stations on the ring and hence the number of slots emptied per slot period is N(p/(nN)) =p /n . As an equi l ibr ium condit ion is assumed, this must also be the number of slots loaded per slot period.

Ring throughput

number of packets deleted per slot period × number of data bits per slot

slot period

P - - k n

(k + h)/C (12)

In an equi l ibr ium situation, the total input (or output) traffic must equal the ring throughput, (N,~n), so

P - - k n

NXm = (13) (k + h)/C

Therefore

pk p - - - (14)

n(k -t-h)

In order to find the access delay, ta, the t ime required to wait for an empty slot to arrive at the given station must first be calculated.

On average, a node will wait for half of a slot period, i.e. (k + h)/2 before the beginning of a slot arrives at that station, and slot synchronization is achieved. The probabi l i ty that a station will have to wai tx slot periods before acquiring an empty slot is (p/n) X ( 1 - p / n ) . Hence, the total time, w, wait ing for channel access for a single slot is

w = + - (k + h) r r 2 r=,

k + h - - - - k ( k + h ) [ p/n. ] (15)

2 [1 - p/n j

Consider now a message of length m which is divided into m/k packets. The first packet will need to wait for slot synchronization and it and all subsequent packets will have to wait for empty slots. Hence, the total access delay before the last packet of the complete message can begin to be loaded onto the line is

-{a k + h m [ o/n 1 - + - - (k + h) (16)

2 k L1 -p/nJ The propagation delay is the t ime required for the packet to travel, on average, half way around the ring and is given by

~.,di -F N A t-p (17)

2

Substituting equations (4), (5), (16) and (17) into equa- t ion (1) produces

m ( k + h ) [ 1 ] i f - p(m +h)_F 1 1 - p k [ k + h \

1 - p~/ / k /

(k + h) ,~c/i + Nz~ + + (m + h) + bit (18)

2 2

Fixe&length register insertion ring

The register insertion technique was originally suggested by Hafner 1° and a more sophisticated version was implemented by Reames and Liu 11. The principle of operation is that a node may access the line whenever the line is free. If data that is to be repeated arrives whi le the station is still transmitting, this data is delayed in a buffer. In order to investigate a system that could be implemented with currently

vol 6 no 4 august 1983 187

available logic, it was decided to examine f ixed-length register insertion with deletion of the packet on return to the transmitter.

To analyse this system, let p be the number of packets currently on the ring and NR be the number of registers currently inserted. By analogy with the slotted ring, it is seen that the number of packets, x, (analogous to the number of slots) deleted per packet t ime is the number of packets currently on the ring (analogous to the number of full slots) div ided by the ring size, in packets (analogous to the total number of slots). Hence

p(k + h) x = ( 1 9 )

NR(k + h) + ,T-,di + (N - NR) /k

Note that the number of registers inserted must be equal to the number of packets on the ring, i.e.

NR = p (20)

The traffic intensity, p, is equal to the number of packets deleted per packet t ime scaled by the proport ion of data contained in each packet. Hence

p(k + h) k p = N R ( k + h ) + X d ~ + ( N - N R ) / k ( k + h ) (21)

To simplify the analysis,/k is assumed to equal 1 bit (22) and using equations (20) and (21) and solving for NR one obtains

p(T-,di + N) NR--

k - p(k + h -- 1) (23)

The access delay has a maximum value of one packet size as a register can be inserted into the line immediately after a packet has passed. Hence, the average access delay wil l vary between zero for zero throughput and (k +h ) /2 for maximum throughput. The maximum traffic intensity that can be achieved when the ring is full is p = k/(k + h) as the throughput is l imited by the overhead h. Thus, the average access delay is

(k +h ) (k +h ) W - - - - - p (24)

2 k

If the message length is m bit, the number of packets requiring access wil l be talk. The first packet wil l suffer an access delay as given by equation (24), but each of the subsequent packets must wait one complete loop delay before the register is available. Note that it is assumed that the register remains in the line and is unloaded and reloaded within a 1 bit period. The total access delay is given by

k P + -- 1 D (25)

Now the total loop delay

D = 7__..dj + NR(k + h) + (N - NR) A (26)

Using this equation with equations (22) and (23) one obtains

(T--,c/i + N)k D (27)

k - - p(k + h - 1)

Hence, from equation (25) it is found that

~ = ( k + h ) 2 [ m l ( ~ - M , + N ) k ~k p + ~ --1 k - p(k + h - 1 ) (28)

and the propagation time, tp, is given by

D 1 (Z~I i + N)k tp . . . .

2 2 k - p ( k + h - 1 ) (29)

Substituting equations (4), (5), (28) and (29) into equation (1), the transfer delay is given by

t, P (m+h ) - t (k+h)2 ( k ) ( Z ' d i + N ) k = - - P + - 1 1 - p 2k k - p ( k + h - 1 )

1 (,T.di + N)k + (m + h) + - bit

2 k - p ( k + h - 1 ) (30)

Carrier sense multiple-access ring

Carrier sense" multiple-access with col l ision-detection (CSMA-CD) is an efficient random access protocol that has been used on broadcast channels. It has the important advantage of requiring no central controller. The ways in which CSMA-CD might be implemented on a ring network are now considered.

Initially, the node monitors the line and if no packet is passing, it may begin transmission. If a packet arrives during transmission, then this constitutes a collision, and there are three courses of action open, as il lustrated in Figure 2.

Node

Incoming - - ~ C % F Line Pocket currently pocket ~ ~ being transmitted

Store and transmit

a later

Node

Incoming ( ' ~ Line Packet currently packet ~x,~,,.~//"~ =' being transmitted

b Dump

Node

Line

Incoming \ / Pocket currently pocket = ~ • being transmitted

/

C Abort transmission and repeat incoming pocket

Figure 2. Possible actions on packet collision for the carrier sense ring

188 computer communicat ions

• The node could cont inue transmission and store the in-coming packet for transmission at the end of the current transmission.

• The incoming packet could be dumped. This is the technique adopted by Clark 12'13 in his 'content ion ring'. This procedure results in the destruction of both packets if the length of the packet is greater than half the loop propagation delay.

• The transmission could be aborted. This is the tech- nique suggested by Tobagi TM for broadcast channels and implemented on Ethernet by Metcalf and Boggs is, and also implemented by West TM.

For this comparative study, it was decided to analyse the second technique because the first technique is effectively register insertion and the third technique is diff icult to implement at the high data-rates con- cerned. It was assumed in the analyses that retrans- mission occurs after a random time delay defined by the binary exponential backoff algorithm used in the Ethernet network ~s. Variable packet size was also assumed.

The analysis is begun by defining the ring traffic intensity, g, as the offered traffic plus the retrans- mission traffic, in which case

p g = ( 3 1 )

1 - - r

where r is the probabi l i ty of collision for a packet. Now consider a packet, B, wait ing to access the line.

The probabil i ty, p, of successful access is the prob- abil i ty of f inding an empty bit on the line. Therefore

P p = l --g =1 - - - - - (32)

1 - - r

If access is denied, the packet must wait 1 bit before attempting access again. Hence, the mean t ime wait ing for access, W, is

W = 0 p + lp(1 - -p) + 2p(1 _ p ) 2 + . . . . bit (33)

Hence

p -- - - bit (34)

1 - - r - - p

For successful transmission, packet B must reach its destination wi thout suffering a collision. Consider a packet A which is at a station on the route of packet B and is wait ing for transmission. The probabi l i ty that packet A successfully accesses the line wi thout suffering a collision with a packet on the line is the probabi l i ty that a space equal to the packet length occurs on the line. I f the traffic on the line is assumed to be Poisson-distributed then this probabi l i ty is

p(o) = e N~'.,/C = e-gin (35)

where X '= X/(1 - r ) is the traffic generation-rate at each station. Hence the probabi l i ty that packet A suffers a collision during the t ime that it is being clocked onto the line is

r = 1 - - e - N 2 ¢ r n / c (36)

The probabi l i ty that this coll ision occurs with packet B is r / N but packet B, which travels completely around

the loop, passes ( N - 1) stations and hence the probabi l i ty that packet B suffers a coll ision on the round trip is ~ r , if it is assumed that N > > 1.

Substituting from (31) in (36) gives

r = l - e x p - C ---

= 1 - exp (37)

If a collision occurs, retransmission is required. The total access delay is taken as the delay before access of a packet that does not suffer a collision. Then

-E~ = d ] + d2 r + d3 r 3 + . . . . dn r n (38)

where dn is the delay associated with the nth attempt at access and is given by

I: 1 n = D + ( n - - 1 ) M + W , n > 1 (39)

where M is the mean value of binary backoff algorithm. From equations (34), (38) and (39) is obtained

L = + D~ - - p - - r (1 - - r - - p )

- 1 + ( l - - r ) 1 r

(40)

The propagation delay, t-p, is

Zdi + N A t o = D / 2 - (41 )

2

As packets are removed by the transmitter, the station delay, A, is assumed to be 1 bit and using this, together with equations (4), (5), (40) and (41) in (1), gives the overall transfer delay

t t ~ - -[- - - "t- ~_..,d i + N + 1 = p 1 - - p - - r 1 - - p - - r

+ - - - 1 + m + h + - - b i t 1 r 1 r 2

(42)

C o m p a r i s o n o f r i n g a c c e s s p r o t o c o l s

Curves of mean transfer delay, tf, against traffic intensity, p, are shown in Figures 3-6. Curves are given for the number of users, (N), = 50; packet lengths, (m), of 1 000 bit and 64 bit; and line rates, (C), of 10 Mbi t /s and 100 Mbi t /s for a 1 km ring. The mean value of binary backoff, M, for the CSMA ring is 1 000 bit.

The token system has a low delay over a wide range of traffic intensity and for all values of packet length and line rate. This is because the token system allows variable length packets to be transmitted and hence incurs no extra overhead for longer packets. In addit ion, the transfer t ime is not greatly affected by line-rate variation.

The slot system chosen does not perform quite as well as the token system because longer packets must be divided into many fixed length slots resulting in a greater overhead.

vol 6 no 4 august 1983 189

I00 000

50 0 0 0 4 0 0 0 0 30 OOO

20 OOO

I0 0 0 0

5 000 4 0 0 0 3 000

,~ 2 0 0 0

1 0 0 0

50O o 400

300

2 0 0

I 00

50 40 30

20

Slot

CSMA Token

Register insertion

10 I I I 1 I I I I 0.0 0.1 0.2 0.3 0 .4 0.5 0.6 0.7 0 .8 0.9

Troff ic intensity, (p)

Figure 3. Transfer delay as a function of traffic intensity for a ? km ring with message length, m, of 64 bit and line rate, C, of 10 Mbit/s

The register insertion system performs well when the packet length is equal to the register size but performance deteriorates when the packet size is much greater than the register size owing to the necessity for long packets to be divided into many smaller regular-size packets. Access for the first of these is rapid but access for subsequent packets is delayed until the previous small packet has completed a full loop and returns to the transmitter where it may be replaced with a new packet. The conclusion to be drawn from this result is that the register should be as large as possible. In addit ion, performance could be improved by al lowing removal of packets at the receiver. This would obviate the necessity of wait ing for the return of previous packets before accessing the line and should result in much faster access.

The carrier sense ring performs well for low traffic levels for all line rates and packet sizes. The reason for this good performance is that variable length packets are used, producing minimal overhead for long packets; in addit ion, access is rapid when very few packets are on the line. Performance deteriorates rapidly with increasing traffic, due mainly to packet collisions producing retransmission traffic.

Consider the selection of one of these protocols for the f ibre-optic ring. The four protocols analysed may be divided into two classes, namely ful ly distr ibuted and part ial ly-distr ibuted control. The token and slot systems have the advantage of distr ibuted access but require a central control ler to generate and monitor the token or slot and hence possess partially distri- buted control. Register insertion and carrier sense are ful ly distr ibuted systems in that no central control ler is required. However, it must also be borne in mind that

implementat ion of any of these protocols at 100 Mbit /s and above requires hardware implementat ion and hardware complexi ty wil l therefore be an important factor in selecting an appropriate network.

It is clear from Figures 3-6 that if a central control ler can be tolerated then the token network offers a superior performance. If it is insisted that the control must be completely distr ibuted to opt imize reliability, then register insertion or CSMA should be chosen. The implementat ion of register insertion is by far the simplest of these two protocols and for this reason it has been selected for a prototype 1 O0 Mbi t /s optical- fibre ring. The ring is currently under construction at the University of Kent, and work carried out so far suggests that the access protocol does indeed result in a relatively straightforward implementat ion. The performance of the completed system will be reported at a later date.

CONCLUSIONS The design of a high-speed optical-f ibre computer communicat ions system has been considered from first principles. The currently used topologies have been examined and it has been suggested that the ring or star structure are the current best choices. A range of common ring access protocols has been investigated and the results show that the token ring offers the best performance over a wide range of packet sizes and line rates; if ful ly-distr ibuted control is required, however, together with simple hardware, then the fixed-length register insertion technique can provide a feasible alternative provided that mean packet sizes are not large in comparison with register size.

50 0 0 0 ~ 40 OO0 Slot

insertion

v

J=4

i@ g.z

g

SMA

40( 30(

20(

10(

5° I 4 0 30

20

10 0.0

Figure 4. for a 1 km ring with message length, m, of 1 000 bit and line rate, C, of 10 Mbit/s

I I I I I I I I 0.1 0 2 0.$ 0 4 0.5 0 6 0 7 0 8 0.9

Traffic intensity, (p)

Transfer delay as a function of traffic intensity

190 computer communications

I00 0 0 0

50 000 40 000 30000

20000

I 0 000

5 000 4 000 5 000

= 2 000

'~ 100(:3

500 "~ 400 c o 30C

g 2oc ~u

I0(

50 4C

50

2C

CSMA / / / S / i o l nserti°n

I 0 I I I t I I I I 0.0 O. I 0 .2 0.5 0.4 O. 5 0 .6 0.7 0.8 0.9

Traffic intensity, (p)

Figure 5. Transfer delay as a function of traffic intensity for a 1 km ring with message length, m, of 64 bit and line rate, C, of 100 Mbit/s

I00000

50 000 40000 30 000

20000

I0 000

5 000 4 000 3 000

2 0 0 0 35

'~ I 000

500 ,~ 400 8 30o

2O0

~ I00

50 40 30 IO

/ / / i R n : g e rSffi:rn

0 ~ ) . 0 I I I I I I I I 0.1 02 03 0.4 0.5 0.6 0.7 08 0.9

Traffic intensity,(p)

Figure 6. Transfer delay as a function of traffic intensity for a 1 km ring with message length, m of 1 000 bit and line rate, C, of 100 Mbit/s

A C K N O W L E D G EM E NTS

British Telecom is gratefully acknowledged for its financial support of research into optical-fibre data networks at the University of Kent. F Abdul Ghani acknowledges the support of the University of Malaya, Kuala Lumpur, Malaysia.

REFERENCES

1 Davies, P A, Adbul Ghani, F and Pung, H K'Optical fibres in Ioacl area networks' Proc. Internepcon UK '82 Brighton, U K (12-14 October 1982) pp 141-146

2 Davies, P A and Abdul Ghani, F'Optical-fibre com- puter communication networks' Proc. Melecon '83 Athens, Greece (24-26 May 1983) pp A1. 10-11

3 Zafiropulo, P'Reliability optimization in multiloop communication networks' IEEE Trans. Commun. Vol 21 No 8 (August 1973) pp 898-907

4 Zafiropulo, P'Performance evaluation of reliability improvement techniques for single loop commun- cation systems' IEEE Trans. Commun. Vol 22 No 6 (June 1974) pp 742-751

5 Kaye, A R'Analysis of a distributed control loop for data transmission' Proc. Symp. Comput. Networks and Teletraffic Polytechnic Institute of Brooklyn (April 1972) pp 47-58

6 Bux, W 'Local area subnetworks: a performance comparison' IEEE Trans. Commun. Vol 29 No 10 (October 1981) pp 1465-1473

7 Liu, MT 'Traffic analysis of the distributed loop computer network (DLCN)' Proc. NTC (1977) pp 31:5-1-7

8 Hayes, J F and Sherman, D N 'Traffic analysis of a ring-switched data transmission system' BSTJ Vol 50 No 9 (November 1971) pp 2947-2978

9 Kleinrock, I_ Queueing systems (Vol 1) John Wiley, USA (1975)

10 Hafner, E Ret a! 'A digital loop communication system' IEEE Trans. Commun. Vol 22 (1974) pp 877-881

11 Reames, C Cand Liu, M T'A loop network for simul- taneous transmission of variable length messages' Proc. Int. Comput. Symp. Taipei, Taiwan (August 1975) pp 7-12

12 Clark, D D eta l 'An introduction to local area networks' Proc. IEEE Vol 66 (November 1978) pp 1497-1517

13 Clark, D D and Svoboda, L 'Design of distributed systems supporting local autonomy' IEEE Compcon (Spring 1980) p 438

14 Tobagi, FA Random access techniques for data transmission over packet switched radio networks PhD thesis, Computer Science Dept. UCLA, USA (1974)

15 Metcalf, R M and Boggs, D R 'Ethernet: distributed packet switching for local computer networks' Commun. ACM Vol 19 No 7 (July 1976) pp 395- 404

16 West, A and Davison, A CNET-- a cheap network for distributed computing Dept. of Computer Science, QMC London, UK Report TR120 (March 1978)

vol 6 no 4 august 1983 191