- Email: [email protected]
Hybrinet: A hybrid bus and token ring network
215
Hybrinet: A Hybrid Bus and Token Ring Network Oliver C. Ibe a n d Pin-Yee C h e n School of Information and Computer Science, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
We propose a hybrid local area network, called the Hybrinet, which combines all the good qualities of the CSMA/CD network and the token ring network. The performance of this network is shown to be superior to that of the token ring network for all offered loads where delay is finite. Furthermore, over the range of offered loads where the CSMA/CD network performs better than the token ring network, it is shown that the Hybrinet's performance is very close to that of the CSMA/CD network,
Keywords: Local area networks, protocols, performance evaluation, network design
..... ~
!
Oliver C. Ibe receivedthe B.Sc. degree in Electrical Engineering from the University of Nigeria, Nigeria in 1975,
the M.S. degree in Electrical Engineering and Computer Science from the Massachusetts Institute of Technol.... ogy, Cambridge, Massachusetts in : 1979, the M.B.A. degree from Northi eastern University, Boston, Massachusetts in 1980, and the Sc.D. degree in Electrical Engineering from the Massachusetts Institute of Technology in 1981. From 1981 to 1983 he held a post-doctoral research position at the IBM Thomas J. Watson Research Center, Yorktown Heights, New York, working on performance analysis of computer systems and local area networks. He is currently an Assistance Professor of Information and Computer Science at the Georgia Institute of Technology, Atlanta, Georgia. His research interests include modeling and analysis of computer systems, computer networks, and local area networks. Dr. lbe is a member of the IEEE, ACM and Sigma Xi.
North-Holland Computer Networks and ISDN Systems 9 (1985) 215-221
1. Introduction In a recent paper, Bux [1] presented an analytical study comparing the performance of different types of local area networks. The study shows in particular t h a t the C S M A / C D network [2] has a better delay performance than the token ring n e t work [2] at low offered loads. However, as the load increases the C S M A / C D network experiences a n increased frequency of collisions which gives rise to unbounded delay. The token ring network, o n the other hand, continues to produce bounded delay long after the delay of the C S M A / C D network has become unbounded. Therefore, it has better delay performance than the C S M A / C D network at high offered loads. The reason why the C S M A / C D network has a better delay performance than the token ring network at low offered loads is because a packet is transmitted almost as soon as it is generated in the C S M A / C D network. A station does not have to wait for a free token to transmit its packet as in the token ring network. Moreover, since a packet suffers at least one-bit delay at each station in the token ring network, the expected dely increases with the number of stations in the network. The one-bit delay does not exist in the C S M A / C D network, thus making transmission to be faster in the C S M A / C D network at low offered loads. ~
l~n.Yee O~en received the Ph.D. de. gree in Computer Science from the University of Illinois at UrbanaChampaign, Urbana, IL, in 1981. He is presently an Assistant Professor in the School of Information and Computer Science, Georgia Institute of Technology, Atlanta, Georgia. Pre' viously, he was a visiting Assistant Professor of Computer Science De~ partment at the University of Illinois at Urbana-Champaign, working on various issues of multiprocessor design; from 1982 to 1983, he was with IBM Data System Division, Poughkeepsie, New York, working on system architectures for very large gcientific and engineering processors. His current research interests include parallel and distributed computer architecture, VLSI architectures and algorithms, modeling and analysis of multiprocessors, computer networks, and local area networks. Dr. Chert is a member of the IEEE and ACM.
0376-5075/85/$3.30 © 1985, Elsevier Science Publishers B.V. (North-Holland)
I~ ~
216
O.C. Ibe, P.-Y. Chen / Hybrinet
The results obtained in [1] for the C S M A / C D and token ring networks thus indicate that if delay performance is the primary criterion for evaluating the two networks, then there is no clear winner since the two networks are complementary to each other. That is, one network is a good candidate for those applications where the other performs poorly, and vice versa. In this paper, we consider a hybrid local area network, called the Hybrinet [3], which combines the token passing protocol of the token ring network and the fast transmission feature of the C S M A / C D network. Preliminary results obtained for this network indicate that its delay performance is very close to that of the C S M A / C D network over the range of the offered loads in which the latter performs better than the token ring network. The results also indicate that the network has a better delay performance than the token ring network for all offered loads where the latter has a finite delay, The rest of this paper is organized as follows. In Section II, we give the description of the Hybrinet. The performance analysis of the network is discussed in Section Ill. Finally in Section IV we present some concluding remarks,
2. Network Description The Hybrinet consists of two cables, one for access control and the other for data transmission, The data transmission cable is a folded unidirectional bus which is divided into two portions: an outbound portion to which the transmitter of each station is connected, and an inbound portion to which the receiver of each station is connected [ 4 ] . Assume that the stations are numbered 1, 2 , . . . , N so that station 1 is the first station on the outbound portion and station N is the first station on the inbound portion. The physical configuration of the Hybrinet is shown in Fig. 1. The access control cable, which connects all stations as in a token ring network with the exception that it does not physically connect station N back to station 1, permits the flow of token from station 1 down to station N. The network operates as follows. Assume that station k has just received the token, 1 < k < N 1. Then if the station has no packet to transmit, it passes the token to station k + 1 along the access control cable. If station k has a packet to trans-
INBOUND BUS ~ ,
~ ~
A,',~ESS
COntROL
l~.~..o . . ~
'
~
CABLE
~ OCTBOU~D BC'S Terminator Fig. 1. Configuration of the Hybrinet.
mit, it transmits the packet on the outbound portion of the data bus. After transmitting the last bit of its packet, it then waits for a time period /3 before passing the token to station k + 1. The choice of/3 depends on how the network is to be operated. For example, in a single-packet operation only one packet is allowed in the network at any time. In this case,/3 will be chosen so that the token is passed to the next station after the packet has cleared all stations on the inbound portion of the data bus. However, due to the topological asymmetry of the network, this operation requires different/3 's for different stations. If one value of /3 is to be chosen for this operation, then we must have/3 = T~a, where T,j is the time required for a packet transmitted by station i to be received by station j, 1 < 0' < N. While such a single value of fl simplifies the analysis and the operation of the network, fl = T~ imposes unnecessary delay on packets since the time required for the last bit to travel from the transmitter of station k, k > 1, to the end of the inbound bus is less than TH. Thus there are periods during which the network is not used while stations may have packets to transmit. Therefore in this study, /3 is chosen so that 0 < fl < TH, and/3 = T~ is the worstcase operation. When the token reaches station N and the station either has no packet to transmit or has completed its transmission as described above, it passes the token to station 1 by broadcasting it on the data bus as a regular packet. All stations hear this broadcast which serves to indicate the beginning of a new transmission cycle. However, only station 1 can accept the token, and a new cycle begins.
O.C. lbe, P.-Y. Chen / Hybrinet
3. Performance Analysis of the Hybrinet In this section, we compare the performance of the Hybrinet and that of the token ring network and the C S M A / C D network. We make the following assumptions for the Hybrinet and the token ring network: (a) There are N stations that are equally spaced, (b) Messages are generated at each station at a Poisson rate of ?~/N packets/second, (c) Packet lengths are exponentially distributed with mean L bits. (d) Because of assumption (a), the walk times w, are identical for the token ring network and for 1 < i < N - 1 in the Hybrinet, where the walk time w~ is the sum of the transmission time and the propagation delay of the token from station i to station i + 1. The measure of the network performance is the mean transfer time, as used in [1]. The mean transfer time is the time interval from the time a packet is generated until it is received by the destination station. The expression for the mean transfer time, T/, is [1]:
ation of the token ring network are considered. For the Hybrinet, we consider the worst-case operation, where /3 = ~, as well as multiple-packet modes of operation with/3 = 0, ~/4, and ~/2. We compare the Hybrinet with/3 = T against both the single-packet token ring network and the C S M A / C D network. The expression for the mean transfer time for the C S M A / C D network is given in [1]. We also compare the Hybrinet with 0
"~(1 - p/U) + ,~
p E [ X 2]
T/=2(1-p)E[XI
217
+E[X]q
2(1-0/
2'
where p = h E [ X] and r = the round-trip delay for the token ring network, and the end-to-end propagation delay for the Hybrinet. For the token ring network, ~- includes the one-bit delay at each station. X has different interpretations for different operations: (1) For the "multiple-token" operation o f the token ring network [1], X is the time to transmit a packet. Here a free token is issued as soon as packet transmission ends. (2) For the "single-packet" operation of the token ring network [1], X is the sum of the time to transmit a packet and the ring round-trip delay r. (3) For the Hybrinet, X is the sum of the time to transmit a packet and/3. We consider a network of 50 stations which generate equal amounts of traffic, by assumption (b). The data bus is assumed to be 2 kilometers long with a propagation velocity of 2 × 10 8 meters/sec. Three different cable capacities are considered: 1 M b / s , 5 M b / s , and 10 M b / s . Both the single-packet and multiple-token modes of oper-
CD
2 km Cable Length 50 Stations
E i.s.
Exponentially
9. ~-
/
Distributed
PacketlLength (mean 1000 bits)
~_~ 8-bit token ~ ° Token Ring,single-pocket c~ "" Hybrinet, fl=l" I--• CSMA/CD r r/ o ,' 2g 5. ," ~ ~ ,'/O .N_ , ~ / / o E o 7 _ ~ ~ . 1. =-","-~U/~,,~i,,,,~,,,,~ ,,,i ,,,h .... ~.... i,,, 0.0 0.2 0.4 O.fi 0.8
°
,j
J
Offered Load,
p
Fig. 2. Delay vs Offered Load Characteristics, 1 M b / s .
.0
218
O.C.
(9
2 km Cable Length
~
E
50 Stations
H
Exponentially Distributed eacketlLength (mean 1000 bits) B - b i t token Token Ring, multiple-token • Hybrinet, /~=0
'-I~ 9. ~(9 CO C (3
lbe, P.-Y. Chen/ Hybrinet
(9
2 km Cable Length
50 Stations
j ~ / / ]~ ]
i-~ 9. L_ (1) CO E (3
=~5.
~5.
-0 (9
-~ (9
._
._~
© Z
O Z
0.0
0.2
0.4
0.6
0.8
Offered Load,
1.0
-
2 km Cable Length 50 Stations
(9
L(9 ~-- 9. CO E E~ I---L
I
N
5. /
E
/
0.2
0.4
Offered
f
/ /
/t
(9
/'
~
N "--
III
/~
o L 0 Z
/
0.0
~' /~
-o5.
/
0 Z
,o
~l /~
• Hybrinet, fl=T
~
~
"'-O
.0
,,W
(mean 1000 bits) B - b i t token • Token Ring, single-packet • CSMA/CD
E £] (9
I t
t (9
0.8
2 km Cable Length 50 Stations Exponentially Distributed Packet Length
E ~
I I I
E D (9
0.6
''''lH''f''''l''''l~'''l'~'l''''l','~l*'''
Exponentially Distributed I Packet Length I (mean 1000 bits) I 8 - b i t token I • Token Ring, single-packet/ " CSMA/CD "" Hybrinet, ~=1-
0.4
Fig. 5. Delay vs Offered Characteristics, 5 Mb/s.
I I
.
0.2
Offered Load,
I
_
1 3.
9.
0.(
p
Fig. 3. Delay vs Offered Load Characteristics, 1 Mb/s.
L (D ~-CO E O L I--
j i / ]/ /
cD (9
(3 (9
(9 E ._ I~
i"
Exponentially Distributed Packet Length (mean 1000 bits) B - b i t token • Token Ring, multiple-token • Hybrinet, fl=O
0.6 Load,
Fig. 4. Delay vs Offered Characteristics, 5 Mb/s.
0.8 p
.0
J /
.,.."
,/ . 4 . - "
1. 0.0
,,~ .... ~.... i .... ,,,, 0.2 0.4 0.6 0.8 Offered Load, p
Fig. 6. Delay vs Offered Load Characteristics, 10 Mb/s.
.0
219
O.C. Ibe, P.-Y. C h e n / Hybrinet
of the Hybrinet improves as fl decreases. And in particular, the worst-case performance of the Hybrinet (which occurs when fl -- % is superior to the multiple-token token ring network. The figure also shows that as the offered load increases the performance of the Hybrinet tends to be identical for all values of ft. Figs. 4 through 7 show similar results for the 5 M b / s and 10 M b / s data buses. The results for the 5 M b / s data bus are similar to those for the I M b / s databus, One of the assumptions made in the analysis of the performance of the Hybrinet and the token ring network is that the stations are symmetrically located on the network. In the remainder of this section, we show that if the stations are symmetrically located in the Hybrinet as we define below, then the mean walk time used in the performance analysis is identical for both the Hybrinet and the token ring network. Furthermore, the mean time between the end of packet transmission and the time the packet is completely received at the destination station is smaller in the Hybrinet than in the token ring network. Hence certain features of the Hybrinet which further enhance its performance were not fully taken into consideration in
,'1 .... ~.... ~.... t .... ~.... ~'".' ~.... ~.... ~.... ~0
2 km Cable Length
E ~_
(1)
50 Stations Exponentially Distributed Packet Length (mean 1000 bits)
~ [I [
- 8-bit token
~'~ 9,
c~O
[
" Token Ring, multiple-token
[
• Hybrinet, 0=0
[
the performance analysis discussed above. For the token ring network, locating the stations symmetrically in the network means that they are separated by the same distance from their neighbors. For the Hybrinet, stations are said to be located symmetrically on the network if the transmitter of station i is equidistant from both the transmitter of station i - 1 and the transmitter of station i + 1, 2 < i < N - 1. In addition, the transmitter of station N is located midway between the transmitter of station N - 1 and the midpoint of the cable. The above description also applies to the receivers of the stations. This configuration of symmetrical location of stations in the Hybrinet is shown in Fig. 8. With symmetry defined this way for the Hybriner, the mean walk time is given by N - 1 ( w0 + t, ) 1 Wavg- N + N ( w° + ½~ + t,.) ~= w° + t,. + 2 N ' where wo is the propagation delay between two adjacent transmitters i and i + 1, 1 < i < N - 1, and t,. is the time to transmit the token. Now wo = ~-/(2N), giving Wavg= T / N + t,, which is the walk time for the token ring network with stations symmetrically located around the network. The mean transfer time includes the time taken by a packet to travel from its source to its destination. If we define the "transport time" as the propagation delay between the source station and the destination station, then for the token ring network, the mean transport time is given by
Y-N-11
c"
[w+(2w+A)+(3w+2A)
+ ... + ( ( N - 1 ) w + ( N - Z ) A } ]
£3 (D
y
:~
y
/, ................... Y
I~
/,
"
ZIP
c-
-O
i
7
0.0
0.2
0.4
Offered
0.6 LoQd,
0.8 /9
Fig. 7. Delay vs Offered Load Characteristics, 10 Mb/s.
1.0
y
-.
......... Y
. . . . .y. . . . .
Fig. 8. The Hybrinet with Stations Symmetrically Located.
O. C. lbe, P.- Y. Chen / Hybrinet
220
where A refers to the one-bit delay the packet suffers at each station, and w is the propagation delay between two stations. The above expression means, for example, that if a packet is destined for a station two hops away, then it will suffer at least one-bit delay at the first station in addition to the propagation delay between the two stations, hence the 2w + A entry above. The denominator N - 1 means that the station is equally likely to communicate with any of the remaining N - 1 stations in the network. Thus with A = 1 / C , where C is capacity of the cable, N-2 Y = 2 + 2C For the Hybrinet, define the transport time ~ j as the propagation delay between the transmitter of station i and the receiver of station j. Then the mean transport time for station i is 1
Y,j
j,i and the mean transport time for the network is 1 N Y = ~ ~ Y~.
km, Y = 29 #s for the token ring network with 1 M b / s bus; Y = 14.6 #s when bus capacity is 5 M b / s , and 7.4 #s when data bus capacity is 10 M b / s . For the Hybrinet, Y = 5.1 /~s for all the three cases above. Thus when the Hybrinet has symmetrically located stations, in the sense defined above, its mean walk time equals that of the token ring network, and the mean transport time is less than that for the token ring network. Therefore, the expression for the mean transfer time of the token ring network is slightly biased against the Hybrinet. That is, the performance of the Hybrinet is somewhat underestimated by the expression we used. Furthermore, the configuration of the Hybrinet permits the use of shorter token lengths than those used in the token ring network. We assume that only a free token is transmitted on the access control cable. Therefore, a station after receiving the token, does not have to check the status of the token to see whether it is a free or busy token as in the token ring network. Thus, the size of the token can be made much smaller than that of the token ring network. This again will contribute to the improved performance of the Hybrinet.
iEx 1 NOW
4. Conclusion
1
Y'-N-
l[(½r+(N-1)w°}
+ { ½¢ + ( N - 2)w 0 ) + . - = ½'r + ½Nw0.
+ { ½z + w0 }]
The above expression can be explained as follows. The time Ylk taken for a packet to travel from the transmitter of station 1 to the receiver of station k is one half of the end-to-end delay ¢ (required to traverse the outbound portion of the bus) plus the time ( N - k + 1)w0 required to travel from the midpoint of the cable to the receiver of station k (which consists of ( N - k + 1 ) w a l k times), 2 < k < N. It can be shown that in general, 7 Nwo ( N - 2)(i - 1)w 0 Y' = -2 + 2 N - 1 ' i = 1, 2 . . . . . NI Thus Y = - ~e + w 0 Since w0 = ~'/(2N), Y is independent of the cable capacity. For 50 stations and a cable length of 2
In this paper we have proposed a hybrid local area network, the Hybrinet. Its operation and performance analysis have been discussed. Its operation is similar to that of the token ring network. However, instead of using a single cable for passing the token and data transmission, the Hybrinet uses two cables; one cable is a point-to-point connection for token passing, and the other is a bus structure for packet transmission. The advantage of this arrangement is to eliminate the one-bit delay which a packet suffers at each station in the token ring network. Based on the analysis, the performance of the Hybrinet is superior to that of the token ring network. Furthermore, the Hybrinet's performance is comparable to that of the C S M A / C D network over the range of offered loads in which the C S M A / C D network performs better than the token ring network. Finally, there are other features of the Hybrinet that can be exploited to further improve its performance. These features which include the possibility of using shorter tokens, were not considered in
O.C. Ibe, P.-Y. Chen / Hybrinet
the performance analysis because the aim of this paper is to compare the Hybrinet against other networks under the same operating conditions, Finally, we make come comparison between the Hybrinet and the token bus network. Like the Hybrinet, the token bus network combines the fast bus transmission features of the token passing protocol. However, the token bus network has certain features which may cause its performance to be inferior to that of the Hybrinet. First, in the token bus network the token contains the address of the next station in the logical ordering to receive the free token. Therefore, the minimum length of the token packet is constrained by the number of stations in the network, a feature that is absent in the Hybrinet. Secondly, when a station which has no packet to transmit receives a free token, in the token bus network the station has to generate a new token including the address of the next station. This takes more time than in the Hybrinet where the station merely passes the token to the next station. Thirdly, since logical neighbors may be located anywhere in the token bus, the scan
time will be longer in the token by network than in the Hybrinet. The above observations are in agree-
221
ment with the conclusions reached in [5] where the performance of the C S M A / C D , token ring and token bus networks is studied. These conclusions include the fact that of the three networks, the token bus network has the greatest delay under light load. Also it cannot support as much traffic as the token ring network under heavy load, because it requires a higher overhead than the token ring network. From the above discussions, we conclude that the Hybrinet performs much better than the token bus network at any offered load. References [11 w. Bux, Local-Area-Subnetworks: A Performance Cornparison, IEEE Trans. Comm. 10 (1981) 1465-1473. [2] D.D. Clark, K.T. Pogran and D.P. Reed, An Introduction to Local Area Networks, Proc. IEEE, 66 (1978) 1497-1517. [3] O.C. Ibe and P.Y. Chen, Design of A Hybrid Bus and Ring Local Area Network, Tech. Report GIT-ICS-84/06, School of Information and Computer Science, Georgia Institute of Technology, Atlanta, GA, (1984). [4] R. Rom and F.A. Tobagi, Message-Based Priority Functions in Local Multiaccess Communication Systems, Computer Networks 3 (1981) 273-286. 15] B.W. Stuck, Calculating the Maximum Mean Data Rate in Local Area Networks, Computer (May 1983) 72-76.
Hybrinet: A Hybrid Bus and Token Ring Network Oliver C. Ibe a n d Pin-Yee C h e n School of Information and Computer Science, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
We propose a hybrid local area network, called the Hybrinet, which combines all the good qualities of the CSMA/CD network and the token ring network. The performance of this network is shown to be superior to that of the token ring network for all offered loads where delay is finite. Furthermore, over the range of offered loads where the CSMA/CD network performs better than the token ring network, it is shown that the Hybrinet's performance is very close to that of the CSMA/CD network,
Keywords: Local area networks, protocols, performance evaluation, network design
..... ~
!
Oliver C. Ibe receivedthe B.Sc. degree in Electrical Engineering from the University of Nigeria, Nigeria in 1975,
the M.S. degree in Electrical Engineering and Computer Science from the Massachusetts Institute of Technol.... ogy, Cambridge, Massachusetts in : 1979, the M.B.A. degree from Northi eastern University, Boston, Massachusetts in 1980, and the Sc.D. degree in Electrical Engineering from the Massachusetts Institute of Technology in 1981. From 1981 to 1983 he held a post-doctoral research position at the IBM Thomas J. Watson Research Center, Yorktown Heights, New York, working on performance analysis of computer systems and local area networks. He is currently an Assistance Professor of Information and Computer Science at the Georgia Institute of Technology, Atlanta, Georgia. His research interests include modeling and analysis of computer systems, computer networks, and local area networks. Dr. lbe is a member of the IEEE, ACM and Sigma Xi.
North-Holland Computer Networks and ISDN Systems 9 (1985) 215-221
1. Introduction In a recent paper, Bux [1] presented an analytical study comparing the performance of different types of local area networks. The study shows in particular t h a t the C S M A / C D network [2] has a better delay performance than the token ring n e t work [2] at low offered loads. However, as the load increases the C S M A / C D network experiences a n increased frequency of collisions which gives rise to unbounded delay. The token ring network, o n the other hand, continues to produce bounded delay long after the delay of the C S M A / C D network has become unbounded. Therefore, it has better delay performance than the C S M A / C D network at high offered loads. The reason why the C S M A / C D network has a better delay performance than the token ring network at low offered loads is because a packet is transmitted almost as soon as it is generated in the C S M A / C D network. A station does not have to wait for a free token to transmit its packet as in the token ring network. Moreover, since a packet suffers at least one-bit delay at each station in the token ring network, the expected dely increases with the number of stations in the network. The one-bit delay does not exist in the C S M A / C D network, thus making transmission to be faster in the C S M A / C D network at low offered loads. ~
l~n.Yee O~en received the Ph.D. de. gree in Computer Science from the University of Illinois at UrbanaChampaign, Urbana, IL, in 1981. He is presently an Assistant Professor in the School of Information and Computer Science, Georgia Institute of Technology, Atlanta, Georgia. Pre' viously, he was a visiting Assistant Professor of Computer Science De~ partment at the University of Illinois at Urbana-Champaign, working on various issues of multiprocessor design; from 1982 to 1983, he was with IBM Data System Division, Poughkeepsie, New York, working on system architectures for very large gcientific and engineering processors. His current research interests include parallel and distributed computer architecture, VLSI architectures and algorithms, modeling and analysis of multiprocessors, computer networks, and local area networks. Dr. Chert is a member of the IEEE and ACM.
0376-5075/85/$3.30 © 1985, Elsevier Science Publishers B.V. (North-Holland)
I~ ~
216
O.C. Ibe, P.-Y. Chen / Hybrinet
The results obtained in [1] for the C S M A / C D and token ring networks thus indicate that if delay performance is the primary criterion for evaluating the two networks, then there is no clear winner since the two networks are complementary to each other. That is, one network is a good candidate for those applications where the other performs poorly, and vice versa. In this paper, we consider a hybrid local area network, called the Hybrinet [3], which combines the token passing protocol of the token ring network and the fast transmission feature of the C S M A / C D network. Preliminary results obtained for this network indicate that its delay performance is very close to that of the C S M A / C D network over the range of the offered loads in which the latter performs better than the token ring network. The results also indicate that the network has a better delay performance than the token ring network for all offered loads where the latter has a finite delay, The rest of this paper is organized as follows. In Section II, we give the description of the Hybrinet. The performance analysis of the network is discussed in Section Ill. Finally in Section IV we present some concluding remarks,
2. Network Description The Hybrinet consists of two cables, one for access control and the other for data transmission, The data transmission cable is a folded unidirectional bus which is divided into two portions: an outbound portion to which the transmitter of each station is connected, and an inbound portion to which the receiver of each station is connected [ 4 ] . Assume that the stations are numbered 1, 2 , . . . , N so that station 1 is the first station on the outbound portion and station N is the first station on the inbound portion. The physical configuration of the Hybrinet is shown in Fig. 1. The access control cable, which connects all stations as in a token ring network with the exception that it does not physically connect station N back to station 1, permits the flow of token from station 1 down to station N. The network operates as follows. Assume that station k has just received the token, 1 < k < N 1. Then if the station has no packet to transmit, it passes the token to station k + 1 along the access control cable. If station k has a packet to trans-
INBOUND BUS ~ ,
~ ~
A,',~ESS
COntROL
l~.~..o . . ~
'
~
CABLE
~ OCTBOU~D BC'S Terminator Fig. 1. Configuration of the Hybrinet.
mit, it transmits the packet on the outbound portion of the data bus. After transmitting the last bit of its packet, it then waits for a time period /3 before passing the token to station k + 1. The choice of/3 depends on how the network is to be operated. For example, in a single-packet operation only one packet is allowed in the network at any time. In this case,/3 will be chosen so that the token is passed to the next station after the packet has cleared all stations on the inbound portion of the data bus. However, due to the topological asymmetry of the network, this operation requires different/3 's for different stations. If one value of /3 is to be chosen for this operation, then we must have/3 = T~a, where T,j is the time required for a packet transmitted by station i to be received by station j, 1 < 0' < N. While such a single value of fl simplifies the analysis and the operation of the network, fl = T~ imposes unnecessary delay on packets since the time required for the last bit to travel from the transmitter of station k, k > 1, to the end of the inbound bus is less than TH. Thus there are periods during which the network is not used while stations may have packets to transmit. Therefore in this study, /3 is chosen so that 0 < fl < TH, and/3 = T~ is the worstcase operation. When the token reaches station N and the station either has no packet to transmit or has completed its transmission as described above, it passes the token to station 1 by broadcasting it on the data bus as a regular packet. All stations hear this broadcast which serves to indicate the beginning of a new transmission cycle. However, only station 1 can accept the token, and a new cycle begins.
O.C. lbe, P.-Y. Chen / Hybrinet
3. Performance Analysis of the Hybrinet In this section, we compare the performance of the Hybrinet and that of the token ring network and the C S M A / C D network. We make the following assumptions for the Hybrinet and the token ring network: (a) There are N stations that are equally spaced, (b) Messages are generated at each station at a Poisson rate of ?~/N packets/second, (c) Packet lengths are exponentially distributed with mean L bits. (d) Because of assumption (a), the walk times w, are identical for the token ring network and for 1 < i < N - 1 in the Hybrinet, where the walk time w~ is the sum of the transmission time and the propagation delay of the token from station i to station i + 1. The measure of the network performance is the mean transfer time, as used in [1]. The mean transfer time is the time interval from the time a packet is generated until it is received by the destination station. The expression for the mean transfer time, T/, is [1]:
ation of the token ring network are considered. For the Hybrinet, we consider the worst-case operation, where /3 = ~, as well as multiple-packet modes of operation with/3 = 0, ~/4, and ~/2. We compare the Hybrinet with/3 = T against both the single-packet token ring network and the C S M A / C D network. The expression for the mean transfer time for the C S M A / C D network is given in [1]. We also compare the Hybrinet with 0
"~(1 - p/U) + ,~
p E [ X 2]
T/=2(1-p)E[XI
217
+E[X]q
2(1-0/
2'
where p = h E [ X] and r = the round-trip delay for the token ring network, and the end-to-end propagation delay for the Hybrinet. For the token ring network, ~- includes the one-bit delay at each station. X has different interpretations for different operations: (1) For the "multiple-token" operation o f the token ring network [1], X is the time to transmit a packet. Here a free token is issued as soon as packet transmission ends. (2) For the "single-packet" operation of the token ring network [1], X is the sum of the time to transmit a packet and the ring round-trip delay r. (3) For the Hybrinet, X is the sum of the time to transmit a packet and/3. We consider a network of 50 stations which generate equal amounts of traffic, by assumption (b). The data bus is assumed to be 2 kilometers long with a propagation velocity of 2 × 10 8 meters/sec. Three different cable capacities are considered: 1 M b / s , 5 M b / s , and 10 M b / s . Both the single-packet and multiple-token modes of oper-
CD
2 km Cable Length 50 Stations
E i.s.
Exponentially
9. ~-
/
Distributed
PacketlLength (mean 1000 bits)
~_~ 8-bit token ~ ° Token Ring,single-pocket c~ "" Hybrinet, fl=l" I--• CSMA/CD r r/ o ,' 2g 5. ," ~ ~ ,'/O .N_ , ~ / / o E o 7 _ ~ ~ . 1. =-","-~U/~,,~i,,,,~,,,,~ ,,,i ,,,h .... ~.... i,,, 0.0 0.2 0.4 O.fi 0.8
°
,j
J
Offered Load,
p
Fig. 2. Delay vs Offered Load Characteristics, 1 M b / s .
.0
218
O.C.
(9
2 km Cable Length
~
E
50 Stations
H
Exponentially Distributed eacketlLength (mean 1000 bits) B - b i t token Token Ring, multiple-token • Hybrinet, /~=0
'-I~ 9. ~(9 CO C (3
lbe, P.-Y. Chen/ Hybrinet
(9
2 km Cable Length
50 Stations
j ~ / / ]~ ]
i-~ 9. L_ (1) CO E (3
=~5.
~5.
-0 (9
-~ (9
._
._~
© Z
O Z
0.0
0.2
0.4
0.6
0.8
Offered Load,
1.0
-
2 km Cable Length 50 Stations
(9
L(9 ~-- 9. CO E E~ I---L
I
N
5. /
E
/
0.2
0.4
Offered
f
/ /
/t
(9
/'
~
N "--
III
/~
o L 0 Z
/
0.0
~' /~
-o5.
/
0 Z
,o
~l /~
• Hybrinet, fl=T
~
~
"'-O
.0
,,W
(mean 1000 bits) B - b i t token • Token Ring, single-packet • CSMA/CD
E £] (9
I t
t (9
0.8
2 km Cable Length 50 Stations Exponentially Distributed Packet Length
E ~
I I I
E D (9
0.6
''''lH''f''''l''''l~'''l'~'l''''l','~l*'''
Exponentially Distributed I Packet Length I (mean 1000 bits) I 8 - b i t token I • Token Ring, single-packet/ " CSMA/CD "" Hybrinet, ~=1-
0.4
Fig. 5. Delay vs Offered Characteristics, 5 Mb/s.
I I
.
0.2
Offered Load,
I
_
1 3.
9.
0.(
p
Fig. 3. Delay vs Offered Load Characteristics, 1 Mb/s.
L (D ~-CO E O L I--
j i / ]/ /
cD (9
(3 (9
(9 E ._ I~
i"
Exponentially Distributed Packet Length (mean 1000 bits) B - b i t token • Token Ring, multiple-token • Hybrinet, fl=O
0.6 Load,
Fig. 4. Delay vs Offered Characteristics, 5 Mb/s.
0.8 p
.0
J /
.,.."
,/ . 4 . - "
1. 0.0
,,~ .... ~.... i .... ,,,, 0.2 0.4 0.6 0.8 Offered Load, p
Fig. 6. Delay vs Offered Load Characteristics, 10 Mb/s.
.0
219
O.C. Ibe, P.-Y. C h e n / Hybrinet
of the Hybrinet improves as fl decreases. And in particular, the worst-case performance of the Hybrinet (which occurs when fl -- % is superior to the multiple-token token ring network. The figure also shows that as the offered load increases the performance of the Hybrinet tends to be identical for all values of ft. Figs. 4 through 7 show similar results for the 5 M b / s and 10 M b / s data buses. The results for the 5 M b / s data bus are similar to those for the I M b / s databus, One of the assumptions made in the analysis of the performance of the Hybrinet and the token ring network is that the stations are symmetrically located on the network. In the remainder of this section, we show that if the stations are symmetrically located in the Hybrinet as we define below, then the mean walk time used in the performance analysis is identical for both the Hybrinet and the token ring network. Furthermore, the mean time between the end of packet transmission and the time the packet is completely received at the destination station is smaller in the Hybrinet than in the token ring network. Hence certain features of the Hybrinet which further enhance its performance were not fully taken into consideration in
,'1 .... ~.... ~.... t .... ~.... ~'".' ~.... ~.... ~.... ~0
2 km Cable Length
E ~_
(1)
50 Stations Exponentially Distributed Packet Length (mean 1000 bits)
~ [I [
- 8-bit token
~'~ 9,
c~O
[
" Token Ring, multiple-token
[
• Hybrinet, 0=0
[
the performance analysis discussed above. For the token ring network, locating the stations symmetrically in the network means that they are separated by the same distance from their neighbors. For the Hybrinet, stations are said to be located symmetrically on the network if the transmitter of station i is equidistant from both the transmitter of station i - 1 and the transmitter of station i + 1, 2 < i < N - 1. In addition, the transmitter of station N is located midway between the transmitter of station N - 1 and the midpoint of the cable. The above description also applies to the receivers of the stations. This configuration of symmetrical location of stations in the Hybrinet is shown in Fig. 8. With symmetry defined this way for the Hybriner, the mean walk time is given by N - 1 ( w0 + t, ) 1 Wavg- N + N ( w° + ½~ + t,.) ~= w° + t,. + 2 N ' where wo is the propagation delay between two adjacent transmitters i and i + 1, 1 < i < N - 1, and t,. is the time to transmit the token. Now wo = ~-/(2N), giving Wavg= T / N + t,, which is the walk time for the token ring network with stations symmetrically located around the network. The mean transfer time includes the time taken by a packet to travel from its source to its destination. If we define the "transport time" as the propagation delay between the source station and the destination station, then for the token ring network, the mean transport time is given by
Y-N-11
c"
[w+(2w+A)+(3w+2A)
+ ... + ( ( N - 1 ) w + ( N - Z ) A } ]
£3 (D
y
:~
y
/, ................... Y
I~
/,
"
ZIP
c-
-O
i
7
0.0
0.2
0.4
Offered
0.6 LoQd,
0.8 /9
Fig. 7. Delay vs Offered Load Characteristics, 10 Mb/s.
1.0
y
-.
......... Y
. . . . .y. . . . .
Fig. 8. The Hybrinet with Stations Symmetrically Located.
O. C. lbe, P.- Y. Chen / Hybrinet
220
where A refers to the one-bit delay the packet suffers at each station, and w is the propagation delay between two stations. The above expression means, for example, that if a packet is destined for a station two hops away, then it will suffer at least one-bit delay at the first station in addition to the propagation delay between the two stations, hence the 2w + A entry above. The denominator N - 1 means that the station is equally likely to communicate with any of the remaining N - 1 stations in the network. Thus with A = 1 / C , where C is capacity of the cable, N-2 Y = 2 + 2C For the Hybrinet, define the transport time ~ j as the propagation delay between the transmitter of station i and the receiver of station j. Then the mean transport time for station i is 1
Y,j
j,i and the mean transport time for the network is 1 N Y = ~ ~ Y~.
km, Y = 29 #s for the token ring network with 1 M b / s bus; Y = 14.6 #s when bus capacity is 5 M b / s , and 7.4 #s when data bus capacity is 10 M b / s . For the Hybrinet, Y = 5.1 /~s for all the three cases above. Thus when the Hybrinet has symmetrically located stations, in the sense defined above, its mean walk time equals that of the token ring network, and the mean transport time is less than that for the token ring network. Therefore, the expression for the mean transfer time of the token ring network is slightly biased against the Hybrinet. That is, the performance of the Hybrinet is somewhat underestimated by the expression we used. Furthermore, the configuration of the Hybrinet permits the use of shorter token lengths than those used in the token ring network. We assume that only a free token is transmitted on the access control cable. Therefore, a station after receiving the token, does not have to check the status of the token to see whether it is a free or busy token as in the token ring network. Thus, the size of the token can be made much smaller than that of the token ring network. This again will contribute to the improved performance of the Hybrinet.
iEx 1 NOW
4. Conclusion
1
Y'-N-
l[(½r+(N-1)w°}
+ { ½¢ + ( N - 2)w 0 ) + . - = ½'r + ½Nw0.
+ { ½z + w0 }]
The above expression can be explained as follows. The time Ylk taken for a packet to travel from the transmitter of station 1 to the receiver of station k is one half of the end-to-end delay ¢ (required to traverse the outbound portion of the bus) plus the time ( N - k + 1)w0 required to travel from the midpoint of the cable to the receiver of station k (which consists of ( N - k + 1 ) w a l k times), 2 < k < N. It can be shown that in general, 7 Nwo ( N - 2)(i - 1)w 0 Y' = -2 + 2 N - 1 ' i = 1, 2 . . . . . NI Thus Y = - ~e + w 0 Since w0 = ~'/(2N), Y is independent of the cable capacity. For 50 stations and a cable length of 2
In this paper we have proposed a hybrid local area network, the Hybrinet. Its operation and performance analysis have been discussed. Its operation is similar to that of the token ring network. However, instead of using a single cable for passing the token and data transmission, the Hybrinet uses two cables; one cable is a point-to-point connection for token passing, and the other is a bus structure for packet transmission. The advantage of this arrangement is to eliminate the one-bit delay which a packet suffers at each station in the token ring network. Based on the analysis, the performance of the Hybrinet is superior to that of the token ring network. Furthermore, the Hybrinet's performance is comparable to that of the C S M A / C D network over the range of offered loads in which the C S M A / C D network performs better than the token ring network. Finally, there are other features of the Hybrinet that can be exploited to further improve its performance. These features which include the possibility of using shorter tokens, were not considered in
O.C. Ibe, P.-Y. Chen / Hybrinet
the performance analysis because the aim of this paper is to compare the Hybrinet against other networks under the same operating conditions, Finally, we make come comparison between the Hybrinet and the token bus network. Like the Hybrinet, the token bus network combines the fast bus transmission features of the token passing protocol. However, the token bus network has certain features which may cause its performance to be inferior to that of the Hybrinet. First, in the token bus network the token contains the address of the next station in the logical ordering to receive the free token. Therefore, the minimum length of the token packet is constrained by the number of stations in the network, a feature that is absent in the Hybrinet. Secondly, when a station which has no packet to transmit receives a free token, in the token bus network the station has to generate a new token including the address of the next station. This takes more time than in the Hybrinet where the station merely passes the token to the next station. Thirdly, since logical neighbors may be located anywhere in the token bus, the scan
time will be longer in the token by network than in the Hybrinet. The above observations are in agree-
221
ment with the conclusions reached in [5] where the performance of the C S M A / C D , token ring and token bus networks is studied. These conclusions include the fact that of the three networks, the token bus network has the greatest delay under light load. Also it cannot support as much traffic as the token ring network under heavy load, because it requires a higher overhead than the token ring network. From the above discussions, we conclude that the Hybrinet performs much better than the token bus network at any offered load. References [11 w. Bux, Local-Area-Subnetworks: A Performance Cornparison, IEEE Trans. Comm. 10 (1981) 1465-1473. [2] D.D. Clark, K.T. Pogran and D.P. Reed, An Introduction to Local Area Networks, Proc. IEEE, 66 (1978) 1497-1517. [3] O.C. Ibe and P.Y. Chen, Design of A Hybrid Bus and Ring Local Area Network, Tech. Report GIT-ICS-84/06, School of Information and Computer Science, Georgia Institute of Technology, Atlanta, GA, (1984). [4] R. Rom and F.A. Tobagi, Message-Based Priority Functions in Local Multiaccess Communication Systems, Computer Networks 3 (1981) 273-286. 15] B.W. Stuck, Calculating the Maximum Mean Data Rate in Local Area Networks, Computer (May 1983) 72-76.