1 0 o�--5�0--�1 0�0-�1� 50�-2�
0�0 EXTENT OF THE RING (TOTAL FIBER LENGTH I N KILOMETERS)
Figure 7 Maximum Access Delay as a Function of the Extent of the Ring
Total Number of Stations
The total number of stations on a ring includes active and inactive stations. In general, increasing the number of stations adds to the ring latency because of the additional fiber length and addi
tional station delays. Thus, the number of stations affects the efficiency and maximum access delay in a way similar to that of the extent; a ring that con
tains a larger number of stations than another has a lower efficiency and a longer maximum access delay. In addition, a large number of stations on a ring increases the bit-error rate. Consequently, large rings are not desirable.
Number of Active Stations
As the number of active stations, i.e . , MACs, increases, the total load on the ring increases.
Figures 8 and 9 show the ring performance as a function of the number of active MACs on the ring.
We considered a maximum-size ring with a TTRT value of 8 ms for the analysis. The figures show that increasing the number of active MACs has a sl ight positive effect on the efficiency, but considerably increases the maximum access delay. Therefore, it is preferable to keep a minimal number of active stations on each ring by segregating small groups on separate rings.
Frame Size
Frame size does not appear in the simple models of efficiency and maximum access delays, because
Vol. 3 No. 3 Summer 1991 Digital Technical journal
80
f=' 60
z w (.) a:
w e;_
>- 40
(.) z UJ u u:: u. 20 w
0
Figure S
250 500 750 1 000 NUMBER OF ACTIVE MAGs
Efficiency as a Function of the Number of Active MACs
frame size has l ittle impact on FDDl performance.
In our analysis, we assumed that transmission stops at the instant the THT expires; however, the standard allows stations to complete the trans
mission of the last frame.
The extra time used by a station after THT expiry is cal led asynchronous overflow. Assuming al l frames are of fixed size, let F denote the frame transmission time. D uring every transmission opportunity, an active station can transmit as many as k frames:
1 00,000
>-:5 1 0,000 w � O (J)
� �
w O
8 &l
1 ,000< � :;2 j
� �
x <
::2 1 00
k =
�
T; 1
TTRT = 8 MILLISECONDS RING LATENCY = 1 .773
MILLISECONDS
NUMBER OF ACTIVE MAGs
Figw·e 9 Maximum Access Delay as a Function of the Number of Active MACs
Digital Technical jow-nal Vol. 3 No. 3 Summer 1991
Performance Analysis of FDDI
Here,
I l
is used to denote rounding up to the next integer value. The transmission time is equal to k times F, which is slightly more than T minus D.With asynchronous overflow, the modified effi
ciency and maximum access delay formulas become
Efficiency = n (kF nkF + D) + D
Maximum access delay = (n - 1 ) (kF + D) + 2D
Notice that substituting kF = T - D in the above equations results in Equations (1) and (2).
Figures 10 and 1 1 show the efficiency and the maximum access delay as functions of the frame size. Frame size has only a slight effect on these metrics. Larger frame sizes do have the following effects:
• The probabil ity of error is greater in a larger frame.
• Since the size of protocol headers and trailers is fixed, larger frames require less protocol overhead.
• The time to process a frame increases only sl ightly with the size of the frame. A larger frame size results in fewer frames and, hence, in less processing at the host.
Overall, we recommend using as l arge a frame size as the reliability considerations allow.
1 00 TYPICAL
BIG f=' 75
z LARGEST
w (.)
a: w
e;_ TIRT = 8 MILLISECONDS
>- 50
(.) z w u u:: u. 25 w
0 1 000 2000 3000 4000 FRAME SIZE (OCTETS)
Figure 10 Efficiency as a Function of the Frame Size
87
Network Performance and Adapters
>-4:
100,000 u:J 1 0,000
o w UJ O UJ Z w o 0 0
� �
1,000::::; :J
� ='
� �
X
�
1 00TTRT = 8 MILLISECONDS
---LARGEST
---BIG
---TYPICAL
10 o�--�10�0�0--�2�0L00��30L0�0--�4�00�0
FRAME SIZE (OCTETS)
Figure 11 Ma.Yimum Access Delay as a Punction of the hame Size
Summary
Although many parameters affect the performance of an fDDI ring network, the target tokt:n rotation time (TTRT) is the key parameter that network managns can control to optimize this perfor
manct:. We analyzed the effect of other paranwters such as the extent of the ring (the length of the cable), the total number of stations, the number of active stations, and frame size. From our data we concluded the following:
• Rings with a large extent and those with a large number of stations are undesirable because they yield a longer maximum access delay ami have only a sl ight positive effect on the ctficiency of the ring.
• It is preferable to minimize the number of active stations on a ring to avoid increasing the maxi
mum access delay.
• A large frame size is desirable, taking into consid
eration the acceptable probability of error.
The value of TTRT does not significantly affect the response time unless the load is near saturation.
Under very heavy load, response time is not a suitable metric. Instead, maximum access delay, i .e., the time between wanting to transmit and being able to do so, is more meaningfu I.
A larger value of TTHT improves the efficiency, but it also increases the maximum access delay. A good trade-off is provided by setting TTRT at 8 ms.
Since this value provides good performance for all ranges of configurations, we recommend that the default value of TTRT be set at 8 ms.
88
References
1 . F. Ross, "An Overview of FDDI: The Fiber Distributed Data Interface," IEEE journal on Selected Areas in Communications, vol. 7, no. 7 (September 1989): 1043-51.
2. Fiber Distributed Data Interface (FDDI) Token Ring Media Access Control (1HAC), ANSI X3.139-1987 (New York: American National Standards Institute, 1987).
3. Token Ring Access Method and Physical Layer Specifications, ANS!/I EI�E Standard 802.5 -1985, ISO/DIS 8802/5 (New York: The Institute of E lec
trical and Electronics Engineers, Inc . , 1985).
4. R. Grow, "A Timed-token Protocol tor Local Area Networks," Proceedings of tbe IEEE Electro '82 Conference on Token Access Protocols, Paper 17/3, Boston, MA (May 1982).
5. R. Jain, The Art of Computer Systems Perfor
mance A nalysis, ISBN 0-471-50336-3 (New York:
john Wiley & Sons, 1991).
6. D. Dykeman ancl W Bux, "Analysis and Tuning of the FDDI Media Access Control Protocol," IEEE journal on Selected Areas in Communications,
vol. 6, no. 6 Ouly 1988) : 997- 1010.
7. ]. Ulm, "A Timed-token Ring Local Area Network and Its Performance Characteristics," Proceed
ings of the Seventh IEEE Conference on Local Computer Networks (February 1982): 50-56.
8. R. Jain, "Error Characteristics of Fiber Distrib
uted Data Interface (FDDI)," IEEE Transactions on Communications, vol. 38, no. 8 (August 1990): 1244- 1252.
9. W Bux and H . Truong, " Mean-delay Approxi
mation for Cyclic-service Queueing Systems,"
Performance Evaluation, vol. 3 (Amsterdam:
North-Holland, 1983): 187- 196.
Vol. j No. 3 Summer 1991 Digital TecbtJical Joun�al