2.3 Frame Overhead


2.3 Frame Overhead

Unlike Ethernet, which requires the use of fill characters to produce a minimum length frame of 64 bytes, there are no such restrictions on a Token Ring frame. Although this makes the computation of the Token Ring frame overhead more direct, this is only true when a frame does not contain a routing information field. If it does, then the length of that field, which can be up to 18 bytes, will affect the overhead of frames that traverse more than one Token Ring network.

If we focus our attention on frames limited to being carried on a single network, as indicated in Figure 2.6c, the overhead of a Token Ring frame is 21 bytes. This overhead is applicable to both 4- and 16-Mbps Token Ring networks.

Table 2.7 illustrates the overhead of Token Ring frames as the information field varies in length from 1 to 16,384 bytes. As previously noted, the maximum information field length is 4500 bytes for a 4-Mbps Token Ring network and 18,000 bytes for a 16-Mbps Token Ring network.

Table 2.7: Token Ring Frame Overhead

Information Field (bytes)

Ratio of Frame Overhead to Total Bytes

Percent Overhead

1

21/22

95.45

32

21/53

39.62

64

21/85

24.71

128

21/149

14.09

256

21/277

7.58

512

21/533

3.94

1024

21/1045

2.01

2048

21/2069

1.01

4096

21/4117

0.51

4500

21/4521

0.46

8192

21/8213

0.26

16384

21/16405

0.13

In examining the overhead associated with information fields whose lengths are under 512 bytes, we can express similar concerns to short Ethernet data fields with respect to network performance. That is, if your organization is in the process of developing client/server applications, network performance can be improved by developing client/server displays and transmitting a large grouping of information to the server rather than a series of short groups of data. Although there is no predefined time gap between Token Ring frames as there is with Ethernet frames, the lower operating rate of a 4-Mbps Token Ring frame increases the probability of poor performance with a lesser number of clients accessing a server and repeatedly transmitting short responses to server application program queries. Thus, you may be able to improve the performance of your Token Ring network, as well as extend its ability to service additional users without degrading network performance, by designing application programs to take advantage of the structure of network frames.




Enhancing LAN Performance
Enhancing LAN Performance
ISBN: 0849319420
EAN: 2147483647
Year: 2003
Pages: 111
Authors: Gilbert Held

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