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Determining Optimal Packet Size for CTP Bundles Overview

You can specify the size of IP packets that are created from data received at the serial port. The CTP device uses packet size along with the serial interface rate to calculate the packet rate; that is, the rate that packets are created. The maximum packet rate is 1500 packets per second.

To determine the optimal packet size, consider the following:

  • Bandwidth for transporting serial data
  • Packet creation delay
  • Performance of the IP network

For example, larger packet sizes are more bandwidth-efficient, but introduce more delay during packet creation.

Bandwidth for Transporting Serial Data

When considering bandwidth in relation to deciding packet size, add overhead for both the Layer 2 encapsulation and the IP header. The IP header comprises 20 bytes; and the encapsulation overhead varies, but is typically either 6 or 8 bytes on serial links. This overhead causes smaller packets to be less efficient and result in serial data requiring more bandwidth.

Calculate the bandwidth required for a serial bit stream as follows:

IP Bandwidth = [Packet Size (bytes) + 20 (bytes) + 2 (bytes) + Encapsulation Overhead (bytes) x [Packet Rate (pps)] x 8

Packet Creation Delay

Serial data received at the CTP interface must be buffered long enough to allow a packet to be created. The delay to create the packet increases as either the size of the packet increases or as the rate of the serial interface decreases. Generally, this delay is minimal except when the rate of the serial interface is low and the packet size is large. We recommend that you set the packet size to a smaller value for lower-speed serial interfaces. Table 3 provides examples of serial interface packet creation delay in milliseconds.

Table 3: Packet Creation Delay for Serial Interfaces

 Serial Interface Delay (msec)
Packet Size (bytes)
Interface Rate (Kbps)12825651276810241400

64

16.0

32.0

64.0

96.0

128.0

175.0

128

8.0

16.0

32.0

48.0

64.0

87/5

256

4.0

8.0

16.0

24.0

32.0

43.8

1024

1.0

2.0

4.0

6.0

8.0

10.9

1544

0.7

1.3

2.7

4.0

5.3

7.3

2048

.5

1.0

2.0

3.0

4.0

5.5

Performance of the IP Network

The number of packets created (packet rate) is inversely related to the packet size configured. For example, smaller packets result in a greater packet rate. When you configure the packet size, consider the packet-forwarding performance of the attached router and network. Table 4 provides examples of packet rates for various packet sizes and serial interface rates.

Table 4: Packet Rate for Various Packet Size and Serial Interface Rate Settings

 Packet Rate (Packets per Second)
Packet Size (Bytes)
Interface Rate (Kbps)12825651276810241400

64

62.5

31.3

15.6

10.4

7.8

5.7

128

125.0

62.5

31.3

20.8

15.6

11.4

256

20.0

125.0

62.5

41.7

31.3

22.9

1024

1000.0

500.0

250.0

166.7

125.0

91.4

1544

1507.8

753.9

277.0

251.3

188.5

137.9

2048

2000.0

1000.0

500.0

333.3

250.0

182.9

Modified: 2018-01-29