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Estimating LAN Performance Requirements

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  1. Estimating LAN Performance Requirements
  2. Example 2: Characterizing Network Performance
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Choosing your physical LAN installation requires you analyze the network traffic and the characteristics of that traffic. This article describes an approach for that analysis, helping you choose the technology that best supports your network load.
Choosing your physical LAN installation requires you analyze the network traffic and the characteristics of that traffic. This article describes an approach for that analysis, helping you choose the technology that best supports your network load.

This content is excerpted from Barry and Marcia Press’ book, Networking by Example (2000, Que).

Common wiring and signaling technology lets your LAN run at 1, 2, 10, or 100Mbps. Table 1 summarizes the network technologies that you can use at each of those data rates.

Table 1

LAN Data Rates and Technology

Data Rate

Net

Technology Choices

Kbps

KBps

KBps

 

115.2

11.52

10

Serial direct cable connection

1,000

125

100

Telephone line network (HPNA standards), IEEE 802.11 wireless LAN

2,000

250

200

IEEE 802.11 wireless LAN

10,000

1,250

1,000

10BASE-2, 10BASE-T, 10BASE-FL Ethernet telephone line network (HPNA standards), IEEE 802.11 wireless LAN, USB direct cable connection

100,000

12,500

10,000

100BASE-T Ethernet

The 10Mbps (10,000Kbps in the table) categorization for USB and wireless is only approximate, but this is close enough for planning network speeds. There are two rate columns in Table 1—one expressed in the usual kilobits per second, and the other expressed in megabytes per second. You convert from kilobits per second to kilobytes per second by dividing by 8 (because there are 8 bits in a byte) for everything but the serial direct cable connection. Convert bits to bytes for the serial cable technology by dividing by 10 to account for the 2 bits that the serial port hardware adds to every byte transmitted.

There's a third column in Table 1, too, one that uses a rule of thumb that maximum LAN throughput is about 80 percent of the raw data rate, to suggest the actual maximum sustained data rate that you'll see on a LAN using each technology.

The other information that you need to decide the LAN data rate that you want is the size of the blocks of information that you'll transfer across your LAN and the rates that this information is available for transfer across network (fast for disks, slow for modems). Table 2 shows some of the possibilities for how much information you're likely to transfer.

Table 2

Data Transfer Sizes by Type of Data

File Size Range (KB)

Type of Data

1–100

Text and email

10–2,000

Graphics

1,000–5,000

Sound

5,000–10,000

Video

1,000–100,000

Combined text, graphics, sound, and video

1,000–3,000,000

Software installations

Using the information in the two tables, you can estimate how long data transfers of each type are likely to take using the different LAN technologies. Now we'll cover how to do the calculations.

Example 1: Calculating LAN Data Transfer Times

Suppose that you regularly work with large data files—perhaps documents containing text and photographs. From Table 2, you'd estimate file sizes in the range of 100KB to 100MB. If you're considering 10BASE-T Ethernet, Table 1 shows you that you can calculate using a 10Mbps data rate and suggests that you'll get a maximum of 1MBps transferred across a network.

If you divide the data size estimates by the data rate estimate, you'll see that the transfer is likely to take 0.1–100 seconds if there's no other activity on the LAN at the time.

The calculation in Example 1 applies if you're sharing files across the LAN and perhaps (depending on the speed of your computer and the complexity of the document) if you're printing to a shared LAN printer. The rates in Table 1 could be irrelevant if the data transfer is information coming off the Internet because your modem speed is probably slower than your network. Table 3 shows the maximum data rates that you're likely to achieve using the common Internet access technologies.

Table 3

Typical Maximum Internet Access Data Rates

Data Rate (Kbps)

Internet Access Technology

Incoming

Outgoing

 

33.6

33.6

Analog modem (V.34 specification)

53

33.6

56Kbps digital modem (V.90 specification)

64

64

Single-channel Integrated Services Digital Network (ISDN)

128

128

Dual-channel ISDN

7,168

1,088

Rate Adaptive Digital Subscriber Line (RADSL), a common version of ADSL

10,000

2,000

Cable modem

You're likely to see lower rates than in Table 3 because your telephone lines or cable network might not support the maximum rate or because you've chosen a limited rate to reduce cost. Your immediate conclusion from Table 3 should be that, unless you have an ADSL or cable modem Internet connection, the speed that your LAN runs at is irrelevant for information coming in over the modem.

After you've identified all the information that you need using Tables 1, 2, and 3, divide the data size estimates by the data rate estimate as in Example 1 to calculate data transfer times typical of how you expect to use your network. The next section shows a typical case.

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