Preventing Network Pocket Collisions
Your
100-Mb network should be blazing along, but users complain that it's
slow. Mismatched inter frame gap settings among the network adapters
could be the problem. Learn how to fix this by adjusting the TCP/IP
protocol and inter frame gap settings.
Are you not getting the
performance you’d expect from a network running at 100 Mb, even
though you're using 100-Mb network adapters with TCP/IP as your
network protocol? If so, 100-Mb network adapters not adhering to the
IEEE 802.3 specification for Ethernet networks could be the cause for
this slip in performance. The IEEE 802.3 specification calls for a
96-bit-times inter frame gap between the time the adapter switches
from transmit mode to receive mode.
When adapters that adhere
to this specification are used on the same network with adapters that
don’t, timing problems occur that eventually cause collisions on
the network. This problem is compounded on networks that use the
TCP/IP protocol because of the scheme it uses for acknowledging the
receipt of data packets. In this Daily Drill Down, I’ll explain how
you can solve this problem by adjusting the inter frame gap settings
and the way that the TCP/IP protocol works with Windows 2000
Professional and Windows XP Professional clients.
The
inter frame problem in a nutshell
The IEEE 802.3 specification
calls for a 96-bit-times inter frame gap. Basically, this inter frame
gap is a period of silence between the moment a particular station
sends a data packet across the network and the time that the other
station responds. For a 100-b network card, this 96-bit-times
inter frame gap comes out to 960 nanoseconds.
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Note
For the sake of comparison, the 96-bit-times inter frame
gap is 9.6 microseconds for a 10-Mb network card, 96 nanoseconds for
a 1-Gb network card, and 9.6 nanoseconds for a 10-Gb network card.
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The inter frame gap is designed to give the adapters involved
in the communication time to switch from transmit mode to receive
mode. In other words, once a network card has transmitted a data
packet, the card needs a few nanoseconds before it is ready to
receive a response.
When it comes to the speed of today’s
computer technology, you may think that 960 nanoseconds is a long
time—and it is. However, when the specification was first hammered
out, using a formula based on 96 bit times was necessary for the
network cards of the day to properly switch from transmit mode to
receive mode.
While the 96-bit-times inter frame gap rule has
remained set in stone, network cards have continued to evolve as the
technology has improved and network speeds have increased. The newer
100-Mb cards no longer actually require a 96-bit-times inter frame gap
to switch from transmit mode to receive mode.
Considering
this, some 100-Mb network card manufacturers are ignoring the
standard and configuring their products with a much smaller
inter frame gap to pump up the data transfer rate. However, other
100-Mb network card manufacturers continue to adhere to the
96-bit-times standard.
If you're using 100-Mb network cards
from the same manufacturer in all the systems on your network, you’ll
be fine. But a more common scenario is an organization that uses
100-Mb network cards from multiple manufacturers. If you administer
in this type of configuration, it's possible your network is
suffering due to an inordinate amount of network collisions caused by
differing inter frame gap settings.
The TCP/IP headache
While
the differences in the inter frame gap settings are a big enough
performance problem to deal with, the situation can get worse when
you’re using TCP/IP as your main protocol.
The TCP/IP
protocol specification calls for sending an ACK (acknowledgment) for
every two data packets that a network station receives. If the
network cards in the two systems are using different inter frame gap
settings, it’s possible that a network station that has received
two TCP/IP data packets could be sending its ACK at the same time the
sender station is sending another data packet. This would compound
the network collision problem already initiated by the mismatched
inter frame gap settings.
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Use a protocol analyzer to detect collisions
If you’re
encountering network performance problems, you can easily test your
network with a protocol analyzer, or a similar device, to measure
collisions, runt packets, or CRC (cyclic redundancy check) errors.
Keep in mind that under regular operating conditions, some collisions
are normal. But a high percentage of collisions, runt packets, or CR
errors indicates a serious performance problem that could very well
be related to the differing inter frame gap settings of the various
network cards on your network.
Adjusting inter frame gap
settings
To begin, you need to have on hand a list of all the
brands and versions of 100-Mb network cards installed on your
network. (This is where a network inventory will come in very handy.)
Next, check the network card documentation for a specification on the
inter frame gap setting. You may need to contact the manufacturer
directly, or via the company's Web site, to track down the inter frame
gap setting. Also find out if the network card provides a method of
adjusting the inter frame gap setting.
The goal here is to
have all the cards on your network use the same inter frame gap
setting. So, if those cards that have nonstandard inter frame gap
settings provide a mechanism for adjusting the setting, you’ll want
to bring them in line with all of the other cards on your network.
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Other terms
As you’re searching for information relating to
your specific network card’s inter frame gap setting, keep in mind
that this setting has several aliases. Some manufacturers call it the
inter frame space or inter-frame space. I’ve also seen it referred
to as the inter packet gap. It also goes by the acronyms IFG, IFS, and
IPG.
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Adjusting the TCP/IP protocol
You can also alleviate the
problem by adjusting the way the TCP/IP protocol works. As I
mentioned, the default configuration for the TCP/IP protocol calls
for sending an ACK for every two data packets that a network station
receives, which can cause a higher volume of network collisions on a
network using network adapters with mismatched inter frame gap
settings. The adjustment I’ll explain here will configure the
TCP/IP protocol to send an ACK for every data packet received. To do
so, you’ll need to edit the registry and add a special parameter
called the TcpWindowSize to the TCP/IP key.
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Note
Since editing the registry can be dangerous, consider
performing a full backup before you attempt this operation.
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To begin, select the Run command on the Start menu and launch
the Registry Editor by typing regedit.exe in the Open text box. Once
you have the Registry Editor up and running, double-click on the key
HKEY_LOCAL_MACHINE. When this subtree is visible, open each of the
following subtrees in succession:
SYSTEM
CurrentControlSet
Services
Tcpip
Parameters
Pull down the Edit menu
and select New | DWORD Value. Then, assign the new value the name
TcpWindowSize and press [Enter] twice. When you see the Edit DWORD
Value dialog box, type 2920 in the Value Data text box. Finally,
select the Decimal option in the Base panel. Your system will be set
to send an ACK with every data packet received.
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Note
Keep in mind that adding this TcpWindowsSize parameter
to the registry can degrade network performance for Wide Area
Networks (WANs) because there is already some extra latency
introduced by this type of connection.
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Conclusion
When you use 100-Mb network adapters from
various manufacturers and are using TCP/IP as your network protocol,
you might not be getting the performance you’d expect from a
network running at 100 Mb due to mismatched interframe gap settings.
I’ve explained how to solve the problem by tracking down
information on your network cards' interframe gap settings and how to
adjust the setting, if needed. In addition, I’ve explained how to
alleviate the problem by adjusting the way the TCP/IP protocol works.
