Performance Issues

Performance Issues

Congestion

The network is a finite resource

  • 1000BASE-T is one gigabit per second
  • It can’t go any faster

A busy network may attempt to send 2 gigabits per seconds

  • Contention brings packet queueing, buffering, etc.

There are only so many resources

  • Buffers will fill
  • Some data may be dropped

Increase the size of the road

  • Or decrease the number of cars

Bottlenecks

There’s never just one performance metric

  • A series of technologies working together

I/O bus, CPU speed, storage access speed, WAN bandwidth, local network speeds, etc.

  • One of these can slow all the others down

You must monitor all of them to find the slowest one

  • This may be more difficult than you might expect

Resolving the network bottleneck:

Bandwidth usage

The fundamental network statistic

  • Amount of network use over time

Throughput

  • The amount of data successfully transferred through the network

Many ways to monitor

  • SNMP, NetFlow, sFlow, IPFIX protocol analysis, software agent

Throughput capacity

  • Total throughput has a maximum value
  • Based on the slowest link

Latency

A delay between the request and the response

  • Waiting time

Some latency is expected and normal

  • Laws of physics always apply

Examine the response times at every step along the way

  • This may require multiple measurement tools

Packet captures can provide detailed analysis

  • Microsecond granularity
  • Get captures from both sides

Packet loss

Discards, packet drops

  • No errors in the packet, but system could not transmit or receive the data

Packets are lost

  • Corrupted during transmission
  • Dropped after validation

Data must be retransmitted

  • Overall communication is delayed
  • Uses additional resources

Jitter

Most real-time media is sensitive to delay

  • Data should arrive at regular intervals
  • Voice communication, live video

If you miss a packet, there’s no retransmission

  • There is no time to “rewind” your phone call

Jitter is the time between frames

  • Excessive jitter can cause you to miss information, “choppy” voice calls

  • Shows the inconsistency of packet delays
  • Lower jitter means better stability of the connection
  • High jitter means stutter in live video or voice/video calls

Wireless Issues

Wireless Interference

There is a limited amount of frequency

  • Everyone can’t talk at once
  • Similar to a wired network

An increasing number of wireless devices

  • They all want to talk
  • Nearby access points using the same frequencies would cause problems

Most APs can monitor frequency usage

  • Can move automatically to unused space
  • Manual configuration is an option

Band selection and bandwidth

Managing channel usage

Disable legacy, low speed support

  • Use the fastest possible speeds and configurations

Check your channels

  • Avoid overlap between access points

Adjust the output power

  • Avoid conflicts with other access points
  • Interference can steal valuable network time

Split the network

  • You might need additional frequencies and access points

Overlapping channels

One misconfiguration, and you have overlapping channels:

Attenuation

Wireless signals get weaker as you move farther from the antenna

  • The attenuation can be measured with a Wi-Fi analyzer

Control the power output on the access point

  • Not always an option

Use a reception antenna with a higher gain

  • Capture more of the signal

Some power is lost in the antenna cable coax

  • Most applicable at higher frequencies
  • Also check for damaged cables, especially outside

Insufficient Wireless Coverage

Determine existing wireless landscape

  • Sample the existing wireless spectrum

Identify existing access points

  • You may not control all of them

Work around existing frequencies

  • Layout and plan for interference

Plan for ongoing site surveys

  • Things will certainly change

Heat maps

  • Identify wireless signal strengths

Client Disassociation issues

A denial-of-service attack

  • Takes advantage of older 802.11 management frame transmission

Device keeps dropping from the wireless network

  • Or never connects

Disassociation frames can be clearly seen in a packet capture

  • Grab the 802.11 frame information with Wireshark

Remove the device performing the disassociation

  • Or upgrade to a new 802.11 standard

Roaming misconfiguration

A wireless network often has a single name

  • SSID (Service Set Identifier)
  • Appears as one big wireless network

There might be multiple access points supporting an SSID

  • Extend the network

Users will move to the best access point

  • These must have identical configurations
  • Users will be dropped if configurations differ