Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Using IP phone internal switch


As the title says my question is in regards to using the internal switch in IP phones to provide the LAN connection to a PC. A co-worker is claiming that using the internal 10/100 switch for PC data at the same time as a call is going to degrade the quality of the call, even with QoS. He also argues that having two different classes of packets (voice and data) traveling on the same access link is slower than having all packets on a port of the same class.

Seeing as the bandwidth used by even the least compressed codec is less than 100Kbps, it seems to me that the most impact of using the internal switch for PC LAN connection would be the serialization delay. If my calculations are correct with full 1500B packets on a 100Mbps link this should still only amount to 120 microseconds. Is there any other major source of delay caused by using the internal switch that would degrade the quality of the call?

Is there any truth to his argument that having two different classes of data on the same link is more processor intensive such that a difference in call quality would be noticed? Or is it more accurate to say that a switch is checking the class of every incoming packet anyway (with QoS enabled) and having more than one class of data on a link would only result in the packets being sent to different queues?

Lastly I notice that gigabit IP phones are mostly only available as higher-grade phones. Should I expect to see gigabit switches in most IP phones soon and would I expect that much less delay twith using the internal switch for PC LAN connection?

Thank you for any information.

Hall of Fame Super Gold

Re: Using IP phone internal switch

Your coworker is wrong.

First, cisco has sold some million of phones and nobody has meet any issue in voice quality when the PC port is used actively.

Second, both phone and switches do prioritize the voice vlan over the data one, this only. happens in very unlikely event that the queue is full.

Third, as per you calculation, you can keep a voice packet waiting for some tens of milliseconds (not microseconds), and nothing bad happens.