Packet Loss and Discards Explained

Let's explore Lost Packets and Packet Discards.  What are they, and what do they mean?

First, to determine if you have a packet loss or discard problem, visit www.testyourvoip.com and run a test.  If you are on Vonage or interested in Vonage, test using "Boston" as your location, because Vonage is located in New Jersey so Boston will give you the most accurate and realistic results.

Now, for a little background on what VoIP is, and what lost or discarded packets will do to VoIP.

VoIP (Voice over Internet Protocol) works by encoding your voice as packets of data.  These packets are then numbered, packaged, and sent over the network to a remote machine that receives the packets.  Since the Internet routes packets one at a time, some of these packets may be delayed for various amounts of time or even lost in transmission.

The remote machine is tasked with the job of taking these packets, sorting them, and re-creating the original voice.  It does this by assembling the packets as they come in, and waiting some predetermined amount of time to play back the packets.  Any packets that come in AFTER they should have been worked into the voice stream are discarded (thrown away), because they are useless - the need for them has already passed by.  Any packets that never make it at all are called "Lost" packets.

So, discarded packets are packets that have been delayed long enough to be useless.  Lost packets are packets that never made it at all.  Both of them are problems to VoIP, as the missing data can cause choppy audio ("Burst Loss"), or loss in voice quality ("Random Loss"), depending on whether large blocks of packets are lost all at once (causing choppiness and silences), or the packet loss is steady (causing a steady voice, but with occasional artifacts or a "metallic" or distant/synthetic sound that some people call "toilet bowl audio").

How VoIP Works:
How Packets Travel in VoIP
In the above diagram, packet 4 was lost entirely ("LOST") and packet 2 made it, but it was too late ("DISCARDED").

Packet discards can be fixed by either:
Packet LOSS can only be fixed by figuring out where the packets are being lost, and fixing it.  In some cases, if network congestion gets severe, a packet will get thrown away (lost) by a router en route to the destination.

Solving Latency (packet discards):
    Packets can be delayed or lost anywhere on your network. How do you find out where this is happening?

Simple.  Using any good Route Tracing tool.  When your packets are sent to a server, they go through the Internet through a series of routers (also commonly called "hops") to reach their destination.  Delays can occur anywhere along that path, including your local network.  To find out where delays may be happening, you can either use the DOS-based tool "TRACERT", which is included on every Windows operating system (and Linux, Unix, and other operating systems as well).  Or you can download a trial of a visual tool called "PingPlotter" from http://www.pingplotter.com  and get your results in a more visual GUI display. If you use it and prefer it, please do the authors a favor and buy it.

One good site to trace to, for example, would be sip.vonage.net - this is the server that handles the job of telling Vonage devices that they have an incoming call, or routing out the calls that Vonage users make (this is NOT the server the actual calls are handled by - that will vary depending on where your call comes from or goes to, and Vonage is not likely to give you that information).

Results from TRACERT will look like this:
    1 <1 ms <1 ms <1 ms 192.168.1.1
    2 50 ms 13 ms 9 ms host-1 [11.11.11.11]
    3 10 ms 13 ms 62 ms host-2 [22.22.22.22]
    4 15 ms 19 ms 74 ms host-3 [33.33.33.33]
        and, eventually,
    14 * * * Request timed out.

When you start seeing "Request timed out", you have hit a firewall that doesn't want you looking inside its network.  Press CTRL-C to cancel the test.

Each line has the following data in order:
 - Hop Number
 - Lowest Latency
 - Average Latency
 - Highest (worst) Latency
 - Host Name (if available)
 - IP Address (in brackets if host name was provided)

"Latency" is a measure of the amount of delay introduced by that specific hop.  The higher the number, the more delay.  It is measured in "ms" or milliseconds, which are 1/1000 of a second.  So a 200 ms delay means 2/10ths of a second.  Route delays are cumulative, meaning they "add up" as you go through the network.  5 hops with a 200 ms delay each means a delay of one second from source to destination.  They are also transient, meaning they will vary wildly from moment to moment based on the amount of traffic they are handling at that moment.

To see the TOTAL trip time to a given server, ping it ("PING (servername)").  The "time=" portion of each response will give you an idea of how long it took for your test packet to make it to the server and back.  Many servers are set up not to respond to ping requests, unfortunately.  sip.vonage.net is one of them.  

The first hop will be your router (if you have one).  Your router should be easy to identify, since it does not have a host name, and probably has an IP address you recognize if you know your router at all.  If you see high latency here, then you have a router problem.  Your router is on your Local Area Network (LAN), and should generally have response times within a few ms, and most of the time should be within 1 ms ("<1 ms" as shown above).

The second hop (or the first if you do not have a router) is your modem or Internet connection.  If you see high latency here, your network is either saturated (try doing the test with only one computer hooked up), or your modem has a problem.  QoS might fix your issue if your problem is caused by saturation (trying to send too much data upstream on the router).  

Any large latency numbers after your modem are the sole responsibility of your ISP, or their upstream providers.  Your ISP may or may not be able to do anything about them, but you can call and ask.

If you are on an idle network and your modem is still giving you high latency numbers, then you should probably call your ISP and ask them to do a line test. There may be something wrong with your cable modem, or your connection.  You can also try a new patch cable between your modem and router or computer, and checking to make sure the wire coming on from the ISP (RJ11 for DSL, Coax for Cable) is free of corrosion and plugged in firmly.  If everything looks good, and you want more information about your cable modem, you can also read the next section on "Interpreting your Cable Modem Data".  But I'd spend that time calling your ISP if I were you.  Maybe you can read it while you wait on hold.  Then you won't be so bored listening to their terrible music.

INTERPRETING CABLE MODEM DATA:

If you are experiencing a problem, and you've gotten this far without success, I'm going to assume you are calling your ISP.  You can read this while you are on hold.  Really.  It's that non-riveting.  

First, please understand that I am not an expert in Cable Modems.  In fact, if you are, and you see gaps or mistakes, I'd like to know about them.  Really.  See the bottom of this page for feedback information.  

However, if you really want to know, you can probably find out some of the details about the quality of signal between your Cable Modem and the Mother Ship.  This can help you determine if there MAY be a cabling problem in your coax, almost as easily as turning on the TV and looking for clear picture.

This information may prove useful when calling Customer Service for your ISP, though their technicians can probably retrive it faster than you can read it off to them.  But drop words like "low SNR" and "insufficient downstream power", and they'll treat you with newfound respect until you mangle up a term, and then they'll laugh at you and start making up terms to test your knowledge (hint:  a cable modem does NOT come equipped with a flux capacitor).  :)

Anyway, enough disclaimers.  You're either curious or have a masochistic streak, so I'll do my best.  Plus you've probably got some time left on hold.

Many modems will show their status on a local page, like http://192.168.100.1 (which works on my Cable Modem - a Linksys BEFCMU10).  Addresses may vary by modem manufacturer and/or model, and the DOCSIS standard says that your ISP can shut down this display if they want to.  If your modem was supplied by your ISP, contact them for the address.  Otherwise, a Google search on your modem model may yield the address you need, if it exists.

Here are a few of the important bits of data you can get from your Cable Modem.  I've put the data from my Cable Modem here for reference.

Standard Specification Compliant       DOCSIS 1.0

DOCSIS is the industry standard for the way modems communicate.  Nowadays, it is VERY RARE to find a Cable ISP who does not use DOCSYS.  There are three levels of this specification.  DOCSYS 1.0 is hideously ancient and many people will tell you that Vonage will simply not work on DOCSIS 1.0.  Well, I get crystal-clear audio on DOCSIS 1.0 but that does not mean it's not a problem for anyone.  Slightly newer is DOCSIS 1.1, and the latest is DOCSIS 2.0.

Some modems can be upgraded to newer DOCSIS via a firmware upgrade.  As per the DOCSIS standard, ONLY your Cable ISP is allowed to load new firmware releases on your modem.  For example, my modem is capable of 1.1, but my ISP uses Axxis equipment, which is not capable of pushing firmware updates to anything but their proprietary modems.  Given that they want $50 to replace my modem with a proprietary Axxis one, and given that they are scheduled to be bought out by Comcast in three months, I'll wait and see what Comcast does.

 Downstream (indicating data sent to you from the ISP):
    Max Bit Rate      3000000 bps
    Downstream Power      -4.34 dBmV
    SNR      36 dB

Max Bit Rate is the speed cap your Cable company has placed on your modem.  While you may be able to change this using techniques commonly found on the Internet, your cable company WILL find out, and they WILL be displeased with you.  So just back away from that concept and move on, OK.  If this is on your modem, then this is the fastest your modem will ever send packets to you.

Downstream Power indicates the amount of signal (or, technically, signal loss) between you and your ISP.  Cisco's web site claims that this number should be -5 dBmV or greater, so as you can see, my modem is supposedly in range.

SNR refers to "Signal to Noise Ratio", or how loud the actual data is as compared to background noise and interference.   My cable signal is 36 times louder than any detected interference.  Yay.  This is good.  I have no clue what "Bad" would be.   Less than 1 would be "very bad", as in you would not be on this web site looking at this tutorial.

 Upstream (data sent to your ISP from your modem)
    Max Bit Rate      256000 bps
    Upstream Power      46.00 dBmV

Max Bit Rate is the same as the one above, except this is your upstream limit (data you send to your ISP).  For VoIP applications, this is the amount of data you can send out, and determines the quality of voice that others hear when you speak.  This is the only area where QoS is effective, and this number (or actually the real world tested value) is critical to QoS.

Upstream Power is the level of signal your Cable Modem is pumping out to your ISP.  I could not find any specs on this, except that it needs to be at least 25dB higher than any interference.

If you really can't sleep, here's a reference with more detail than you'll ever want on the subject:  http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/cable.htm


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