Guidelines for providers offering VOIP service over Amplex’s network

Amplex provides service to customers using a fixed position wireless technology.  The network is capable of providing excellent performance for voice services – provided that the guidelines in this post are followed.  Failure to follow these recommendations will likely result in poor call quality and significant customer dissatisfaction.

#1. Before selling or purchasing voice services please check with Amplex to determine if the existing Internet service is appropriate.   We do not recommend deploying VoIP services to locations served by 900Mhz or 802.11 equipment.  In some cases we may be able to upgrade service to accommodate VoIP services.   Significant costs may be encountered if  additional height is needed for the antenna.  The type of equipment in use can not be determined by looking at our invoices – you must contact us.

#2. Notify Amplex of the IP address(es) of the service providers VOIP gateway.  Amplex runs a ‘Quality of Service’ (QoS) enabled network.  This means that we classify and prioritize traffic flowing over the network.   For traffic entering the network at our external borders (from the Internet) any existing QoS is reset to default priority. Traffic is then classified into several categories.  For the purpose of this discussion RTP (the voice part of a IP call) from KNOWN PROVIDERS is set to high priority (DSCP 46).   If you notify Amplex of the VOIP gateway address we will mark traffic as high priority. As of the publication date Buckeye Telesystems is the only provider to have supplied this information.

#3. Notify Amplex of the amount of traffic at the customer location that is needed for the number of call sessions * codec bandwidth.  Based on this information we will enable and set a high priority CIR (committed information rate) for the desired amount of bandwidth. Do not request more bandwidth than is needed as excessive bandwidth reservations will negatively affect other network traffic.

#4). Set the correct DSCP headers in outgoing RTP traffic.   Outgoing traffic to be handled as high priority is determined by the CPE (Amplex’s Customer Premise Equipment) based on the low latency bit of the DSCP header – specifically bit 3 of the 6 bit DSCP header.  We suggest using DSCP code point 46 (or 101110 in binary, some equipment may refer to this as ‘EF’ for Expedited Forwarding).  Note that DSCP is a 6 bit field that is part of an 8 bit IP header.  Equipment that requests a 8 bit value should use 10111000 as a binary value.  Either the customer VIOP gateway or the SIP phones must set DSCP appropriately.

#5)  The customer router MUST clear, at a minimum, the low latency bit of the DSCP field of all non-voice traffic. Failure to accomplish this step will allow other common traffic (SCP, SSH, video) to consume high priority upload bandwidth resulting in poor call quality.  Manipulation of the DSCP values can be handled by many business class routers and firewalls. We have found the Juniper SRX series to be cost effective and capable.

#6.  Check with Amplex to verify that we are seeing traffic flowing in the high priority queue of the radio, in both directions, during a VOIP call.

Following these guidelines should result in a quality VOIP experience. If you are considering either purchasing or selling voice service over Amplex’s network we strongly encourage you to ensure that the VOIP provider and your internal networking team (or consultant) is capable of understanding and following the recommendations in this whitepaper.

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Why does my router keep locking up?

Alternate title:  Why do you keep telling me I have a virus when my Internet quits working?

Home routers are technically not really routers at all – they are network address translation (NAT) boxes.  So what is NAT and why do I care?

NAT was developed in order to conserve address space.  NAT is used in consumer routers as it conserves address space, is easy to configure, and provides some firewall protection to the computers.

How NAT works is pretty simple.  The router (what we are going to call the NAT box) has an outside and a inside interface.  The ‘outside’ is the side connected to the Internet.  The ‘inside’ is the side connected to the computers in your house.

For demonstration purposes lets have 2 computers that we call “A” and “B” in the house.

When computer A connects to a site on the Internet the router makes an entry in the NAT table that says “computer A is talking to Google”.  Computer B wants to connect to Yahoo.  The router makes an entry in it’s NAT table to remember that B is talking to Yahoo.  So far so good.  When responses come back from Google the router knows that ‘Google’ traffic goes to computer A and that “Yahoo” traffic goes to computer B.  The router now has 2 entries in it’s NAT table – one for computer A to Google and one for computer B to Yahoo.  Computer A now goes to a different site – this adds another entry in the NAT table.

So – each new connection from a computer to a site on the Internet uses up one slot in the NAT table (actually it uses several as web pages are composed of multiple images, text, advertising, etc.).  Most consumer routers have NAT tables that can hold a few thousand entries.  How does the router decide when to discard the NAT table entries?  If the connection between the computer is terminated cleanly (the TCP protocol has a way to do this) the entries are removed from the NAT table. Entries that are not cleanly terminated (and some protocols do not have a method to indicate they are done transferring data) are eventually timed out of the table.  Many routers will also start discarding the oldest entries if the NAT table is full or close to full.

So what happens when the NAT table is full?  The router no longer has a place to store information required to process the data coming back from the Internet.  The computer will not be able to establish a connection and the connection will time out.  Since web sites are actually composed of many items when the NAT table is nearly full parts of the page may load while the remainder loads slowly or not at all.  Some routers (that don’t expire entries when the NAT table in nearly full) will appear to lock up at this point and need to be rebooted.  Others will reboot spontaneously or recover if the computers are shut off.

So why would a NAT table be full?  The most common reasons a NAT table is full (or overloaded) is that the computers are trying to talk to too many sites and/or the connections are not being properly terminated (and therefore not being removed from the NAT table).  What kinds of software tries to talk to large numbers of computers on the Internet? Peer-to-Peer file sharing and Viruses.  Let’s take each one separately.

Peer-to-Peer networks are programs that enable you to share files from your computer with others on the Internet who would like to download them.  This is most commonly used for (illegally, but that’s another matter) downloading music and video files from others.  The Wikipedia page has a good description of how peer-to-peer networks work. Depending on the configuration of the peer-to-peer software the program may not limit the number of computers it is sharing files with and/or may not limit the amount of bandwidth being used.  All of the programs we have seen have options for limiting the number of concurrent connections and the amount of bandwidth.  We suggest setting those as low as possible if you are having lockup issues.

Viruses:  Pretty much by definition viruses try to propagate themselves by attacking other computers. Once a computer has been taken over by a virus or other malware it is impossible to say what it is going to do – but they often try establishing so many connections that they quickly overload the NAT table.

So what is Amplex looking at when I call in? Amplex also uses NAT in our customer premise equipment (CPE). The NAT table in our equipment is limited to 4096 entries. When a customer calls in with a connection issue one of the first things we check is to see if the NAT table in the CPE is full. If it is and the customer says they are not running file sharing we are going to assume it is a virus issue. If you are running file sharing we are going to suggest turning it off or adjusting it’s settings.

When we tell you we are seeing signs of virus activity it is not that we are looking at your computer or even seeing the specific traffic . We are seeing the large number of entries in the NAT table of the CPE.

How does an end user figure out which computer is causing the problem? It can be difficult as viruses do their best to hide themselves. Easiest is usually to try turning off one computer at a time and see if the problem goes away. Keep in mind more than one computer may be infected.

But but but.. we don’t want Amplex to do NAT.  I want to have a transparent connection to the Internet! Ok – no problem, just let us know. You will need to understand how to set a static IP address on your router. Please research how to do that before contacting us and we will happily disable NAT on our CPE.

Update on new tower sites

Seems like projects always take longer than they should.   In any case…

The Gibsonburg site is up and running.   I am not completely happy with the coverage area we are getting from the 2.4Ghz sector at the site but the 5.7Ghz transmitter is working very well.    As soon as we have the funds we will swap the 2.4 for a couple of sectors which should improve coverage in the area.

The Dirlam Road site just east of Bowling Green is up and running – we will be converting many of the 900Mhz customers south of SugarRidge and/or north-east of the Bays Rd tower to the new site over the next couple of weeks.   This will result in a significant performance increase for those customers.

Rising Sun is on the back burner for the winter – I do not expect to have equipment at Rising Sun until spring 2010.

The North Baltimore / Hoytville site is a project for late December or early January – funding and weather may delay this though.

New tower sites coming soon!

Amplex has received permission from Countyline Co-Op to use the grain silo’s at Gibsonburg and RisingSun.   This will give us much better coverage in the western and central portions of Sandusky and south-eastern Wood county.   We expect to have the Gibsonburg site operational by October 19th and the Rising Sun site by November 15th.

We have purchased the rights to place our equipment on a tower just east of Bowling Green on Dirlam Rd.   This tower should be operational in early November.   This tower will provide improved service to customers south of SR-105 between Bowling Green and Pemberville.

A new water tower is being constructed by the Northwest Water and Sewer District at  Hoytville.   This tower is located at the new rail yard being constructed west of North Baltimore.  This tower should be operational by late November.

The Northwest Water and Sewer District water tower at Weston is being replaced with a newer (and much larger) water tower in the spring of 2010.    Amplex will be moving our equipment from the existing tower in Weston to the new tower in the spring.

A new Northwest Water and Sewer District tower east of Luckey will be operational in the spring of 2010.

POP3 versus IMAP mail

Methods to check your mail:

Amplex supports several different ways to access your email:

  • POP3 (Post Office Protocol #3)
  • IMAP (Internet Message Application Protocol)
  • Webmail

The major difference between POP3 and IMAP is where the messages are stored.

When retrieving messages with POP3 the default behaviour is to:

  1. Retrieve from the mail server (at your ISP) the number of new messages on the server.
  2. Transfer the messages from the ISP to your computer
  3. Delete the messages from the mail server.

When checking a mailbox using IMAP a completely different thing happens:

  1. Compare the list of messages at the server and the local computer to determine message state (new, read, deleted, replied to, etc.)
  2. Show the current state of the mailbox.  Synchronize the state of the messages on the server and the local computer.

The big difference between the two is that POP3 REMOVES the messages from the server once it has transferred them to your local computer.  That POP3 removes the messages from the server is very important to understanding the difference between the two accounts.  IMAP leaves the messages on the server until they are deleted by you.
Webmail is simply a way of using a web browser to read your mail using IMAP.  Webmail interacts with your mailbox using IMAP.
Nearly all mail client software (Outlook, Outlook Express, Thunderbird, Incredimail, Entourage, Vista Mail, etc.) can be set up to check mail using either POP3 or IMAP but all default to POP3 unless told otherwise.

So why would you want to use POP3 or IMAP?   Which one should you choose?

If you always check your mail from the same computer then POP3 is a good choice.   Since POP3 transfers the mail to your computer you always have a copy of your mail and you can read it when you are not connected to the Internet.  Remember – POP3 will transfer the mail and then delete it from the server.   Once you retrieve your mail using POP3 it is erased from the ISP’s mail server.
If you check your mail from multiple computers then IMAP is a better method.   Since IMAP keeps the mail on the server along with the state of the mail (read, unread, replied to) it makes it much easier to check your mail from multiple computers.   If you have a computer at work and at home both set up to check the same account using IMAP you will see the same messages on both computers.   When you read a message on one computer and then check the other one the message will show up as having been read already.
If you set up two computers to check mail using POP3 then something really confusing happens.   If both computers are set to check mail every 10 minutes (the default) then the first computer to check after a new message arrives retrieves it and deletes it from the server.   Let’s say for example   your ‘home’ computer is checking for messages using POP3 at 5, 15, and 25 minutes after the hour.   Your ‘work’ computer is checking at 0, 10, and 20 minutes after the hour.   When a new message arrives at 2 minutes after the hour it will show up only on the home machine.   A message that arrives at 8 minutes after the hour ends up only on the work machine.   A message arriving at 12 minutes would only show up on the ‘home’ machine.   Very confusing if your at work waiting for a message to arrive.
Things can get very  confusing if you are using both IMAP and POP3 at the same time.  Keep in mind that Webmail is really an IMAP client.  Let’s assume your home computer is set up to use POP3 and you leave it running and it’s checking for new mail every 10 minutes.   If you’re at work and decide to check your mail using webmail you log in and don’t see any messages – because your home computer is retreiving and deleting the messages from the server every 10 minutes.  Or you get lucky and catch the message before your home computer retrieves it – and then you check again 15 minutes later and it’s gone -  because your home computer just retrieved it and deleted it off the server.

So what’s the moral of this story?

Pick a method of checking mail and stick with it – if you use webmail then always use webmail.

If you want to use both webmail and a mail program like Outlook then set it up to check mail using IMAP.

If you want to use POP3 to check your mail then make sure you DO NOT leave it running when you are not using it.

If your messages all suddenly disappear off webmail it’s a safe bet that somewhere you have a computer checking your mail using POP3 and all of your mail was transferred to that computer.

Are there exceptions to the above discussion?

Yes – there are options available in most mail clients to tell POP3 not to delete messages off the server, to delete them after a certain amount of time, or based on other criteria.  These options are available to make POP3 behave more like IMAP – but they are something of a kludge – your probably better off using a protocol like IMAP.

Two other things occasionally happen with mail:

When using POP3 if the connection to the server is interrupted before all of the messages are retrieved the next time you connect  you will get another copy of all the message you already received.   The is because messages are not deleted until after all the messages are transferred.
When using both POP3 and IMAP the POP3 client will occasionally show a message in your mailbox that says “DO NOT DELETE THIS MESSAGE – INTERNAL MESSAGE DATA”.   This message is stored on the mail server and contains information used by IMAP.   Occasionally a POP3 client accidentally retrieves this message.   You can safely delete the message without hurting anything.