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IP telephones.

Started by Stephen Furley, July 16, 2009, 05:11:18 PM

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Stephen Furley

At work we're gradually converting to Mitel IP telephines; we're up to about 40% IP now, and about 60% analogue.  These IP telephones are very different to conventional analogue ones, and totally unlike traditional rotary dial ones, but I thought some people here who haven't used IP telephones might be interested to know a bit about them.  There's a data sheet for the 5304 here:

The model which we're supplying to most users is the mitel 5304, almost the bottom of the range.  The difference between these and the more expensive models which we supply to a few users is that the more expensive ones can be used as a speakerphone, have extra buttons, and can take a headset; I would say that between 5 and 10% of our users need these facilities.

So, what's so special about these 'phones, and why are we changing over to them?  The first thing about them is that they are expensive.  You might want to grab hold of something while I tell you what they cost; at the moment the 'phones themselves cost us 100 pounds.  To get them to work requires a device licence, and to assign a number to them requires a user licence.  These licences cost an additional 81 pounds.  181 pounds for an entry-level telephone isn't exactly cheap, to put it mildly.  The higher up models in the range don't actually cost a great deal more, and the licences are exactly the same.  I think the 5312 is about 130 pounds, plus headset if required.  At those prices there had better be some advantages to these things!

The first really fundamental difference between these and conventional analogue telephones is the way they are connected; they don't use conventional telephone cabling; they talk through Ethernet networks.  A conventional telephone needs a pair of wires, one and a half pairs if earth recall is being used; these things use two pairs for data, and usually another two for power, though some power over Ethernet power sourcing equipment supplies power over the data pairs; the 'phones, and anything else which uses PoE, can accept either.  So, we need four pairs rather than one.  The difference is that while conventional analogue 'phones each need their own pair all the way back to the PABX room, one Ethernet cable can serve many 'phones.  Voice is sampled at 8 kHz., with a resolution of 8 bits, and with one channel.  To transmit this uncompressed needs 64 kb/s, with a bit of overhead let's say 100 kb/s; a single Ethernet cable can carry literally hundreds of calls, and while an analogue 'phone needs its pair even when it is in use, the IP 'phone only needs its bandwidth when it is in use.  In practice, the amount of bandwidth required for voice is so small that we could remove it totally from the network, or double it, or increase it ten-fold, and we'd hardly notice the difference.

All of the current models of Mitel IP Phones have a built in Ethernet switch, with a second port, to which the user's pc can be connected, so only one drop of cable is needed to each desk.  Since we already have a data connection to each desk, the voice traffic effectively goes along for free.

It's no longer necessary to have a pair of wires and a socket for every 'phone; as long as there's one network point in a room you can just plug in  PoE mini-switch, and then connect several 'phones to that.  This is very useful whee a number of 'phones are needed in a room for a short period; in a couple of weeks I've got to set up a six station call centre in a room for clearing; it's only needed for a few days.  With analogue 'phones this would involve getting an engineer in, having cable run to the room from the nearest distribution point, and possibly all the way from the frame if there weren't enough spare pairs available in the dp, and sockets installed in the room; an expensive business, and only of any use in that room; we couldn't just move it somewhere else the next time.  We can put a switch and several 'phones into a room for a few days, or even a few hours, for an event at a cost of almost nothing, and then take it away, and move it somewhere else.  Calling an engineer in to install one additional telephone socket in a room costs more than an IP 'phone and its licences.

While the IP telephone has to be connected directly to a copper Ethernet cable, and by that cable to a suitable switch, beyond that point the data can travel over anything capable of carrying a network; fibre optics, wireless bridges, laser links up on the roof to a building on the other side of the road; the 1Gb BT fibre link to another building which we have a couple of km away; even send in through a VPN over the Internet to anywhere we want to put a phone.  They can also pass through all standard switches, routers etc. We no longer need a separate voice infrastructure, and this will lead to big savings in the longer term.

While the number of a conventional telephone is actually assigned to the pair of wires to which it is connected, with IP 'phones the number is associated with the telephone itself.   Strictly speaking, it's associated with the MAC address, and this association is held in a table in the central controller.  This means that if you unplug a telephone and take it to another location, the number will move with it.  On the other hand, if you replace a telephone with another one it would have to be configured to have the same number, but that's not difficult to do.

As the 'phone starts up it goes through quite a complex sequence of events; it's possible to watch these on the display, which is useful for troubleshooting if a 'phone won't start up properly.

With analogue 'phones all switching is done centrally at the PABX; with IP 'phones the switching is distributed; a telephone has to talk to the controller to set up and clear down a call, but the voice traffic itself on a call from one IP 'phone to another will travel directly between those devices, by the most direct route.  Of course it has to go via the PABX if the call is to an analogue extension, or to a trunk for an external call.  Of course, we're now dealing with packet switching, rather than circuit switching, but Ethernet switches are much cheaper than conventional telephone switching equipment, and if you don't already have an existing analogue system, or it's reached the end of its life, the cost saving in not buying a new one could be significant.  There are disadvantages of course; all analogue 'phones are powered centrally from the PABX, with IP 'phones not only is the switching distributed around the building, but so is the power; a UPS will be required at each switch location if the telephones are required to work during a mains failure.

Those are the main differences from our point of view; how about that of the users?  For most of them there's not much difference.  They can make, or answer, a call.  it's a bit easier to set up conference calls and a few other things, but few of them do this.  There's a 'message waiting' indicator for the voicemail system, and it's possible to look up numbers in the directory and see if your 'phone is diverted, and where to, using the display.  A small number of our users need special features which would be difficult, or impossible to provide with analogue 'phones.

Almost all of the IP 'phones can take at least two lines and some can take many more than this; I think the limit is something like 96 lines on some models.  Of course, all of these lines are carried on the same wires.

I must admit that I was a bit doubtful about handling voice on packet switched networks at first, about five years ago now, but I'm convinced; they work really well, and make life mush easier when things change as much as they do in our organisation (a large college).  At present, the IP telephony is only within our private system; the trunks out to the PSTN are BT ISDN30 (ISDN PRI) with 30 'B' channels, 1 'D' channel and one synchronising channel, all carried on a 2 Mb/s E1 service.  This is delivered on two conventional copper pairs, and we have two of these services, though the second one only has 13 'B' channels active.  BT is planning to move their network to IP over a number of years, so it's likely that the trunks will eventually be IP as well.

That's about it; I don't think I'll be adding any IP telephones to the collection in the near future, but they work well in a business system.


An interesting description of where landlines are probably all going to go someday for both business and personal use.  At my work, school sites, I have no idea what they are using other than twisted pair copper and traditional office switching equipment.  I found this line of yours of interest:

QuoteThey can make, or answer, a call.  it's a bit easier to set up conference calls and a few other things, but few of them do this.

Seems to be the norm about everywhere.  I've rarely ever seen anyone actually use much of the fancy technology that is at their disposal, I go to endless meetings with senior admin and multiple local agencies and only once did anyone try even a conference call.  (it was a fiasco)  When I first got my cell phone about 9 years ago I thought I'd be using all the spiffy abilities it had daily, but it turns out that I use it for calls, texting, a few pictures, and playing a chess game on rare occasions.  I know a lot of folks with iPhones and they talk a lot about how they have apps that can do whatever, but pretty much all they do is what I do, and surf the web every so often to check their blogs or whatnot on screens that are about the size of a 3 X 5 card.  Even my household cordless phone has a bunch of nifty features that I've never bothered with, and now that I have old rotaries hooked up I very rarely use the cordless at home. 

Someone on this forum, I don't recall who, has a website that they run where they carry info and old products, like a 1937 pair of Levi's 501s.  Anyway they had recently bought an old Gillette single blade razor, the kind that were standard issue 40 years ago, as they got tired of how much it costs to buy modern blades for our Schick Quattro Omolomgotto Turismo Portofino 8 blade vibrating and reciprocating plasma powered razor that cost $18.99 cents for a pack of 4 blades that last a month, tops.  They had bought a couple of hundred blades for the old Gillette and paid about 50 bucks, and use one a week, and were very pleased.  Seems like a lot of modern life is like this, incredibly powerful gizmos that can just about do everything you want, but we hardly use that power and might be better served with simpler methods. 


Stephen, that's an excellent description of how IP phones work. I work for a school system with 30 schools and we just finished an installation deploying IP phones to every school and into every classroom. We use Cisco equipment. I'm not the person in charge of the IP phones, so I can't offer all the technical details, but I do have a few comments.

One amazing phone Cisco offers is a wireless IP phone. All it needs is a charger base. It uses the Cisco wireless access points throughout the building. The same ones we use for laptops. They work fairly well, though you can get cut off while walking around sometimes. We also just deployed the wireless, so it may have some tweaking necessary. We recently bought wireless IP phones for all our principals and assistant principals. That way they can make and receive calls when out in the school.

I already have two IP phones in my collection. They got hit by lightning and were being thrown out. Like you, we don't install full duplex speakerphones in all locations because they're very expensive. Our classrooms in all but one school have the smaller phone with no speakerphone. Only administrators have speakerphones.

Our county in North Carolina has five telephone companies and you can imagine what kind of mess that was with moves, adds and changes to analog phone systems. We also had many different systems that we couldn't troubleshoot, so we were paying the phone companies a lot of money for maintenance. Now we do it ourselves. Your example of setting up a call center was a good one. We can do so much more ourselves now.

We also save in long distance charges because we can install what's called a PRI line at a school in a different telco area than the school central office, so calls can be made to a very large surrounding area. The Call Manager knows by the prefix where to direct the call.

One nice feature is the DID number. A direct dial number right to someone's desk. All the central office personnel have this as do the school principals. FAX machines use what's called an ATA, an analog telephone adapter, and also use a DID number. One thing we wish they would make is something cheaper that we could connect an analog bell ringer to, for cafeterias and other loud areas.

The same contractor who has helped us deploy the phones and iron out some problems also wrote a Windows program that will record a call on your computer if it's plugged into the IP phone's data port. It only works when there are no extra ethernet switches on the line. The phone must be plugged into a wall ethernet jack and the computer must be plugged directly into the phone. It records it as a .wav file.

Another feature we have is a malicious caller button on the phone. When pressed, it sends the call info (extension, line, time, etc.) in an email to four of us, so we can alert the local telco and find where the call originated. It worked very well once when a kid was making a bomb threat from a cell phone while in the boys' bathroom of the high school. They caught him red-handed, as we say in the states. He was suspended for the remainder of the school year.

We can also find out other somewhat sensitive info like how many calls the receptionist sees on a daily basis and who called what number at what time.

For the most part, IP phones have been a change for the better. When the internet goes down at a school, it can be pretty bad, though. We have a 911 line and an analog phone at each school for emergencies. Just last month we had vandals break a single-mode fiber link between two schools and we were able to forward all the calls to the 911 line until the fiber was re-pulled two days later.

For power outages, we have powered switches on UPS units. These mainly go to office areas. But in the majority of classrooms, we decided to eliminate the powered switches and dedicated wiring to next to the hallway door because of the expense. So those phones have power bricks. We can then pull the wire to near the door and replace the switches sometime later when funds permit. Near the door is a good location because the teacher can take the handset into the hall for more privacy, and in an emergency, the phone can easily be located. It also doesn't drop onto the floor and break. Those phones weigh almost nothing.

The Cisco switches offer what's called QOS, which means quality of service, so we can dedicate bandwidth to the phones in case there's a large amount of internet traffic at the school. We have had network problems still take down the phone system inside the school, so it's not perfect. An Apple AirPort with TWO ethernet cables plugged into two ports on a mini-switch will take down a network faster than you can say BOO.

Those are our experiences with IP phones. I hope nobody thinks I'm trying to sell anyone on any particular product--I'm just stating the facts. We do like them and in the long run they will save us time and money.