|Friendly Debate — Cellular vs. Paging
On Sep 16, 2005, at 11:48 PM, Zachary Wozich wrote:
[Zach] Been awhile. Hope all is well with you. I just wanted to make a few comments regarding one of your recent articles in this weeks newsletter. I hope you don't mind, but I am going to copy and paste your paragraph then write comments under the points you made that I am concerned with. Also, I know this is a forum for wireless data and messaging, but as a person who works in cellular, I wanted to respond on some remarks written about cellular or "other technologies". This comes from a understanding and prior background in one way paging as well. Here is what I have to say. . .
[Zach] Cheap and reliable, yes, paging is. Immune to relying on ma bell, floods, weather and "when everyone tries to use them at the same time" ........ paging is NOT! If I remember correctly, people still need to use a wire line (telco, Internet) when generating a page. We remember what happened during 9-11 when people all tried to call NY at the same time.
[Brad] True Paging systems do use phone lines like cellphone systems, but the point being made here is about "mass alerting" or "public warnings." When Katrina blew past New Orleans, there was a Paging Control Terminal installed in in Metairie, Louisiana. It was completely destroyed—they couldn't even find the building—so the traffic was re-routed to other terminals. That meant that no one could dial into the system locally, but then the telephones didn't work there anyway. Traffic from outside the area was still being broadcast because it came in on satellite. Sure several transmitter sites were destroyed but since Paging uses "simulcast" with multiple transmitters there were some transmitters that continued to work and others that only needed AC power which soon came on line via emergency generators.
[Brad] In a recent issue of the newsletter, Ron Mercer very eloquently addressed this topic:
[Zach] Also, most paging systems have their network connected to a central switch with one satellite up-link. This central switch then links the remote switches (ones used in other states to eliminating long distance calls for subscribers) are back hauled using ma bell. This back-hauls prone to many variables that can cause outages for customers. Floods, Back Hoe outages, poor Telco customer service. Also, when a two way paging site is connected to a wire line back-haul such as frame relay, Ethernet, or T-1, its prone to the same variables listed above that cellular is prone to as well.
[Brad] True but this does not affect the ability of the Network Control Center to send out a mass alerting message. These control centers are frequently redundant and are located in buildings that are VERY well protected. (Like practically bomb proof.)
[Zach] And what about the reliability of one way paging? Yes, it probably is the most reliable. Low speed data rates using high power transmitters covering broad landscape with satellite down links in the KU band. However what satellite makes up for with being very inexpensive it lacks in two spots. The fact that the KU band 10 GHz - 14 GHz is prone to rain fades causing outages all the time. This happens especially during hurricanes, thunderstorms, and snowstorms.
[Brad] True but rain fades are brief and as soon as the rain passes, everything starts to work again.
[Zach] Also, I witnessed this my self. A massive satellite outage. When Telstar 4 had a main power buss fail two years ago, it cause my companies 150+ sites to be off the air for 24 hours and it took a week for the rest to be re-pointed to Telstar 5.
[Brad] In my over fifty years in radio communications, this has happened one time. Yes, it was bad but most Paging companies have contingency plans now to get back online if something like this should happen again. The Paging company that I was working for when this happened had satellite antenna mounts that could be quickly re-aligned to a different satellite. Unfortunately, getting to all the satellite antennas to do this re-alignment was not so quick since there were thousands of them all over the United States. I think the chances of a satellite failure and a natural disaster happening at the same time are slim.
[Zach] I have found, the most reliable form of back haul is microwave to my cellular sites. It can be years (and it has at my sites) before a outage can occur on a microwave back-haul. Also since they are usually classified in C Band or 4 GHz to 6 GHz, they aren't vulnerable to rain fades. Yes, there is path fading or cancellation (usually in warm hazy summer nights when the signal passes over mountain terrain or bodies of water), but space diversity receive dishes fixes this problem most of the time.
[Brad] So I guess you have never been in an earthquake zone when a little temblor can knock all the microwave dishes out of alignment. I concede that microwave links are very reliable. They are used in some Paging systems as well.
[Zach] Last, when everybody tries to send One Way pages all at once. They are limited to how fast the page can be generated and sent. The bottle neck (remember low speed data rate) is the low data rate. I have seen 5 - 10 minutes lag time when thousands of pages are in queue waiting to be sent out when massive group messages were being sent out. POCSAG or FLEX.
[Brad] Sorry Zach but you are confusing "terminal-based group call" like the cellular and trunking systems use to call a group, with "common-capcode group call." Most radio communications systems can call groups of users or subscribers, but they have to call each one individually. Even in Paging, before Pagers were sophisticated to the point of being able to have several capcodes, we did variations of "terminal-based group call." In this type of group call, it was necessary to define the group in the control terminal. When a subscriber record was defined as a group call record, it would contain all the members of that group. So if I wanted to send a message to 50 Firemen, each of their individual capcodes would be recorded in a group call record and when a message came in to that number, it would be sent out 50 times—once to each Fireman. This is very inefficient and if this type of group call were still used today, it would not be possible to alert very large groups of people.
[Zach] Actually Brad, this is incorrect when saying "no other technology can [do] that". SMS text messaging has very robust group messaging features. Usually they can be generated at the comfort of your cell phone, or Internet. Voice mail just the same. Also the newest feature PTT, has vast capabilities for "talking" to many people at once, quickly, and efficiently. Also, there are common cell phone numbers as well which send group voice mails to as many people has the switch tech has assigned. I know. I am on one of those list at my company.
[Brad] A "common-capcode group call" is a very simple concept. One message gets transmitted and EVERYONE on that capcode receives it at the same time. It can be 50 Firemen or everyone in New York City. Please see the explanation following about how the Nextel iDen system handles a message to a group of up to a maximum of 100 users by duplicating the voice packets and then sending them to each individual user. If the cell phone systems have incorporated a method of sending a common message to everyone without having to use individual addressing, I would like to learn more about it. I am fairly certain that this cannot be done using voice. GPRS data on GSM systems (and data on other types of systems) is another matter. It may be possible on these systems—using a common signaling channel and possibly a new data device that is not a cell phone. I am sure that someone will enlighten me.
[Zach] I would like to see a million people get notified in one minute with paging. Especially during a crisis situation. With all the variables listed above and the capacity issues with one way paging when everybody is trying to use the system at once. Television, would make more sense since almost everybody has a television set in their house and almost every body in America is notified of currents events nightly. Yes, what makes us turn our TV on in an emergency? Probably the forms of communication listed in these paragraphs. What do millions of people have for their personal communication devices in America? Probably some two way device providing encrypted (free of charge), reliable, voice, data communication device of which there are hundreds of thousands of serving terminals nation wide. Cell Phones.
[Brad] I would like to see it too—that's why I am promoting the concept. Actually on an idle Paging system, one minute would be plenty of time to send a message to several million Pagers (in the same geographic area and on the same frequency). Remember only one capcode has to be transmitted, followed by one message. Even if the message contains several hundred alphanumeric characters, the whole process only takes a fraction of a second—even with the relatively slow data rates of Paging—FLEX or POCSAG. Real-world factors can stretch that minute out. Messages ahead of the emergency alert message—in queue—have to go out first—although there are ways of "dumping the queue" in some Paging systems to get around this. Also, the FLEX system "collapse value" has a direct effect on latency.
A FLEX™ pager looks for its capcode every 2x Frames, where x is the Collapse Value.
[Brad] Yes, television receivers are a great way to get the initial message out. In fact, in the near future televisions will have a special chip in them that will allow the set to be turned on (even if is off) to receive an emergency message. This is a great idea. One of the lessons learned from Katrina is that the requirements for on-going communications AFTER the event—when there is no AC power to run televisions—are very important and can only be met with a battery-operated device. The right kind of national alerting system will doubtless use several different types of equipment—even big signs along highways—like the Amber alerts that have already been deployed in many areas.
[Zach] One point I would like to mention being fair, is that there are capacity issues with cell sites. In CDMA, one T-1 and one frequency can handle about 120 voice only calls. Data will bring that number down. Yes, there is call blocking to the general public when mass amounts of customers want to make a call at the same time. However, there what they call overload classes. A priority 1-15 designated on each phone. 1 being the lowest and 15 being the highest. All police, and fire personal along with any other first response personal have a class of 15. This means, there is always some bandwidth allocated to these personal no matter how many calls are up on the cell site.
[Brad] Thanks for being fair.
[Zach] One other thing about cell phones, is that provide the domino effect. One person makes a call to another. That message get forwarded to next the person and so on. One way paging does not allow that. Of coarse two way does, but the number of two way sites is declining all the time due to lack of customers and infrastructure support.
[Brad] Even if there is no physical damage to a cell phone system or a wire-line telephone system, if everyone tries to use them at the same time (like in an emergency situation—bomb, storm, airplane crash, etc.), they will not work. These systems are designed to work at a specific grade of service with a specific percentage of the total number of subscribers during the busiest hour of the day. The "domino effect" cannot possibly alert one or more million people in the space of a few minutes. Cellphone systems can not handle that amount of voice traffic.
[Brad] The basic advantage that Paging has over telephone technologies is its store-and-forward nature. In telephony, when a call is attempted and the circuits are busy or congested, the call fails permanently and it must be attempted again later. In Paging, as long as a circuit exists to accept the call, it will be placed into queue, and even though the system latency will increase due to a large amount of traffic in times of emergency, it will be transmitted as soon as the channel is clear. This "non-real-time" or "store-and-forward" nature of Paging communications is much better for emergency communications. Of course in One-way Paging the message will be lost if the pager is turned off or out of range when the message gets transmitted, but in Two-way Paging the system will keep trying until it finally locates the pager and gets an acknowledgement back confirming that the message was successfully delivered. There would be no problem in using One-way Paging for mass alerting since the primary location of the receiving device would be on a wall in a conspicuous place and it would be clear long before the emergency that the location was good or not (for receiving messages). Daily or weekly tests could be transmitted automatically. In Israel they have found that the only way to be sure the device will work during an emergency, is to send it routine messages. It can't just sit there for months or even years like a smoke detector, it must be exercised periodically.
[Zach] Thanks Brad for taking the time to read this. Any comments will be gladly accepted and this message can posted on your website using my name. Zach
[Brad] Thank you Zach. I think this exchange of our ideas and opinions will be beneficial for anyone having the patience to read through all of this.
I have been on both sides of the fence on this topic and just decided to pass along some facts, opinions, and comments. Cellular has come a long way in the past few years.
As professionals, I know you will respond in due time with some facts, opinions, and comments as well while looking at both sides objectively. And if you don't, that's OK too. That's the great thing about America. I will still read your news letter :)
Hope all is well with you personally and professionally. Regards, Zach
How does the walkie-talkie feature on a Nextel phone work?
Nextel phones offer a service called Direct Connect that allows you to push a single button and connect with another Nextel user. This service is typically free as long as the other user is in the local coverage area. You can even specify a group of Nextel users (up to 100) that you can connect to all at once, similar to a dispatcher's radio like police or taxi companies use. This makes Nextel a very popular provider for companies with a workforce that can be spread out, such as construction work.
Nextel is unique among service providers because it has an entirely separate special cellular network that has its own frequencies and equipment in addition to the normal cell network shared with other providers. This network is based on Motorola's Integrated Digital Enhanced Network (iDEN) and makes Direct Connect possible. It uses the 800 MHz portion of the radio spectrum assigned to specialized mobile radio (SMR) service. Nextel has purchased a large segment of these frequencies in a significant number of the national and international cellular service markets.
iDEN uses TDMA technology to split a 25 KHz frequency into six separate time slots. Using a combination of half-duplex and full-duplex signals, iDEN is able to provide:
The digital two-way radio service uses a half-duplex signal. A normal cell phone call uses two separate frequencies, one to send and one to receive, for each call while a Direct Connect call uses only a single frequency. Direct Connect relies on the proven technology of Push To Talk (PTT), commonly used in dispatch radio systems. PTT requires the person speaking to press a button while talking and then release it when they are done. The listener then presses their button to respond. This way the system knows which direction the signal should be traveling in. To enable Direct Connect, Nextel configures your phone to use the dispatch call service to reach the person or persons you specify. This person (or group) must use Nextel's service also. When you make a Direct Connect call to someone, here's what happens:
As you can see, the Direct Connect feature still relies on cellular technology to connect to the recipient(s). A normal "walkie talkie" style two-way radio will only operate if the radios are within a certain distance of each other. Systems using the Direct Connect feature can communicate anywhere within the service area specified by Nextel, typically a large urban area or even an entire state.
Why Paging? — A Review
Why is One-way Paging is the BEST technology to use when it is necessary to alert many people in a short time?
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