Cellular Network Components

Home , Cellular network, Wireless network

Table of Contents

Cellular Network Technologies -1 ...... 2

Cellular Network Technologies -2 ...... 6

Cellular Network Technologies -3 ...... 9

Cellular Network Technologies -4 ...... 11

Notices ...... 13

Page 1 of 13 Cellular Network Technologies -1

Cellular Network Technologies -1

Cellular Signal Considerations • Base station transmitter size and power • Antenna strength of low-powered mobile devices • Channel availability within cell • Wireless Network Standard — GSM ~ 20 miles — CDMA ~ 40 miles • Signal Attenuation — Contributing environmental factors

**032 Cellular Signal Considerations. So what is the size of the base station? How much power does it have? How big is it? How big are the antennas it's using? What's the antenna strength of the device connecting to the network? The signal strength to my laptop, the Wi-Fi in my laptop, is probably much stronger than the signal strength to my cellular phone. Because the computer allows for a bigger antenna to be placed inside of it. Whereas with a phone, you only have a certain area where the antenna can place- be placed.

Page 2 of 13 In addition to that, we have stuff like Apple's Antennagate. When the iPhone 4 first came out, and they realized if you hold it a certain way you actually block the antenna. Well that's because the antenna was placed on one of the sides of the phone. So if you're holding the phone, the antenna is covered. And the original thing they came out with was don't hold the phone that way. How else am I supposed to hold the phone?

So eventually they came out with yes, we'll give you a free case to use with your phone to prevent you from blocking the cellular signal by holding it there.

There's a lot of thought that comes into these devices; and where antennas are placed in it, and how many antennas are placed in it. And the FCC actually does tests to make sure the antenna isn't transmitting on frequencies it shouldn't be transmitting on. It can get very indepth.

Channel Availability. So how many people are already on the network? Is there enough space to add another person to it? If not, what can we do to get more people on this- or more people on our network?

We might have to add a cell tower. We might have to add something like a picocell or a femtocell; which I'll discuss in a second.

Wireless Network Standards. GSM in perfect conditions-- flat area, nothing blocking the signal-- can reach

Page 3 of 13 approximately 20 miles. CDMA actually has no upper limitation; but in the real world you're probably only going to be able to see one tower get up to 40 miles away.

Again, if you're in New York, if you're in San Francisco, you might have a tower every quarter mile, every half mile. It just depends on how many users are there and how much the signal is blocked by other buildings; or hills for that matter.

Signal Attenuation. Again, environmental factors determine how far the signal can reach.

Now picocells and femtocells are actually very interesting devices. So a cellular tower is considered a macrocell. It provides a signal to a very wide range or wide area.

A picocell is a step down where it might provide a signal to one building. A former company I worked at actually had a picocell for that building because the walls were so thick the nearby cellular tower couldn't reach inside the building. So they had to get a localized cellular antenna to cover just the building.

My house is the perfect place to do cell phone investigations because it acts like a Faraday cage almost; where I swear I go outside, I have five bars. As soon as I walk in the house, it's zero to two bars. Something to do with the material used in the house; line of sight, something, prevents the cell signal from getting in the house.

Page 4 of 13 I'm a Sprint customer. I called Sprint and said, "I have no service in my house. I don't want to switch from you but what can I do in order to get a signal?" And they actually recommended I buy a femtocell; which is a small little box-- it almost looks like a router-- that I connected to my wired network, and within my house I now have full connectivity. So I connect to it. It connects over the LAN to Sprint's network, and I now have five bars in the house. So I can actually send and receive phone calls.

I think they charge like five dollars a month. They add like a phone line charge in order for me to use it. But for me it was worth it because I didn't want to necessarily switch from Sprint service.

I would expect going forward these type of devices will become more and more ubiquitous. I would not be surprised to see office buildings and malls be designed with those devices to remove some of the issues we see with saturated towers. So now we have them on a localized tower. So you now have more bandwidth available for the full tower.

Page 5 of 13 Cellular Network Technologies -2

Cellular Network Technologies -2

Cell towers and mobile devices use low-power transmitters to contain signals within cell area. • No signal overlap - same frequencies can be used in other cells • Calls are “handed off” to adjacent cell towers as user moves in and out of range 832 radio frequencies provide 395 full-duplex voice channels. • Allows two users to talk at the same time by using one channel for each participant in the conversation

**033 They use low- cell towers and mobile devices use low power transmitters to contain the signal within the cell area. There's not supposed to be any signal overlap between cellularareas.

The calls are handed off to adjacent towers as users move from or in and out of range of a specific tower.They don't want it to be where you're on the border of two towers and your phone keeps swapping between the two towers. Because then you're going to start draining your battery as it switches between one and the other. It wants it to pick one, and itwaits until your signal attenuation becomes a certain level before it switches over to the other one.

Page 6 of 13 There's 832 radio frequencies that provide 395 full-duplex voice channels.

So again, the precursor to the cellular technology was really the walkie- talkie technology where you might have one antenna in an area, and that antenna could cover somewhere between 10 and 20 miles, with mobile device units connecting to that tower.

That tower did not connect normally to any other towers in the area. So you could only talk to other people on your walkie-talkie that were on the very same tower. And it was half-duplex. So one person got to talk and then the other person got to talk; and back and forth, until your conversation was concluded.

Well with cellular technology, they have many more frequencies, and each conversation uses two of the frequencies. That's where we come up with the 395 full-duplex. So each phone conversation is actually using a send frequency and a receive frequency.

It allows two users to talk at the same time. Well I was going to draw something but the thing is blanked out and I don't see anything. So I'll draw it later.

The way this works is the cells are normally shaped like a hexagon-- so six sides-- with a tower in the middle; it covers that area. And each tower only uses one-seventh of the available frequencies.

Page 7 of 13 And then there's six more cells surrounding it. Each of those use a different one-seventh of the frequencies. That way, as I'm moving between cells, my phone actually- my phone conversation actually switches to a new frequency as it gets to the new tower. It's how they prevent bleed over. That's how they allow as many users as possible to talk using only these 395 frequencies.

But during the handoff process, that's when you're more than likely dropping conversations. So as you're driving down I-10 let's say-- which is the freeway that goes from Los Angeles to Georgia, I believe-- you're going between different cellular regions. If you're on a phone call, the weakest point of your phone conversation is going to be right at the point where you're handing off from one tower to the other. And that's normally when you're going to drop a phone call.

Page 8 of 13 Cellular Network Technologies -3

Cellular Network Technologies -3

The BSC (Base Station Controller) manages hand-offs between cell towers. • Mobile user can move from cell to cell without session loss. • Cell tower automatically selects a frequency for mobile device in adjacent cells. • Switch to new frequency is done upon entering cell without interruption in service. • Mobile devices use low power to prevent overlap into adjacent cells.

**034 So what are the technologies that actually make up this hand off process?

The first piece you have is the base station controller. It actually manages the handoff between cell towers. Mobile users can move from one cell to another, and they're not supposed to lose the session. Normally it works. But if you're going to a new tower and that tower is saturated already, you might get there and your call gets dropped because it just can't handle any more additions.

Page 9 of 13 They automatically select which frequency your device is going to use. It switches to the new frequency when you enter the new cell; when your signal attenuation to the old tower becomes weak enough that the new tower is required to pick up the new cellular- or the cellular call.

And they use low power to prevent overlap into other cells. So you don't want your device to have such a strong signal that it never releases from the tower you're leaving. Plus if the signal's too strong, it's actually using more battery, and it's probably going to run out of power much sooner.

Page 10 of 13 Cellular Network Technologies -4

Cellular Network Technologies -4

The MSC (Mobile Switching Center) routes calls and data to the PSTN (Publicly Switched Telephone Network) for landline access. • The BSC controls the link between the cell tower and MSC. Sat phones communicate with cell towers via satellite uplink.

Base Station / Cell Tower Base Station / Cell Tower

BSC MSC PSTN

**035 Then we have the Mobile Switching Center. The Mobile Switching Center routes calls to the publicly switched telephone network, or the wires you see along the side of the road.

So if I'm making a call to a landline, or if I'm making a call to somebody in another city, that call does not travel wirelessly the entire way. It will hit the MSC-- there's usually one per city. I'm guessing bigger cities could have more than one. I have not verified whether that's true or not. It switches there and then the MSC then routes it to a landline.

Page 11 of 13 It gets to the MSC on the other side; and then it goes back on to the cellular infrastructure, if you're calling another cell phone.

The BSC actually controls the link between the cell tower and the MSC. I know there's a lot of acronyms, and they all sound alike. I didn't invent the cellular infrastructure.

Satellite phones communicate with the cell tower via satellite uplinks. So you have your satellite phone with your little satellite unit. It bounces off a satellite. It hits another sat or it-- yes it hits a satellite dish on the other end and then gets routed on to the Mobile Switching Center. And that's how you make a call with a satellite phone.

Page 12 of 13 Notices

Notices

This material has been approved for public release and unlimited distribution except as restricted below. This material is distributed by the Software Engineering Institute (SEI) only to course attendees for their own individual study. Except for the U.S. government purposes described below, this material SHALL NOT be reproduced or used in any other manner without requesting formal permission from the Software Engineering Institute at [email protected].

This material is based upon work funded and supported by the Department of Defense under Contract No. FA8721-05-C-0003 with Carnegie Mellon University for the operation of the Software Engineering Institute, a federally funded research and development center. The U.S. Government's rights to use, modify, reproduce, release, perform, display, or disclose this material are restricted by the Rights in Technical Data-Noncommercial Items clauses (DFAR 252-227.7013 and DFAR 252-227.7013 Alternate I) contained in the above identified contract. Any reproduction of this material or portions thereof marked with this legend must also reproduce the disclaimers contained on this slide. Although the rights granted by contract do not require course attendance to use this material for U.S. Government purposes, the SEI recommends attendance to ensure proper understanding. NO WARRANTY. THE MATERIAL IS PROVIDED ON AN “AS IS” BASIS, AND CARNEGIE MELLON DISCLAIMS ANY AND ALL WARRANTIES, IMPLIED OR OTHERWISE (INCLUDING, BUT NOT LIMITED TO, WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE, RESULTS OBTAINED FROM USE OF THE MATERIAL, MERCHANTABILITY, AND/OR NON-INFRINGEMENT). CERT® is a registered mark of Carnegie Mellon University. .

Page 13 of 13