wireless and mobile data

8/2/2019 Wireless and Mobile Data

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Ch. 14 - Wireless and Mobile Data

Characteristics

1 Wireless vs Mobile

2 Wireless Characteristics

3 Generic Wireless Network

4 Micro- and Macro Mobility

5 Mobile IP

6 Connectivity

7 Packet Switched vs Circuit Switched

8 Wireless Data Communications Systems

9 Wireless LAN and MAN

10 Mobility and Bitrate map


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Ch. 14 - Wireless and Mobile Data Characteristics

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Many people use the terms wireless and mobile as synonyms. This is not strictly true. In

this table we try to illustrate the differences between the terms wireless and mobile.

Those users that are not wireless and not mobile are easy to come up with. This is what you

can call "traditional data communications" and consists of conventional LAN, MAN and

WAN technology, such as ethernet, FDDI or X.25.

At the other extreme, there are users that are both wireless and mobile. So here we can have

taxis using radio modems, field service engineers (recall that Mobitex's first application was

for Telia's field service engineers), transport and public safety users.

What about the users that are using wireless technology but are not mobile? These are the

traditional "telemetry" applications that were mentioned in chapter 13. These include

parking or gas meters and coke machines.

The last set of users, those that are mobile but not wireless are a little harder to define.

Imagine the following scenario, I sit in my office in Gothenburg and work on my laptop PC

connected to my local network. Then I pick up my laptop, jump on a plane and fly to

Stockholm. There I plug my PC into the LAN in Stockholm and carry on working as if I was

on my home network. In this case I'm using wired technology - the two Local Area

Networks, but in some sense I'm mobile because I move around the country. This type

of access is called Nomadic Computing. An example of technology that allows this kind of

movement is Mobile IP, which will be explained in more detail later.


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Because wireless data uses an unguided medium it suffers from certain problems that do not

affect its wired counterpart. One must remember that a wireless user is constrained by the

network coverage. If the user moves to an area without coverage, then the user's

terminal becomes useless. Although network operators try to minimize these "blackspots"

they are inevitable, especially for new networks that are in the process of being deployed.

Multi-path fading, log-normal fading, inter-symbol interference and attenuation are all

characteristics of the radio medium. Coupled with the limited amount of frequency spectrum

available to wireless applications these attributes mean that the raw bit error rate is greater

than that found in guided mediums. This means that greater overheads are required to

maintain acceptable error rates, thus reducing the amount of user data that can be sent per

second.

So, wireless systems almost always offer lower bit rates than their wired alternatives.

Wireless systems also tend to have a higher latency than wired systems. For most protocols

this is not a big issue, but is something that should be addressed when setting protocol

parameters (e.g., timeouts should generally be longer due to the slower response times).


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Although all wireless networks are different they do share some common characteristics.

Here we try and illustrate those parts, which most systems share.

Most wireless applications consist of a user wishing to communicate with a server at some

remote location. The user's terminal is connected to the wireless network via a radio link to a

base station. The server is connected to a network, be it the Internet or some corporate

Intranet. Now, each base station is connected to something called a serving node. The

terminology differs depending on which mobile network you look at, but the idea is almost

always the same. Note that one serving node can be attached to several base stations. On the

other side of the diagram we have the network attached to some gateway. Finally, tying the

pieces together we have the backbone network to which both the gateway and the serving

node are connected. Note that the backbone can be connected to several serving nodes and

several gateways. Using this network a wireless user can communicate with a remote server.

Other things are also attached to the network backbone. For example, there is some kind of

subscription register to store information on the mobile users. There is also some operations

and maintenance equipment to allow network monitoring, charging etc.


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Micro and macro mobility are terms used to describe different methods of dealing with

mobility management. Micro mobility techniques are used when mobiles move between

individual base stations - micro means small and so micro mobility techniques are used for

small moves.

Macro means big, so macro mobility techniques are used for large moves between different

networks. So, for example, when you take your GSM telephone from your home network in

Sweden, and move to a new network in a different country, for example in England, you are

moving to a new network and therefore the mobility management is solved using macro

mobility techniques.

Now, you may have spotted there is a grey area in this definition. If micro mobility is

moving between base stations and macro mobility is moving between networks, what term

do we use for moving between different serving nodes?

There is no right and wrong answer here; it depends on the techniques implemented in the

network. For example, in a GSM network the same techniques are used when mobiles move

between base stations and between serving nodes. So, GSM solves the problem of moving

between serving nodes with micro mobility techniques. Conversely, PDC, the Japanese

mobile telephone standard, uses macro mobility techniques for mobiles moving between

serving nodes, so moving from one serving node to another and moving from one network to

another both use the same technique. It's simply a question of how the network is

standardized.


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What is Mobile IP? Well, it's a macro mobility technique that is it can be used to allow

movement between two interconnected networks. In this example we are using Mobile IP to

allow a portable user to move between two different LANs interconnected by the Internet.

Imagine this scenario. You normally work in you lovely air-conditioned office in Dallas. One

day your boss informs you that you have to go to Ericsson Stockholm to give a

presentation. You spend a few hours preparing the presentation, which remains on a

server on your home network. You then take your lap top computer across the Atlantic to

your meeting in Stockholm. There you plug in to the LAN. Without having to change any

configuration parameters or addresses, you can immediately access your home LAN in

Dallas and can easily give your presentation. Also, external users can still contact you with

your original IP address, as that has moved with you. This transparent mobility is the job of

Mobile IP.

So, how does it work? Well, conceptually it's quite simple. Here you can see your home

network in Dallas and your foreign network in Stockholm. When you are on your home

network you are addressed using the IP address 193.234.210.74. Users on your own

network or on other networks connected to the Internet can talk to your computer using that

IP address, 193.234.210.74. The router on your home network ensures that packets bearing

your IP address are delivered to you.

Now, what happens when you pick up your laptop, spend 10 hours on a plane and arrive at

Ericsson in Stockholm? Well, when you plug your laptop into this new foreign network

your laptop talks to the router on the foreign network, which is called the Foreign Agent.


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Your laptop basically sends a message to the Foreign Agent saying, "Hi, I'm laptop

193.234.210.74 and I'd like to use your network." The Foreign Agent then sends a message

to the router on the home network, the Home Agent, and says "The mobile with IP

address 193.234.210.74 isn't on your network anymore, but it can be reached by sending

packets to me at IP address 193.17.213.64." The Home Agent accepts this request and then

we're ready to roll.

So, what happens when some host out on the Internet starts to send packets to your

laptop? Well, the routers on the Internet have no knowledge of your boss's decision to send

you to Stockholm, so they forward the packets to your home network as usual. The router

on your home network, the Home Agent, sees these packets and remembers the care-of-

address it received from the Foreign Agent. So, it takes your IP packets and sends them

through an IP tunnel over the Internet to the Foreign Agent. The Foreign Agent looks at the

packets and thinks, "This IP address isn't on my network, but I remember receiving

information about it, so I can forward it on directly." In this way packets reach the mobile

node.

What happens when the mobile node wishes to send packets out to the Internet? Well, this

causes no problem whatsoever, the laptop builds IP packets with the desired destination

address and the usual source address (that is, 193.234.210.74) and sends them directly out to

the Internet as usual.

Of course Mobile IP isn't perfect. One of the biggest drawbacks is the routing inefficiency it

introduces. Packets destined for our laptop pass through the Internet to our home network

and then back out onto the Internet to our foreign network. This doglegged routing means

that received packets suffer from higher latency than normal. It also means that the Internet

is loaded more as the same packets must pass through it twice. There are also some

important security implications to consider with Mobile IP. The mobile node must

be carefully verified to ensure that packets are not forwarded to impostors intent on stealing

others information.

Mobile IP is only one of a host of macro mobility techniques. Others, such as Dynamic

DNS, L2TP and IP security are also capable of solving the problem of nodes moving

between interconnected networks.


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In most cases a mobile node wishes to talk to some remote computer. For users and

applications, the way these computers communicate doesn't matter that much. Using a wired

or wireless network makes no difference as long as the packets arrive. This is why these

industry standard protocols, such as TCP and IP, are used. If the applications see some

standard interface then they don't need to know if they are using a wireless network or a

fixed network, they just see a standard interface. These standard interfaces are extremely

important to both application and network designers.


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It is extremely important to understand the difference between packet switched and circuit

switched systems, a difference, which is illustrated here.

You can compare a circuit switched data to a train and the physical medium is the train line.

Before the train (data) can use the line it has to ask and be granted access. Once this is done

the train can then proceed on the line, but it has exclusive access to the line. No other trains

can use the line while our train is there. In a data communications environment,

circuit switchings main drawbacks are the call setup overhead (that is, asking for and being

granted access to the line) and the low utilization of the physical medium (only one train at a

time). One advantage of circuit switching is that the call setup procedure allows users to

be allocated and guaranteed a certain bandwidth. A good example of circuit switching is the

analogue telephone network. Before you can talk to anyone you have to dial their number (a

call request). The call is physically switched through the network until you had an end-to-

end circuit between your telephone and the telephone you are calling. Only when the phone

is answered at the other end (a call accept) can communication begin.

What about packet switching? Well, in this case our data can be imagined as cars and the

physical medium as a motorway.

Instead of sending one big chunk of data, like we did with a circuit switched train, we send

smaller chunks of data, with many users on the same channel simultaneously. The advantage

with this is that we don't need any call set-up procedure, we just jump in the car and go.

Packet switching also gives a much higher channel utilization, because many people can use

the same motorway simultaneously.


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The disadvantage is that because we have no way of knowing how much data other users are

going to send there is no guarantee of bandwidth if too many people try to use the

motorway at the same time it will slow down and eventually stop. The best example of a

packet switched network is the global Internet.


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There are lots of different wireless data communication systems. To help us understand

them all we divide them into different categories, Local Area Networks, Metropolitan Area

Networks and Wide Area Networks. Some of the techniques used for wireless LANs and

MANs are shown here, but we'll discuss those in a little more detail in the next slide.

Wide Area Networks have been divided up into two distinct groups here circuit switched

and packet switched. On the circuit switched side are Satellite phones, the American mobile

telephone standard AMPS, GSM and PDC. HSCSD is a new data oriented circuit switched

service for GSM, which will be discussed a little more in chapter 16.

The oldest of the packet switched services is Mobitex; a dedicated packet switched network

that has been available since 1986.

The other three systems are conceptually very similar, they all build on existing circuit

switched mobile telephony infrastructures and they all add a packet switched service. GPRS

provides packet data services by building on the GSM infrastructure, PPDC provides packet

data services by building on PDC and CDPD provides packet data services by building on

the AMPS system.


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In the wireless LAN marketplace there are two main solutions, using infra-red or radio

waves. None of these solutions pretends to offer mobility - they merely offer a slightly more

flexible way of coupling your desktop computer to the LAN. In some cases this

wireless flexibility can be very desirable - laptop users do not need to continually plug andunplug network cables, they can simply move into the covered area and start working. In

some older buildings, pulling cable to every desktop can be expensive, so using a wireless

solution can save money. Of course, the usual drawbacks apply - the bit rate is much lower

than that offered by other LAN technologies. Typically IR and RF based LANs offer a bit

rate of about 4 Mbps. Compared with Ethernet, which is over 25 years old and yet manages

10 Mbps, that can be a considerable drawback. IR based nets are also further limited by the

fact that their connections must be "line-of-sight" based. This is not a problem for RF based

LANs.

An interesting solution to the MAN question has been developed by an American companycalled Metricom. They've developed a wireless system that breaks away from the usual

wireless network topology that we saw in chapter 14.3. Instead of building a hierarchical

system consisting of base stations, serving nodes and a backbone, Metricom build a semi-

intelligent mesh consisting of small boxes that sit on top of street light and utility poles. One

or more of these pole top boxes is equipped with a fixed connection to the global Internet.

When a user sends data it goes to the nearest pole top box. This box then independently

routes the information to a box that has a fixed connection. All this routing is done

automatically by the meshed network, there is no human intervention and no need for

hand crafted routing tables.


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Metricom's system currently offers a bit rate of 38 kbps, a speed they hope to increase to

56 kbps some time in 1999.

They offer a public Internet access service in three American urban areas, the Bay Area

in California, Seattle and Washington DC. Metricom's solution is also being used by several

campus based networks and by Sun Microsystems (among others) to cover their corporate

campus. The system uses the license free portion of the RF spectrum and utilizes a

patented frequency hopping technique to provide security and maintain good link quality.

The pole top boxes are positioned every 400 to 800 meters and take approximately 5

minutes to install. Metricom believe that their system has almost unlimited scalability, and

could cover the whole of the states if necessary.


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This diagram allows you to compare the systems we've mentioned in this lesson. The

horizontal-axis uses a logarithmic scale to show the bit rate a system offers. The vertical-axis

shows the amount of coverage a system offers, from a single room to the whole world.

As you can see, those systems with the lowest mobility, the wireless LAN solutions, offer

the highest bit rates, whilst those with higher mobility, such as GSM, offer lower bit rates.

The red lines indicate the future developments. For example, GSM currently offers coverage

spanning the globe and a bit rate of 9.6kbps. Enhancements, such as GPRS and HSCSD will

increase this bit rate.

AMPS offers coverage in several countries across the world with a bit rate similar to that

found in GSM. Future enhancements, such as circuit switched data, will offer higher bit rates

than are currently available. Metricom's system, which was discussed earlier offers a higher

bit rate than current GSM or AMPS systems, but only provides mobility at the

metropolitan level.

Universal Mobile Telecommunications System, UMTS, will offer a much higher bit rate than

any of todays systems. Because of the expense of introducing mobile networks, the first

implementations will offer patchy coverage. As time progresses, hopes are that UMTS

will cover the globe.

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