wireless and mobile data

1 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 Ch. 14 - Wireless and Mobile Data Characteristics 2 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. Ch. 14 - Wireless and Mobile Data Characteristics 3 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). Ch. 14 - Wireless and Mobile Data Characteristics 4 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. Ch. 14 - Wireless and Mobile Data Characteristics 5 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. Ch. 14 - Wireless and Mobile Data Characteristics 6 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. Ch. 14 - Wireless and Mobile Data Characteristics 7 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. Ch. 14 - Wireless and Mobile Data Characteristics 8 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. Ch. 14 - Wireless and Mobile Data Characteristics 9 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. Ch. 14 - Wireless and Mobile Data Characteristics 10 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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Switched 8 Wireless Data Communications Systems 9 Wireless LAN and MAN 10 Mobility and Bitrate map Ch. 14 - Wireless and Mobile Data Characteristics 2 Many people use the terms wireless and mobile. 1 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. 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