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Wireless Networks and Mobile Networks

Beginning and Evolution

This article describes the development of wireless networks and mobile, its evolution, characteristics that differ from wired networks, but it describes the current wireless technologies (2008). We will highlight the evolution of mobile Internet access, pointing out the key technologies present in each generation, and managing mobile networks and mobility offered by wireless networks today.

1. Advancement of Wireless Networks and Mobile Networks

In recent years, the wireless equipment for Internet and mobile services such as mobile phones, grew rapidly, given the emergence of new technologies in this area, it became clear that wireless networks and mobile services they enable, have taking an increasingly larger over the years due to the immense benefits provided by them.

However, there are still many challenges posed by these networks, but there is no doubt that this is a great development and should become even more in coming years.

2. Elements of a Wireless Network

Unlike wired networks (wired), wireless networks provide the existence of mobile hosts, despite the technical challenges that arise in wireless networks and mobile networks, many of those not addressed in wired networks. According Kurose [3], a wireless network has as elements, hosts that may or may not be mobile and wireless links, base station and the infrastructure network.

2.1. Host Wireless

The equipment of end systems that run applications, as well as wired networks, however wireless host can not be mobile phones and PCs, or mobile such as laptops, PDAs and palmtops.

2.2. Wireless Links

They are the means by which end systems connect to each other or to a base station, may have different transmission rates and can transmit at different distances. However, there is the possibility of occurring bit errors caused by different reasons which will be demonstrated below.

2.3. Base Station

It is responsible for sending and receiving data to and from a wireless host associated with it, is responsible for coordinating the transmission of this host. Base stations can be cell towers (in cellular networks) and access points (in a wireless LAN). A host may be associated with a base station in two ways:

It is within the reach of wireless communication base station;

It uses the base station to relay data between it and the larger network.

A host mode operates in the infrastructure when it is associated with a base station, thus all basic services are provided to it by the network to which it is associated. The hosts may make transfers (handoff), changing its point of connection to the larger network when moving out of range of a base station.

2.4. Infrastructure Network

It is the larger network with which a wireless host may want to communicate.

3. Features Links and Wireless Networks

There are important links between wired and wireless, that may be related to reduced signal strength, interference from other sources, multipath propagation or multipath can distort the signal received by the recipient.

Physical obstructions in the environment such as buildings or mountains can prevent a listen to another station during a broadcast station, this is caused by the weakening of the strength of a signal as it propagates through the wireless problem known as hidden terminal.

3.1. CDMA Protocol

The multiple access code division (CDMA) is a type of access protocol to shared media, and technologies prevalent in wireless LANs, and also used in cellular technologies. In this protocol each bit being sent is encrypted by multiplying the bit by a signal (code) that changes at a speed much higher than the original sequence of data bits, this speed is known as a chipping rate.

The CDMA protocol is a partition, that is, it breaks the code space and assigns each node a dedicated portion of the space code.The CDMA codes must be chosen carefully so that CDMA receivers can extract the signal of a code, considering the difficulty in getting the forces of the signals received from multiple transmitters are the same. The CDMA protocol will be discussed again later in section seven, when we'll discuss the cellular access the Internet.

4. IEEE 802.11

With the advancement of communications over the last few years, several technologies seeking to meet real needs of users, the wireless started gaining ground since the use of satellites to personal use, the latter was considered an expensive technology but the time is becoming increasingly cost-effective existing ones. Believing in this new technology the IEEE (Institute of Electrical and Electronics Engineers) has established research groups for this technology becomes a reality more concrete, born in 1990 and the IEEE 802.11 standard that had its beginnings in the low speed, but over time their speed was increasing along with the investments.

Currently the focus of wireless technology is the WLAN (Wireless Local Area Network), despite the significant increase in the rate of data transfer, which rose from a few kilobits per second to 2 Mbps, WLANs not meet satisfactorily the need for bandwidth business . Thus, the IEEE has invested in improving the 802.11 standard (which became known as 802.11b), with the same architecture and technology, but with throughput of large data between 5 and 11 Mbps, instead of pushing the technology and stimulating the scientific and industrial communities to standardize, to design and produce products for these networks. There are several versions of IEEE 802.11, in the sections below discuss the most common.

4.1. 802.11a

Can reach speeds of 54 Mbps within the standards of IEEE and 72 to 108 Mbps by non-standardized. This network operates at a frequency of 5 GHz and initially support 64 users per Access Point (AP).

Its main advantages are speed, free of charge frequency that is used and the absence of interference. The biggest drawback is the incompatibility with standards regarding the 802.11 b and g access points as well as customers, the 802.11a standard is compatible with both 802.11b and 802.11g in most cases, already becoming standard in the manufacture of equipment.

4.2. 802.11b

It reaches a speed of 11 Mbps standardized by the IEEE and a speed of 22 Mbps offered by some manufacturers not standardized. Operates in the 2.4 GHz frequency initially support 32 users per access point.

A weakness in this standard is the high interference for transmitting and receiving signals, because they run at 2.4 GHz equivalent to mobile phones, microwave ovens and Bluetooth devices. The positive aspect is the low price of their devices, the bandwidth free and the free availability around the world. 802.11b is widely used by providers of wireless Internet.

4.3. 802.11g

It is based on compatibility with 802.11b and offers speeds of 54 Mbps. It works within the 2.4 GHz frequency has the same drawbacks of the 802.11b standard (incompatibility with devices from different manufacturers). The advantages are also the speeds.

Uses static WEP authentication already accepting other types of authentication as WPA (Wireless Protected Access) encryption with dynamic (method of TKIP and AES). It is sometimes difficult to configure, such as Home Gateway because of its radio frequency and other signals that can interfere with the transmission of the wireless network.

4.4. 802.11n

In the final stages of approval, operates in the bands of 2.4 Ghz and 5Ghz, promises to be the standard for wireless media distribution, it will offer, through the MIMO (Multiple Input, Multiple Output - which means multiple inputs and outputs), higher rates transmission (up to 300 Mbps), more efficient signal propagation (with a coverage area up to 400 meters outdoor) and a large backward compatibility with other protocols. The 802.11n meets both the needs of wireless transmission standard for HDTV, as a highly shared, business or not.

4.5. 802.11v

It is the standard management of wireless networks for the IEEE 802.11 family, but is still in the initial proposals. V The Task Group of IEEE 802.11 (TGV), the group responsible for defining the standard 802.11v, is working on an amendment to the 802.11 standard to allow the configuration of client devices connected to 802.11 networks. The pattern may include management paradigms similar to those used in cellular networks.

5. IEEE 802.11 Architecture

The IEEE 802.11 standard defines an architecture for wireless networks, based on the division of the area covered by the network in cells. These cells are called the BSA (Basic Service Area). According to Smith [6] the size of the BSA (cell) depends on the characteristics of the environment and the power of the transmitters / receivers used in the stations.

Access Point (Access Point - AP): special stations are responsible for the capture of broadcast stations carried by the BSA, to other stations located in BsAs, relaying them, using a distribution system;

Distribution system: it represents an infrastructure of communication that connects multiple BsAs to allow the construction of networks covering areas larger than one cell;

ESA (Extended Service Area - or Extended Service Area): represents the interconnection of various BsAs by the distribution system through the AP;

ESS (Extended Service Set - or Extended Service Set) is a set of stations formed by the union of several BSSs connected by a distribution system.

Figure 1 - Union of two BSS forming an ESS

The network ID in the following way: each of the ESSS receives an identification called ESS-ID, within each ESSS, each BSS receives an identification called BSS-ID. Then, the set formed by these two identifiers (the ESS-ID and BSS-ID), form the Network-ID of a wireless network standard 802.11.

The distribution system, and interconnect multiple access points can provide the resources needed to interconnect the wireless network to other networks, and he, the distribution system is usually represented by a system of wired (copper or fiber).

When there is the existence of APs in a wireless LAN, we call wireless LANs infrastructure, because there is some form of central control devices in that network, though each device can be grouped with others around you and / or no Internet connection forming an ad hoc network. A key element in the architecture of wireless LAN infrastructure with the access point to Smith (1995) [6] it performs the following functions:

Power Management: allows stations operate saving energy through a so called power save;

Synchronization: ensures that the stations associated to an AP are synchronized by a common clock;

Authentication, association and reassociation: allows a mobile station even going out of their cell of origin remains connected to the infrastructure and do not lose communication.

According Kurose [3] to create an association with an AP, a station must authenticate a number of ways the most commonly used by businesses are allowing access via MAC address that another way is to use user name and password in all cases the AP must communicate with an authentication server that is responsible for verifying if this station can be associated with this network, and this is accomplished through protocols such as RADIUS or DIAMETER. According to Alencar [1] the function that allows you to maintain continuity of communication when a User is a cell to another is called handoff.

5.1. 802.11 MAC Protocol

After a successful association several stations can begin to send and receive data simultaneously, so there must be some protocol that controls the transmission and receipt of data frames. The IEEE has defined a protocol for media access (MAC sub-level data link), called DFWMAC (Distributed Foundation Wireless Medium Access Control), which supports two access methods, a basic distributed method, which is mandatory and a method centralized, which is optional, these two methods can coexist if the IEEE 802.11a protocol medium access of 802.11 networks also addresses problems with stations moving to another cell (roaming) and stations lost (hidden node ).

The method of distributed access is the basis upon which we built the centralized method. The two methods, which can also be called coordination functions (Coordination Functions) are used to support the transmission of traffic asynchronous traffic delayed or limited (time bounded). A coordination function is used to decide when a station is allowed to transmit.

In the distributed coordination function (Distributed Coordination Functions - DCF), that decision is made by individual points of the network and can thus collide. In the role of centralized coordination, also called the function point (Point Coordination Function - PCF), deciding when to transmit is centered on one particular point, a station with frames to transmit, should feel free medium for a minimum period of silence , IFS (Inter Frame Space), before using it. Using different values for that period. The DFWMAC sets three priorities for media access:

Distributed Inter Frame Spacing (DIFS) - space between frames of DCF (Distributed Coordination Function), this parameter indicates the largest delay time, so the lowest priority, it monitors the environment, waiting at least an interval of silence to convey data ;

Priority Inter Frame Space (PIFS) - the space between frames (PCF Function Coordination Point), a lag time between the DIFS and SIFS (medium priority), is used to access services delayed, or an access point controlling other we just need to wait a PIFS time to access the medium;

Short Inter Frame Space (SIFS) - is used for transmission of frames carrying immediate responses (short), as ACK that have the highest priority.

Represents the method for basic access protocol DFWMAC. Is a function known as CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) with recognition. The DCF function works similarly to CSMA / CD technology wired LAN (Ethernet standard 802.3), with only one difference: the CSMA / CD Ethernet control collisions when they occur, while the CSMA / CA standard without wire just try to avoid collisions.

In the method CSMA / CA collisions can occur and this method does not guarantee correct delivery of data. Thus, one station after transmitting a frame, you need a return receipt (ACK) to be sent by the destination station. The destination station sends the receive signal after waiting only for a SIFS time, if such notice does not arrive in time considered, the source station performs re-transmission of the frame.

To improve data transmission, the protocol adds DFWMAC method CSMA / CA with recognition, an optional mechanism that involves the exchange of control frames RTS (Request To Send) and CTS (Clear To Send) before transmitting data frames . This mechanism works as follows:

1. A station, after waiting for DIFS time and their removal random before actually transmitting the data frame, transmits an RTS control frame, which carries an estimate of the length of time in the future transmission of data frame and ACK to this one but the recipient of the transmission of data to come;

2. The destination station to receive the control frame RTS hit its Vector Allocation Network (NAV). NAV specifies when a station tries to access the medium again. In response to the RTS, the receiver sends a CTS control frame stating that it is ready to receive the data frame. The CTS informs the other stations on the transmission to take place, making them also acertem their NAVs. Now, all stations are informed about the transmission and will wait to access the medium, we can then say that the RTS and CTS reserve medium for transmission. Only then, the transmitting station sends the data frame after SIFS, which must be answered with an acknowledgment (ACK) sent by the receiving station;

3. In this case, a collision will happen only if two stations send an RTS at the same time. The RTS should only be used when we have a bigger picture, because its use entails a network overhead (overhead).

This is an optional feature that can be inserted in the protocol DFWMAC has been built on a distributed coordination function (DCF) for asynchronous transmission frame, and is implemented through a mechanism for orderly access to the medium that supports the transmission of traffic delayed or limited asynchronous traffic.

For the integration of these two functions - punctual and distributed - we use the concept of superblock, so that the protocol can work in a way that the function off to take control of the transmission, to prevent the occurrence of collisions. For this, the protocol DFWMAC divides time into periods called superblocks, which consists of two consecutive time intervals, which are used as follows:

1. In the first half, controlled by the PCF, the access is ordered, which prevents the occurrence of collisions, after waiting PIFS, the point of coordination leads to the first station, which can respond after SIFS. After waiting more SIFS, the coordinator gives time for the second season and so on. When a station does not respond after SIFS, the coordinator waits for PIFS and passes the turn to the next;

2. In the second half, controlled by the DCF, the access is based on the ownership dispute over the medium, collisions can occur;

5.2. MAC frames

The frame MPDU (MAC Protocol Data Unit) of the MAC sublayer is composed of the following basic components:

MAC header, contains control information frame, duration, address and control information sequence;

Body frame size: variable contains information specific to the type of frame;

Information redundancy FCS (Frame Check Sequence).

Control framework: The first two bytes indicate the protocol version, frame type (control, data maintenance), if the frame was broken or not, privacy information and two bits of the distribution system.According to these two bits, the address fields acquire different meanings;

Duration ID: Used to reserve a virtual environment, using RTS / CTS. This field indicates the length of occupancy of the transmission medium;

Addresses 1 to 4: Each of these fields contain default MAC address (48 bits) as well as in other 802.x LAN.Its meaning depends on two bits of the distribution system;

1 The address is the MAC address of the station that should receive the framework;

2 The address is the MAC address of the station that sent the frame;

3 The address is complex because it contains the MAC Router hours when the station is sending a framework for its network, and then return in the table contains the MAC train station which the response frame belongs.

Sequence Control: Used to filter frames that can eventually be duplicated.

The field of control sequence is used to sequence the order of sending pictures, is analogous to TCP and delivering "packages" in this case tables in the order must be received by the receiver.

Data: They range from 0 to 2312 octets;

CRC: 32 bits of error correcting codes common to all LANs 802.x.

5.3. Roaming

According to Alencar [1], Roaming is an important feature of wireless communication. Allows stations to change the cell and continue sending and receiving information, the function of roaming works as follows:

The station, to realize that the quality of the current connection to your access point is very poor, starts searching for another access point;

The station then chooses a new access point based, for example, in signal strength, and sends a request to join the cell of this new access point;

In cell visited the AP that will determine whether the mobile station visits had not previously registered.If this procedure has not been made, that AP will inform the AP of the cell origin on the new position. The new AP sends a response of accession, and the station has belonged to this new BSS;

Thus, the AP cell origin learns the new position of the mobile station, and sends the information intended for them, as if such station was in its own cell.

5.4. Seasons Lost / Hidden

A major problem in wireless networks occurs when a station is incommunicado for a period of time with the AP. There are several reasons why this occurs. The shutdown of the mobile station, the output of the mobile station's area of operations of AP or entry of the mobile station in an area where radio waves from another place does not spread or areas with high signal degradation that can be grounds geographical or environmental.

MAC protocol handles the problem of lost seasons as follows:

When trying to communicate with mobile stations several times with no reply, the AP sends a request for communication for all other mobile stations within its coverage area.Each of these sends a request for communication to the lost season, this in turn sends a response notification to all stating that it is active;

Stations that hear this communication bridge sends a request directly to the AP, which may well find the best option of communication between the AP and the station lost.

If the AP goes down with the bridge and the bridge goes down with the season lost, the AP chooses another bridge between the stations that responded initially. With this method, the AP has a chance to recover a season, which for some therefore became incommunicado with the network.

6. Bluetooth

Bluetooth is an industrial specification for areas of wireless personal area networks (wireless personal area networks - PAN).Bluetooth provides a way to connect and exchange information between devices such as mobile phones, laptops, computers, printers, digital cameras and game consoles via a digital radio frequency short-range unlicensed and globally secure.

It is a standard communication protocol primarily designed for low power consumption with short range (depending on the power: 1 meter, 10 meters, 100 meters) based microchip transmitters low cost on each device. Bluetooth enables these devices to communicate with each other when they are within the range. The devices use a system of radio communication, so do not need to be in line of sight of each other, and can stand up in other environments, as long as the received transmission is powerful enough.

6.1. Bluetooth versus Wi-Fi Network

Bluetooth and Wi-Fi have slightly different applications in offices and homes today, and during the move: setting up networks, printing, or even transferring presentations and files from PDAs to computers.

Wi-Fi Bluetooth differs because it offers high transmit power and covers large distances, but requires more expensive hardware and robust with high energy consumption. They use the same frequency of transmission, but employ different multiplexing schemes. While Bluetooth is a substitute for cable in a variety of applications, Wi-Fi is a cable replacement only for local network access.

7. Cellular Access to Internet

Just as occurred with the fixed (wired) mobile network was extended in order to support not only voice telephony but also wireless Internet. Thus it became possible connection with a fairly high speed and seamless mobility and continuous, allowing access to the Internet while moving, for example, on a bus or a train. This was only possible due to the fact that the phone is present everywhere, in many areas worldwide.

The term cell refers to the fact that a geographical area is divided into several areas of coverage, known as cells. Each cell contains a base station that communicates with mobile stations (sending or receiving signals) within its cell. The range of these signals depends on several factors, such as transmission power of base station transmit power from the mobile station, obstacles such as buildings, in the cell, and height of the antenna base station.

Each base station connects to a central switching unit mobile (Mobile Switching Center - MSC), which manages the establishment and termination of calls to and from mobile users. MSC contains a lot of the features found in a central telephone switching normal, but extended with additional functionality required to manage the mobility of their users [2].

7.1. Approaches FDM / TDM and CDMA

For a cellular service provider can serve multiple users at the same time, it is necessary to share their portion of the radio spectrum. The two approaches are used: (1) A combination of multiplexing frequency division multiplexing (FDM) multiplexing and time division (TDM) and (2) multiple access code division (CDMA).

The first approach combines FDM, which divides the channel into several frequency bands, each band is dedicated to a call, with TDM, whose characteristic is to divide time in frames, where each frame is divided into compartments and each call uses a compartment particular designated roster. With this combination have been a channel divided into several sub-frequency bands, with time divided into frames and magazines. Thus, if the channel is divided into F sub-bands and the time T is divided into compartments, then the channel can support F - T simultaneous calls.

In the second approach, there is no division or frequency or time. All users share the same frequency at the same time. Each User in a cell is assigned a distinct sequence of bits called a chipping sequence. When the sender and receiver use the same sequence of chipping, the receiver can recover the sender's transmission between the simultaneous transmissions of other senders. An important advantage of CDMA is that it eliminates the need for frequency allocation. When a system is used FDM / TDM, the receptors are sensitive to interference from other signals on the same frequency band. Thus, a given frequency can be reused in a system FDM / TDM only in cells located at a sufficient distance to prevent such interference.

7.2. First Generation - 1G

The mobile technology is classified into "generations". The first generation supported only the voice traffic. These systems (1G) was analog and are almost extinct.

7.3. Second Generation - 2G

The second generation (2G), although digital, were also designed for voice communication. The main advantages of the 2G technology in relation to 1G are better able to service within a cell, better security to reduce fraud and more advanced services, such as identification (ID) calls and messaging.

There are several standards and technologies 2G. Among them are: TDMA, GSM and CDMA, all will be discussed in the sections below.

TDMA Interim Standard 136 (IS-136 - Interim Standard IS-136 TDMA). It is a combined FDM / TDM technology that has evolved from 1G and was quickly widely available in North America.

Global System for Mobile communications (Global System for Mobile Communications - GSM), emerged in Europe to replace the 1G. It was released with great success in early 1990 and spread to Asia and North America, and today, the standard cellular communications more widely available. The GSM standard uses FDM / TDM interface combined to AR. It consists of bands of frequency of 200 kHz and each band supports eight TDM calls. GSM encoding human voice at 13 kbps and 12.2 kbps.

The 2G systems such as IS-95, GSM and IS-136 are designed for voice service and do not adapt well to the data. As might be one cell technology suitable for both voice communication and data communication (3G) and taking into account the availability of this technology would take many years, companies have developed protocols and standards that allow provisional data transmission over the infrastructure existing 2G . The following are the main protocols and standards imposed.

General Packet Radio Service (GPRS - General Department of Radio Packet) which evolved from GSM. GPRS provides data service more efficient than GSM, with higher speed. GPRS mobile can use more than one time interval within a given channel, on demand. With it, a certain amount of intervals is reserved for data communication and dynamically allocated for mobile stations as a function of their demands instant.

The standard Enhanced Data Rates for Global Evolution (EDGE - Best Data rates for Global Evolution) is intended primarily to increase the capacity for data rates of a GSM / GPRS, which theoretically can provide users with 384 kbps for data communication .

Since the CDMA 2000, Phase 1 (CDMA 2000, phase 1) has evolved from IS-95 and can provide service data packet to 144.4 kbps and paves the way for the release of the 3G CDMA 2000, Phase 2.

7.4. Third Generation - 3G

3G systems must provide telephony and data communication must transit at speeds of 144 kbps, 384 kbps for stationary use outdoors or speed of the walk and walk, 2Mbps indoors.

There are two competing standards in the 3G Universal Mobile Telecommunications Service (UMTS - Universal Mobile Telecommunications Service Mobile), which evolved from GSM. Inherits the characteristic of GSM, but changes the radio access (FDMA / TDMA for GSM) for a technique called Direct Sequence CDMA Wideband CDMA (DS-WCDMA) and is widely available in Europe. Already in North America and parts of Asia, the standard provided is the CDMA-2000, which evolved from the 2G IS-95 and which also uses CDMA as part of its interface AR.

After reviewing all the technologies available today in terms of wireless Internet, we, users, have desires that can occur in the near future. We would like to access the Internet wirelessly from anywhere in the meet, and that access has the highest speed possible, and standing or moving. The system can automatically select the system that offers a bit rate higher then and there.

When we move from one environment to another, it is done in an automatic change of access technology to another without any User intervention, which is maintained our TCP connections for new calls to continue to reach us while we travel. We would like the system supports voice and real-time video over IP so that we could, for example, do video conferencing, no matter where we were. And, of course, that all this was free or at minimal cost. And it's in study and development in Japan for example, is what the authors call 4G, where mobile devices behave like the computers connected to the global network supporting communication between different types of furniture.

8. Mobility Management

First we must define what is a Roaming User's point of view of the network layer. A User is considered mobile or not depending on how it moves between the points of connection to the network. A User with a laptop inside a building is not considered mobile because it is associated with a single point of remote access. Already a User within a vehicle, running with a certain speed through various access networks and wireless wanting to maintain a continuous TCP connection with a remote application while traveling, it is definitely mobile. Another situation would be a User that take your laptop from one location to another (with different access points) and want to connect to the network at the new location. This User is also mobile, but need not maintain a live connection while moving between points of connection to the network.

To maintain a continuous TCP connection with a remote application while moving between points of remote access is necessary to maintain the same IP address. If the User uses a laptop in one spot, turn it off and turn it on again in another location, the private IP address used is not very important.

In a network environment, the residence of a mobile node (laptop or PDA) is known as a native (home network) and within that network entity that performs management functions on behalf of mobility of the mobile node is known as the home agent (home agent). The network that the mobile node is currently residing is known as the external network (foreign network) or visited network (visited network), and the entity in the foreign network that assists the mobile node in the management of the functions of mobility is known as an extern to (foreign agent). A correspondent is the entity that wants to communicate with the mobile node.

8.1. Addressing

For the User mobility is transparent to network applications, it is desirable that a mobile node keeps its address when it moves from one network to another. When a mobile node resides in an external network, all traffic sent to the permanent address of the node now needs to be routed to the external network.

One possible solution is the external network to announce to all other neighboring networks where the mobile node now resides on your network, ie it has the correct path to route datagrams to the permanent address of mobile node, which could be done by customary exchange of routing information and interdomain intradomnios and would require few changes in the infrastructure of existing routing. When the mobile node out of an external network and join one another, the new foreign network would announce a new route, which is highly specific to the mobile node and the former external network derive their information about the mobile node.

Thus it is possible the location of the mobile node by other networks and is easy to route datagrams to the mobile node, since the tables datagrams direct transfer to the external network. However, the network routers, would have to maintain records on tables of transfer to potentially millions of mobile nodes and update these records as the nodes if one moved.

Another possibility is to move the functionality of the mobile network core to the edge of the network, or through the native mobile network. The home agent in the network native mobile node can monitor the external network in which the mobile node resides. You need a protocol between the mobile node and home agent to update the location of the mobile node.

One of the roles of the external agent is to create the so-called care of address (care of address - COA) or email address given to the mobile node pair, and the network portion of address COA penalize the network portion of the external network address. Thus, there are two addresses associated with a mobile node, its address and permanent address COA, sometimes called external address. A second role played by the external agent is to inform the home agent the mobile node is residing on your network (foreign agent) and has the address given COA.

8.2. Indirect Routing to a Mobile Node

When a correspondent wants to send a datagram to a mobile, it simply sends the datagram to the permanent address of mobile node and waits for confirmation of receipt.

The home agent, and is responsible for interacting with an outside agency to monitor the COA of the mobile node also has the task of verifying the datagrams that arrive and are addressed to us whose native network is the network's home agent, but are currently residing in an external network. The home agent intercepts these datagrams and then forwards to a mobile node by a two-step process. First encapsulate the original datagram and send it to the external agent using the COA of the mobile node, which in turn stripped off and forward the datagram to the mobile node.

The mobile node can address your datagram directly to the corresponding (using your own permanent address as the source address and the corresponding address as the destination address).

8.3. Direct Routing to a Mobile Node

The approach of indirect routing suffers from inefficiency known as triangular routing problem. When a call is in the same network as the mobile node in the worst case, side by side, it must send the datagram to the home agent that forwards back to the mobile node on the external network.

The direct routing overcomes this problem. An agent first queries the corresponding home agent that sends the COA of the mobile node. Now the index can send the datagram directly to the foreign agent the mobile node. But two problems arise: we need a discovery protocol for Mobile User agent to consult the corresponding home agent in order to obtain the COA of the mobile node. Another problem is that when the mobile node moves from one foreign network to another, although the home agent has a new address COA, the staff shall not know the new COA, as he consults the home agent only once, at the beginning of session.

One solution would be to create a protocol to notify the change of the corresponding COA. Another solution, adopted in GSM, is to call the external agent from the external network where the node was found for the first time, the anchor foreign agent. When the mobile node move to another foreign network, it will file with the new foreign agent and that new foreign agent foreign agent provide the anchor the new COA of the mobile node. When the anchor foreign agent receives a tunneled datagram for a mobile node that has left the network, it can then reencapsular the datagram and forwards it to the mobile node using the new COA. If later, the mobile node to move to yet another external network, the external agent in this new visited network will then anchor the external agent to establish transfer to this new external network.

9. Mobile IP

The architecture and Internet protocols to support mobility, known as Mobile IP, which is a flexible protocol that supports many different operating modes, making it a complex protocol.

The main objective of Mobile IP is to have a host can change the subnet without the User notice this change, keeping your IP address. These sub-networks may not be homogeneous, so that the protocol has procedures for the recognition of different technologies. The transparency of the transition from one subnet to another also applies to the upper layer protocol Mobile IP.

9.1. Involved with the Mobile IP

With the mobility were introduced seven entities:

Mobile Agent (Mobile Agent) - a node or host that is part of a network, but that can change the network, but remain with the original network;

Home Agent (Agent Local) - a node that represents the original network of the Mobile Agent. This agent redirects all datagrams addressed to the Mobile Agent;

Foreing Agent (Foreign Agent) - a node or host that represents the network where the mobile agent is temporarily allocated. He is the intermediary between the Agent and the Mobile Agent of Origin;

Home Address - is its original and permanent address that identifies with its network of origin;

Site-link Address - is an address to which the hosts of the home network can communicate with the Mobile Agent without the intermediate routers;

Carrier-of Address - is the address that is associated with the Mobile Agent, when he is not in your home network;

Tunnel - is a path which the packets addressed to mobile host should be encapsulated;

9.2. The IP Routing

The routing of IP datagrams based on network addresses. The devices that perform routing are known as routers, or the terminology for Internet gateways. An IP router is not more than one "host" which has an IP address on more than one network. With the right software can ensure the transfer of datagrams between different networks in which it has the address.

Usually a router has more than one physical interface, ensuring the transfer of datagrams between different link technologies. An IP router can connect two IP networks. In this case a router for their connection to IP networks overlap can ensure their interconnection as a single physical interface in possession of two IP addresses.

A routing table is a set of associations (network, path 1, path 2, ...). Each association records several possible routes to reach the network indicated. Each path is the (next gateway, metric), which indicates the gateway next to where it should be sent the datagram and what the metric associated with this path. The metric is a measurement of the efficiency of the way to its destination can be defined based on various criteria such as:

• Delayed Transmission

• Number of hops (intermediate nodes);

• Line capacity;

• Price link.

Both the "hosts" as the "gateways" generally implement routing tables. These routing tables can be static or dynamic. A static table is defined by the network administrator, and when changes occur in the network topology tables must be updated manually.

The routing information can be exchanged between gateways in order to dynamically update the tables. For this purpose they use routing protocols. The routing protocols used in networks terminals are known as IGP (Interior Gateway Protocols "), the most common RIP (Routing Information Protocol"). The protocols used in the transit networks are known for EGP (Exterior Gateway Protocols ").

IP makes the routing of packets from a source endpoint to a destination by allowing routers to do the following incoming packets for network interfaces output according to routing tables. These tables typically maintain the output interface for each destination IP address, according to the number of networks to which the IP address is connected. The network number is derived from the IP address by setting the last four bits equal to zero. Thus, the IP address carries information that specifies the node IP connection.

To maintain the existing connections of the transport layer as the mobile node travels from one point to another, he has to maintain its IP address. In the TCP (the protocol used on most Internet connections), connections are indexed by all (Source IP, Source Port, Destination IP, Destination Port). Changing any of these four numbers will do with the connection is dropped and lost. On the other hand, correct delivery of packets at the current point of connection of the mobile node depends on the network number contained in the IP address of the mobile node, which changes with each new connection point. To change the routing requires a new IP address associated with the new connection point.

9.3. The solution presented by the Mobile IP

The Mobile IP was developed to solve the problem of IP routing by allowing the mobile node has two addresses. In Mobile IP, home address (local address) is static and is used, for example, to identify TCP connections. The care-of address changes with each new connection point and can be viewed as the topological address of the mobile node, it indicates the network number and so indicates the connection point of the mobile node with respect to network topology. The home address makes it appear that the mobile node is continuously available to receive data on your local network, where Mobile IP requires the existence of a network node known as the home agent (local agent). When the mobile node is not connected to your local network (and is therefore linked to what is known as the foreign network), the home agent receives all packets destined to the mobile node and find a way to deliver them at the current point of connection of the node mobile.

When the mobile node moves to another location, enter its new care-of address in its home agent. To deliver a packet to a mobile node's home network, the home agent sends the packet to its local network to the care-of address. This delivery requires that the package is modified so that the care-of-Addrees appear as the destination IP address. This change can be understood as a redirection. When the packet arrives at the care-of address, the reverse transformation is performed for the package once again, to the local address as destination IP address. When the packet arrives at the mobile node, addressed to the local address is processed properly by TCP or any other transport protocol, which receives the IP layer of mobile node.

In Mobile IP home agent redirects packets from the LAN to the care-of-address building a new IP header containing the care-of-address of mobile node as destination IP address. The new header encapsulates the original packet, causing the local address of the mobile node has no effect on the direction of the tunneled packet until it reaches the care-of address. Such encapsulation is also known as tunneling, which suggests that the packet travels over the Internet, bypassing the normal effects of IP routing.

Figure 2. Solution presented by the Mobile IP

The Mobile IP is then the cooperation of three separate mechanisms:

Discovering the care-of address;

Registering the care-of address;

Establish a tunnel to the care-of address.

The home agents and foreign agents send in broadcast ads for agents (ads that contain care-of addresses) at regular intervals (eg once per second or once every three seconds). If, however, a mobile node must acquire a care-of address and not wait for one of these periodic announcements, can then send in a broadcast or multicast request will be answered by any foreign agent or home agent receiving it.

The use home agents listings of agents to become known, even if they offer any care-of address.Thus, a listing agent performs the following functions:

Allows the detection of mobile agents;

List one or more care-of addresses available;

Informs the mobile node on special abilities provided by foreign agents, for example, alternative methods of encapsulation

Allows mobile nodes to determine the network address and status of their connections to the Internet;

Allows mobile nodes to know whether the agent is a home agent, foreign agent or both and is therefore in your local network or in a pool.

The mobile nodes use the requests of the router to detect any change in the number of mobile agents available at the current point of connection. (In Mobile IP this is called the agent solicitation - solicitation of agents). If the ads are no longer detected by a foreign agent that previously had offered a care-of address to the mobile node, the mobile node should assume that the foreign agent is no longer within range of the network interface of the mobile node. In this situation, the mobile node should start looking for a new care-of address, or possibly use a care-of address known by the announcements that is still receiving. The mobile node may choose to wait for another announcement that has not recently received any care-of-address announcement, or send a request to staff.

Once a mobile node has its care-of-address your home agent must be informed. The registration process begins when the mobile node, possibly with the help of a foreign agent, sends a registration request with information on the care-of address. When the home agent receives this request, it usually adds the necessary information to its routing table, approves the request and sends a response to this request to the mobile node. While the Mobile IP protocol, it is not necessary that the home agent deal with requests for updating the existing entries in its routing table, in doing so provides a natural implementation strategy.

When the mobile node can not contact its home agent Mobile IP has a mechanism that lets the mobile node trying to enroll with another unknown home agent on your local network. This method of automatic discovery of home agent is achieved using an IP broadcast address instead of IP address of home agent as the destination of the application for registration. When the broadcast packet reaches the local network, other home agents of the network will send a message of rejection to the mobile node. However, their rejection notices containing their addresses to the mobile node to use a new attempt to register. Note that this broadcast is a broadcast on the Internet, but a directed broadcast that reaches only the IP nodes in the network.

The mechanism of encapsulation by default that must be supported by all mobile agents using Mobile IP is the IP-within-IP. When using IP-within-IP the home agent, the beginning of the tunnel, enter a new IP header in front of the IP header of any datagram addressed to the home address of mobile node. The new tunnel header uses the care-of-address of the mobile node as destination IP address, or destination of the tunnel. The IP address of the beginning of the tunnel is the home agent and the tunnel header uses the number of higher-level protocol (number 4), indicating that the next header is again an IP header. In IP-within-IP the original IP header is preserved as the first part of the data read from the header of the tunnel. Therefore, to recover the original package the foreign agent only has to remove the header of the tunnel and turning the rest to the mobile node.

Sometimes the header of the tunnel uses the number 55 in the inner header. This happens when the home agent use minimal encapsulation instead of IP-within-IP. Processing the header of the tunnel is at least slightly more complicated than processing in IP-within-IP, because some of the header information of the tunnel is combined with the header information in a minimum package to restore the original IP header. Moreover, the header overhead is reduced.

Figure 3. Tunnel under the Care-of-address

After the mobile node back to your local network, it cancels the registration in its home agent to release the care-of address registered. In other words, it configures its care-of-address back to your home address. The mobile node can do this by sending a request to record directly to its home agent with the lifetime set to zero. There is no need to cancel the registration in the foreign agent because the service will expire automatically when the lifetime of the service ends.

10. Managing Mobility in Cellular Networks

The idea of mobility also applies to hosts on the move, where communication is via radio waves, and consequently has a high probability of errors in transmission.

We will use the architecture to analyze the GSM mobile cellular networks. Just as Mobile IP, GSM adopts an approach of indirect routing, routing the first call corresponding to the native network of the mobile node and thence to the visited network. Native network of the mobile node is called public land mobile network native (native PLMN) also known only as native network. The network is the native provider from which the User's mobile subscriber, the visited network is the network where the mobile node is currently residing. Both have very different responsibilities.

The native network lie a database known as a native of Location Register (HLR) which contains the number of permanent cell phone and the profile information of the subscriber, also contains information about the location of these subscribers. If a User is currently transiting the network of another mobile provider, the HLR will contain sufficient information to obtain an address on the visited network to which should be routed a call or Mobile User, a switch within the native network, known as the Central Switching Portal for Mobile Services (GMSC) is contacted by a correspondent.

The visited network to kill a database known as a location of visitors (VLR), which contains one record for each User that is currently moving in the portion of the network served by the VLR is usually located with the mobile switching center (MSC) coordinating the establishment of a call to and from the visited network.

10.1. Routing calls to a Mobile User

A call to a GSM User consists of some steps:

1. dials the corresponding number of Mobile User, in itself, does not refer to a specific phone line or location. The initial digits are sufficient to identify the overall network native Roaming User. The call is routed from the corresponding passes through the public switched telephone network and wound up in the MSC network native Roaming User.

2. Native MSC receives the call and asks the HLR to determine the location of the Mobile User. In the simplest case, the HLR returns the number of roaming mobile station, this number is different from the permanent number of mobile phone, which is associated with the network's native Mobile User. The roaming number is assigned temporarily to a Roaming User when he enters a visited network. In this case, the MSC will need to consult the native VLR to obtain the number of roaming mobile node.

3. Given the number of roaming, the native MSC provides the second portion of the call through the network to the MSC in the visited network. The call is completed, was routed from the MSC corresponding to native and thence to the MSC visited, and thus to the base station that serves the Mobile User.

When a mobile phone is switched on or enters a part of a visited network that is covered by a new VLR, it must register with the visited network. This is done through the exchange of signaling messages between Mobile User and VLR. The visited VLR, in turn, sends a request to update the location of the Mobile User HLR. This message informs the HLR of the roaming number in which the Mobile User can be contacted or the address of the VLR, which also gets the HRL information about the subscriber's Mobile User and determines which services should be provided to the Mobile User by the visited network.

10.2. Transfer GSM (Handoffs)

A transfer (handoff) occurs when a mobile station changes its association from one base station to another during a call. A mobile phone call is initially routed to the Mobile User through a base station and, after the transfer, through another base station.

Among the reasons for the transfer to occur are: the signal between the current base station and mobile User may have deteriorated then the call is in danger of being dropped, too, a cell may have been overwhelmed, handling a large number of calls to alleviate that congestion is transferred to cell phones User nearby less congested.

As is associated with a base station, a Mobile User periodically measures the potency of a signal emitted by signaling its current base station and nearby stations. These measurements are passed one or two times per second for the base station's current Mobile User. The transfer is initiated by the base station old basis of these measurements, the following steps:

• The old base station (BS) informs the visited MSC to be made a transfer and BS to which the Mobile User should be transferred.
• The visited MSC initiates the establishment of the way to the new BS, allocating the resources needed to carry the call rerroteada and signaling the new BS that a transfer is about to occur.
• The new BS to and activates a radio channel to be used by the Mobile User.
• The new BS returns a signal to the visited MSC and the new BS and the Mobile User needs to join with the new BS.
• Mobile User is informed that he must make a transfer.
• Mobile User and the new BS exchange one or more on messages to fully activate the new channel in the new BS.
• moving the new BS sends a completion message transfer which is transmitted to the visited MSC. Then, the visited MSC rerroteia the current call for the Mobile User, via the new BS.
• The resources reserved along the path to the old BS is released.

GSM defines the MSC visited by the Mobile User right at the beginning of a call as anchor MSC. Throughout the duration of the call it is routed from the MSC native to the anchor MSC and then the anchor MSC to the MSC visited. When a Mobile User is the coverage area of one MSC to the coverage area of another MSC, the call is routed from the anchor MSC to the new visited MSC.

Instead of keeping one jump from the anchor MSC to the MSC chain, simply chains MSCs visited by mobile, making the MSC a former transmit the active call to the new MSC each time the mobile will move into a new MSC .

11. Wireless and Mobility

• Recovery Time: the goal is to recover bit errors when and where they occur. This includes protocols that recover loss or corruption in the data link layer protocols, transport layer that share a TCP connection into two segments, one from the source to the wireless link and one of the wireless link to the destination and protocol link layer aware of the TCP.
• TCP sender aware wireless link is in approaches to local recovery, the TCP sender is unaware that their industries are going through a wireless link. One approach is the sender and receiver become aware there is a d wireless link, in order to distinguish between congestion losses that occur in the wired network and corruption and losses that occur in the wireless link and invoke congestion control only in response to losses in wired network.
• Applications that operate on wireless links, particularly for mobile wireless links, should treat bandwidth as a scarce commodity. Although wireless links propose challenges at the application layer, the mobility that they create also makes possible a rich set of applications with location and context aware.