SEMESTRAL PROJECT 37MK

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1 SEMESTRAL PROJECT 37MK WIRELESS TOPOLOGIES Jméno : Onofre Arce Juan Pablo Introduction.- The most common devices used on WLANs (Wireless Lans) are workstations, which include both laptop and desktop models. Many corporations are supplying their workforce with laptops, instead of desktop models. While in the office, the laptop is typically connected to a docking station with a large display monitor, full size keyboard, and a mouse, for improved ergonomic use. The laptop is easily transported for business or personal use, at home or on the road. This has eliminated the need for two systems for each employee and the need to constantly transfer files between two PCs. Laptops and docking stations eliminate concerns about leaving a needed file on the desktop, while away from the office. Furthermore, corporations are able to cut down on expenditures associated with purchasing and maintaining two devices for each employee. Laptop computers and notebook computers are becoming increasingly popular, as are palm top computers, personal digital assistants (PDAs), and other small computing devices. The main difference between desktops and laptops is that components in a laptop are smaller. Instead of expansion slots, there are PCMCIA slots, where NICs, wireless NICs, modems, hard drives and other useful devices, can be inserted. The card is usually the size of a thick credit card. It is inserted into a PCMCIA slot along the perimeter. The use of wireless NICs eliminates the need for cumbersome adapters, connectors, and cables. Mobile Computing Operating Systems (OS) Several operating systems are used on mobile computers. The primary ones include MS DOS, Palm OS, Symbian OS, Windows Compact Edition (CE), and Windows XP Embedded. DOS is a very basic and efficient OS that will run one program at a time. The other OSs will run multiple programs at once. The Palm OS is an OS that was developed especially for PDAs. Symbian OS is an open standard OS, licensed for use in many mobile computing devices and easily customized with third-party software. Windows CE and Windows XP embedded are simplified versions of Windows. Windows XP Embedded is only for use on x86 CPUs. The look and feel are very similar to that of the Czech Technical University in Prague 1

2 desktop Windows versions. An early version of Windows CE was called Pocket PC. Remember that the mobile computer must be interoperable with the desktop PC protocols, or additional software may be needed. ACCESS POINTS AND BRIDGES The access point (AP) operates at Layers 1 and 2 of the OSI Reference Model. An access point (AP) is a WLAN device that can act as the center point of a stand-alone wireless network. An AP can also be used as the connection point between wireless and wired networks. In large installations, the roaming functionality provided by multiple APs allows wireless users to move freely throughout the facility, while maintaining seamless, uninterrupted access to the network. A Wireless Bridge is designed to connect two or more networks that are typically located in different buildings. It delivers high data rates and superior throughput for dataintensive, line-of-sight applications. The bridges connect hard-to-wire sites, noncontiguous floors, satellite offices, school or corporate campus settings, temporary networks, and warehouses. They can be configured for point-to-point or point-tomultipoint applications. WLANS CATEGORIES WLANs are access layer elements or products. WLAN products fit into two main categories: Wireless in-building LANs Wireless building-to-building bridging WLANs replace the Layer 1 transmission medium of a traditional wired network, which is usually a Category 5 cable, with radio transmission over the air. WLANs also replace the Layer 2 MAC functionality, with wireless MAC drivers. WLAN products can plug into a wired network and function as an overlay to traditional or wired LANs. WLANs can also be deployed as a standalone LAN, when wired networking is not feasible. WLANs allow the use of desktops, portable computers, and specialty devices in an environment where connection to the network is essential. WLANs are typically within a building, and are used for distances up to 305 m. Properly Czech Technical University in Prague 2

3 used WLANs can provide instant access to the network from anywhere in a facility. Users will be able to roam without losing their network connections. Wireless bridges allow two or more networks that are physically separated to be connected on one LAN, without the time or expense of dedicated cables or T1 lines. WIRELESS REPEATER In an environment where extended coverage is needed, but access to the backbone is not practical or available, a wireless repeater can be used. A wireless repeater is simply an access point that is not connected to the wired backbone. This setup requires a 50% overlap of the AP on the backbone and the wireless repeater The user can set up a chain of several repeater access points. However, the throughput for client devices at the end of the repeater chain will be quite low. This is because each repeater must receive and then re-transmit each packet on the same channel. For each repeater added to the chain, throughput is cut in half. It is recommended that not more than two hops be used. When configuring repeater access points use the following guidelines: Use repeaters to serve client devices that do not require high throughput. Repeaters extend the coverage area of the WLAN, but they drastically reduce throughput. Use repeaters when client devices that associate with the repeaters are preferably of the same manufacturer. Different manufacturer client devices sometimes have trouble communicating with repeater access points. Use omnidirectional antennas, like the ones that ship with the access point, for repeater access points. Generally within buildings, the availability of Ethernet connections is fairly pervasive. Repeaters can be used to extend APs from the building edge, to the surrounding outdoor portions of the building, for temporary use. For example, one customer could use repeater-mode APs to extend coverage into the parking lot during spring sales for a grocery store. The client association is assigned to the wired/root AP and not to the AP acting like a repeater. ROAMING A WLAN designer must determine whether clients will require seamless roaming from access point to access point. Czech Technical University in Prague 3

4 As a client roams across the wireless network, it must establish and maintain an association with an access point. The following steps are taken to ensure seamless roaming: The client sends out a request for association and immediately receives a response from all access points within its coverage area. The client decides which access point to associate with based on signal quality, strength, the number of users associated, and the required number of hops to reach the backbone. After an association is established, the client's Media Access Control (MAC) address drops into the table of the selected access point. If the client encounters difficulty, it will roam for another access point. If no other access point is available, the client will lower its data transmission rate and try to maintain connection. After the client roams to another access point, its MAC address drops into the table of the new access point, which sends a broadcast message basically stating that it received "MAC address X". The original access point forwards any data it had for the client to the other access point, which responds by sending the same to the client. The following two factors need to be considered when designing a WLAN with seamless roaming capabilities for devices that are powered on while moving from one point to another: Coverage must be sufficient for the entire path. A consistent IP address should be available throughout the entire path. The IP subnet for each access point could be on different switches and separated by Layer 3 devices. If so, consider using Layer 2 switching technologies such as ATM-LANE, ISL, or 802.1q, to span the VLANs. This will help ensure that there is a single broadcast domain for all access points. Association process When a client comes online, it will broadcast a probe request. An AP that hears this will respond with information about the AP such as RF hops to the backbone, load, and so on. If more than one AP replies, then the client will decide which AP to associate with, based on the information returned from the AP. APs broadcast beacons at periodic intervals. A beacon contains details similar to that in the probe response. The client listens to all APs it can hear and builds an information table about the APs. Re-Association process As the client is moving out of range of its associated AP, the signal strength will start to drop off. At the same time, the strength of another AP will begin to increase. The same type of handoff can occur if the load on one AP becomes too large as long as the client can communicate with another AP. Czech Technical University in Prague 4

5 SCALABILITY Scalability is the ability to locate more than one access point in the same area. This will increase the available bandwidth of that area for all users local to that access point. In the past, this scalability was limited to only frequency hopping spread spectrum (FHSS) products. DSSS products could not change channels without some reconfiguration. This means that they can look for and use the best channel. There are three separate, 11-Mbps channels available. These channels are completely nonoverlapping and non-interfering. Up to a theoretical 33 Mbps per cell can be achieved with b devices. However, users still only operate at a maximum theoretical value of 11 Mbps, since they can only connect to one AP at any given time. In the case of a, there are eight non-overlapping channels, each up to a theoretical bandwidth of 54 Mbps. This means that a maximum of eight discrete systems can reside in the same area, with no interference. Therefore, the highest aggregate total data rate for an a system is a theoretical 432 Mbps, for a given cell area. Remember that any connected user will still only receive up to 54 Mbps. With more APs, users will have a greater chance of higher data rates. Channel usage and interference In metropolitan areas, it is possible to have third-party interference from other companies that are using wireless devices. In this situation, it is important to ensure that different channels are utilized. However, this situation will not be known, until the user actually implements the wireless link. Changing channels is the best way to avoid interference. Remember that the standard uses the unlicensed spectrum and, therefore, anyone can use these frequencies. BASIC TOPOLOGIES There are several basic physical configurations that can be used in a WLAN deployment. This section will discuss the following primary WLAN topologies: Peer-to-Peer (Ad Hoc) Topology (IBSS) A wireless service set can consist of nothing more than two or more PCs, each with a wireless network card. This configuration, which does not include an AP, is called an Independent BSS (IBSS). Operating systems such as Windows 98 or Windows XP have made this type of peer-to-peer network very easy to set up. This topology can be used for a small office or home office, to allow a laptop to be connected to the main PC, or for several individuals, to simply share files. However, coverage limitations are a drawback in this type of a network since everyone must be able to hear everyone else. Czech Technical University in Prague 5

6 Basic Infrastructure Topology (BSS) The basic service set (BSS) is the building block of an LAN. An example is a BSS with three stations that are members of the BSS, in addition to the AP. The BSS covers a single cell, when a device moves out of its BSS, it can no longer communicate with other members of the BSS. A BSS uses infrastructure mode, a mode that needs an access point (AP). All stations communicate through the AP. The stations do not communicate directly. A BSS has one service set ID (SSID). Extended Infrastructure Topology (ESS) An extended service set (ESS) is defined as two or more BSSs that are connected by a common distribution system. This allows the creation of a wireless network of arbitrary size and complexity. As with a BSS, all packets in an ESS must go through one of the APs. Dial-up The dialup topology is designed for the small office/home office (SOHO) market. It allows telecommuters, SOHOs, and home users the convenience of wireless connectivity via an AP. In fact, some APs are less than $50 USD. Shared dialup connectivity allows both wired and wireless devices access to the modem and the Internet. A PC running Windows 2000 or XP Internet Connection Sharing provides the connectivity and also functions as a DHCP server for the wireless clients. Unfortunately, even though dialup is quite inexpensive, it is quite slow compared to the latest broadband technologies. Many consumers are choosing to go with broadband technologies if available. Broadband With greater availability of broadband services such as cable, DSL, wireless and satellite, many users are switching from dialup to broadband. Also, users are now installing home networks in order to share the high speed Internet connection as well as local file and print services. The basic home network topology consists of the "modem", a router, a hub/switch, a wireless AP, and clients. In some cases, the router, switch, and wireless AP are housed in a single unit called a wireless router. There are still many broadband users with a standalone PC directly connected to the Cable or DSL modem. Since users are directly exposed to Internet attacks, this simple but insecure topology should be secured by some type of hardware or software firewall. Without any protection, the PC many "open" TCP ports will soon be compromised by attackers. Even if users do not have any valuable information on the PC, the device can be used to assist attackers to compromise other networks. Wireless routers typically support both Cable and DSL. In this topology, the AP or wireless router will support the wireless clients, while the built in switch supports the Czech Technical University in Prague 6

7 wired devices. Other services such as DHCP and Network Address Translation (NAT) are provided by the router. NAT enables multiple LAN devices using private IP addressing to share a single public IP address. Many vendors are also including firewall features such as access control, filtering and stateful inspection to protect the LAN devices from attackers. In addition to the wireless security issues, home users with an "always on" Internet connection are more prone to attackers. CAMPUS TOPOLOGIES The purpose of a campus WLAN is to serve as an access system that incorporates complete mobility. WLANs allow users to access information from unwired places outdoors, in dining halls or informal study spaces, from classroom seats and, even, the athletic fields. However, campus WLANs should not be viewed as a replacement for a wired environment, but rather as a way to add more functionality to the existing network. A campus-wide wireless overlay easily provides networking in hard to reach or temporary locations. These are places that might have otherwise been left out altogether. One of the biggest benefits of a campus WLAN is the ability for people to sit in common areas and work together, and still easily get network access. In the case of many education institutions, where resources are limited, this could mean that there are fewer users competing for the handful of hard-wired computers. Wireless is quickly becoming a viable and important tool in a variety of business and educational environments. VLAN FEATURES LANs are increasingly being divided into workgroups connected through common backbones to form virtual LAN (VLAN) topologies. VLANs enable efficient traffic separation, provide better bandwidth utilization, and alleviate scaling issues by logically segmenting the physical local-area network (LAN) infrastructure into different subnets so that packets are switched only between ports within the same VLAN. When combined with central configuration management support, VLANs facilitate workgroups and client/server additions and changes. Some common reasons why a company might have VLANs: Security Separating systems that have sensitive data from the rest of the network decreases the chances that people will gain access to information they are not authorized to see. Departments/Specific job types Companies may want VLANs set up for departments that are heavy network users (such as multimedia or engineering), or Czech Technical University in Prague 7

8 a VLAN across departments that is dedicated to specific types of employees (such as managers or sales people). Broadcasts/Traffic flow Since a principle element of a VLAN is the fact that it does not pass broadcast traffic to nodes that are not part of the VLAN, it automatically reduces broadcasts. Access lists (ACL) provide the network administrator with a way to control who sees what network traffic. Some APs only support the 802.1Q Trunking protocol standard. Most of switches and routers can support both the pre-standard Inter-Switch Link (ISL) protocol and 802.1Q, depending on the model and IOS image. Switches will not allow different VLANs to talk to one another. A router will be needed to allow different VLANs to communicate to each other. WLANs can now fit nicely into the larger network because VLANs have been enabled on the APs. This allows WLAN users to roam from access point to access point maintaining connectivity to the proper VLAN. CONCLUSION As a conclusion from this semestral project we can say that user mobility is an increase in productivity. For example, meetings and conferences have become less challenging. Access to resources was typically limited or required valuable time to prepare, such as copying all needed files to the laptop before the meeting. With WLANenabled laptops, users can simply pick up and go, with all of their resources available. Furthermore, users are connected to corporate resources while in the meeting, which means that instant messaging, , printing, file, and Internet access are all easily accessible. If desktops are currently in use, they can easily be converted from wired to wireless systems, by changing the NIC and deploying access points. Wireless NICs are also available as PCI adapters. This may seem a step backward, if 10/100 Ethernet is already installed. However, when the next office reorganization takes place, costly rewiring will not be required. As long as applications do not require bandwidth greater than 54 Mbps, WLANs are a viable option. One great advantage of using the standard is that many laptops are now shipped with compatible wireless NICs pre-installed. Without any modification, these devices can interoperate with any product wich is IEEE compliant device. Product testing in different hardware and software configurations now includes WLAN devices, such as NICs, software clients, and access points (APs). It is important that this phase be completed to ensure that the network meets business requirements even with the great advantages of WLANs, they may not be viable in some situations. Czech Technical University in Prague 8

9 Mobile devices can be based on different wireless standards. It is important to use only compliant devices. The big advantages in doing this include interoperability, speed, reliability, and real-time data communications. Equally important is choosing a software application package that will be compatible with the devices used in a given environment. Czech Technical University in Prague 9