Introduction to Wireless LANs

David O’Reilly

Wireless LANs

Wireless LANs are not necessarily a replacement for wired networks. They may form part of a larger solution.

Radio transmissions are used instead of wires.

Allows a network to be deployed in awkward environments, or where it would otherwise be too expensive to have wired connections.

Wireless LANs are useful where a high level of mobility is required.

Wireless LANs

A wireless network can be installed without altering the building in any way, and can be removed when necessary.

There is a premium for wireless hardware over traditional wired hardware, but this is covered by the savings in cabling and installation costs and the benefits of flexible communications.

Many businesses reorganise on a regular basis, and a wireless network could mean that IT is placed far down the list of obstacles.

Wireless LANs

Wireless networks can provide access to all online resources from anywhere within the organisation.

Wireless networks can be configured in similar ways to wired networks – from ad hoc or peer-to-peer installations from small workgroups, to full infrastructure for hundreds or thousands of users.

The user experience of a wireless LAN is exactly the same as a wired network. No re-training of the user base is required.

Consider the following questions…

Would you need to install excessive cabling to link up distant departments?

Do your premises present special difficulties when it comes to wiring for Ethernet?

Would your business benefit from having ‘drop in’ facilities for roaming staff?

Do you reconfigure your office regularly, cater for varied staff numbers or allowing people from partner companies to work on-site with your staff?

The 802.11b standard

Previous wireless devices had a reputation for being slow and unreliable with poor interoperability with other vendor equipment.

New standards and co-operation are making wireless products available to a wide range of users…

The 802.11b standard

The main standard for wireless LANs is the IEEE 802.11 standard.

Conceived in 1990, is was approved in 1997. It uses the 2.4GHz IMS (Industrial, medical,

scientific) frequency band which does not require a license.

An 802.11 network is based on a cellular architecture.

An individual cell is referred to as a basic service set and is controlled by an access point.

The 802.11b standard

Most installations have several cells, with access points connected through a backbone. The backbone is usually Ethernet.

There are also situations where no access point is required; known as ad hoc networks.

The original 802.11 standard specified data rates of 1Mbit/sec and 2Mbit/sec.

The 802.11b standard specifies the additional rates of 5.5Mbit/sec and 11Mbit/sec.

Interoperability

Wi-Fi branding has been created to guarantee interoperability.

Any product with the Wi-Fi logo is guaranteed to work with other Wi-Fi products. The Wireless Ethernet Compatibility Alliance (WECA) was formed in 1999 to certify the compatibility of Wi-Fi products, and to promote Wi-Fi as the global wireless LAN standard.

Bluetooth

Bluetooth is the name for a wireless technology that allows a wide range of devices to communicate with each other.

A universal short range radio link replaces the need for cables to connect different types of device.

For example, mobile phones, laptops, keyboards, PDAs, fax machines, projectors, and most other peripherals could be connected with Bluetooth.

Bluetooth

Bluetooth uses a rapid acknowledgement and frequency hopping scheme to ensure a robust link.

Bluetooth radios also work in the 2.4GHz band.

The maximum data rate is 1Mbit/sec.

Security

Wireless LANs are shared media and are therefore (theoretically) susceptible to packet sniffing.

There are also a number of wireless LAN specific security issues. For example, if you have wireless LAN coverage in your car park, it is as if you had installed Ethernet ports in your car park as far as an attacker attempting to gain access to your internal network is concerned.

Security

There are a number of security features that typically appear in access points:

SSIDs MAC address filtering WEP (Wired Equivalent Protection)

The Cisco Aironet 350 series also offers a range of new security features based on the EAP (Extensible Authentication Protocol) and IEEE 802.1x standards.

Security

All of these features have weaknesses, ranging from poor security in the case of SSIDs to poor scalability in the case of MAC address filtering.

WEP also suffers from a number of recently published weaknesses.

Security

WEP stands for Wired Equivalent Privacy

It is intended to provide a level of protection that is normally found in a wired network.

Wired networks are normally protected by physical security mechanisms such as controlled access to a building.

Security

Wireless networks, on the other hand, can have coverage areas outside the building, e.g. in the car park.

Since wireless LANs are not necessarily protected by physical security, it was decided to include WEP encryption into the 802.11 standard to provide an equivalent level of protection.

Security

WEP is not intended as a complete security solution, just as physical security is not a complete security solution in the case of a wired network.

It needs to be augmented with additional measures such as access control, end to end encryption, password control, authentication, VPNs and firewalls.

Wireless LAN Components

A number of different wireless LAN components are available each of which serve different purposes.

PC cards, PCI cards, access points, multifunction bridges, workgroup bridges, antennae.

Wireless LAN Components

PC and PCI cards are used to provide an individual laptop, desktop or PDA computer respectively with access to the wireless network.

PC and PCI cards can be purchased with either fixed or detachable antennae.

Currently compact flash wireless LAN NICs are not available, but are in development.

Wireless LAN Components

Access points are the wireless equivalent of an Ethernet hub.

Acts as a bridge between the wired network and the wireless network, transferring information from one to the other.

Different vendors access points can support different numbers of (theoretical) simultaneous users. In practice the limitation on the number of concurrent users arises from contention for the medium.

Wireless LAN Components (Access Points)

Wireless LAN Components

The coverage of an access point depends heavily on the environment in which the access points are to be deployed.

Factors such as thick walls, steel reinforcements, microwave ovens, filing cabinets can all have an effect.

Multiple access points can be deployed in a cellular architecture for coverage of arbitrarily large areas.

Wireless LAN Components

Ethernet bridges are typically used in building-to-building line of sight applications.

Distances of up to 25 miles are possible but this distance is limited in Ireland due to maximum radiated power restrictions imposed by the ESTI.

Wireless LAN Components (Bridge)

Point to point link

Wireless LAN Components (Bridge)

Point to Multi-point link

Wireless LAN Components (Bridge)

Overcoming obstacles:

Wireless LAN Components

Workgroup bridges are used to connect devices to a wireless network for which it would not be possible to provide a direct connection.

Issues such as lack of device driver support or no NIC available would lead to a requirement for a workgroup bridge.

Wireless LAN Components (Workgroup Bridge)

Wireless LAN Components

One of the key benefits of wireless LAN components is that they can all be disconnected and reused at a new location with great ease.

This allows total investment protection in network infrastructure in the case of an office relocation.

Cellular Architecture

Within the 2.4GHz band used for wireless networking, there are 3 non-overlapping sub-bands available.

If two access points using the same sub-band (or overlapping sub-bands) are within range of each other, they will contend for access to the medium.

These sub-bands can be used in a cellular architecture to provide coverage over an arbitrarily large area.

Cellular Architecture

Roaming

The area covered by a single access point is known as the coverage cell of that access point.

Roaming is the ability to move seamlessly from one coverage cell to another.

In order for roaming to work, there must be overlapping coverage cells.

Roaming

Rate Shifting

As the distance from the nearest access point increases, the signal from the access point will weaken.

Under these circumstances, the network throughput may in fact be increased by decreasing the data rate. This is because a slower data transmission rate will lead to fewer retries.

Access points and NICs can usually rate shift between 11, 5.5, 2 and 1 Mbps as required.

When to use wireless LANs

Wireless LANs are not ideal for every situation, and they are not necessarily a substitute for a wired network.

Rather, they should be used to overlay and/or supplement your current wired network.

Wireless LANs allow users the choice between the speed of a wired network and the convenience of a wireless network.

Wireless LANs are also ideal in situations where running cable is either infeasible or impossible.

When to use wireless LANs

When you have a highly portable or mobile workforce. Portable means users who change location regularly whereas mobile means users who are moving as they work. E.g. people using PDAs to manage stock in a warehouse or shop.

Wireless LANs are also ideal in situations where there are multiple buildings where laying cable between the buildings is not an option, for example buildings either side of a public road.

When to use wireless LANs

Outlying buildings represent another situation where wireless LANs are extremely helpful.

Within a building, wireless LANs are extremely useful in cases such as warehouses, hangars, shop floors, open plan offices, and general offices where the convenience of wireless connectivity is seen as helpful.

Offices in listed buildings, leased temporary space, or offices with extremely thick walls are other scenarios where wireless LANs can be used.

Characteristics of wireless LAN communication

In the case of both intra- and inter-building wireless LANs, the data rate is 11Mbps.

This figure depends on distance from the access point or bridge.

Greater distances are possible at lower data rates.

In the case of non-point-to-point links, the medium is shared between all of the nodes who wish to access the network.

Characteristics of wireless LAN communication

This shared nature applies equally in the case of inter-building links.

If two access points with overlapping coverage cells are using the same frequency band they will contend with each other for access to the bandwidth.

This situation can be avoided by using a well designed cellular architecture.

Characteristics of wireless LAN communication

Wireless LANs use CSMA/CA (carrier sense, multiple access with collision avoidance) as opposed to CSMA/CD (carrier sense, multiple access with collision detection) which is used by Ethernet.

There is a protocol overhead associated with CSMA/CA but this is offset by the fact that no bandwidth is wasted on collisions.

Characteristics of wireless LAN communication

On a congested shared Ethernet segment, the expected throughput is about 30% whereas on a congested shared wireless segment, the throughput can be as much as 50%.

This figure will vary from vendor to vendor.

Vendors providing wireless solutions

Enterasys Lucent 3COM Cisco Systems Colubris SpectraLink BreezeCOM Intersil

WRQ NDC Elan Symbol Systems Intermic

Management

As with most network components, there are four different ways to manage the wireless LAN access points and bridges; via a console port, telnet , a built in web server and SNMP.

Some access points support propagation of configuration and firmware upgrades through the network.

Centralised administration, e.g. centralised authentication source (RADIUS), central MAC address filtering database, etc. is missing from most product lines. (except Aironet 350 series)

Integrated voice and video over wireless LAN

Voice over IP will work over a wireless network, if your existing network infrastructure supports VoIP functionality.

SpectraLink systems produce a 802.11 wireless integrated telephone and data system.