Wireless Network Design Principles

Mobility

Addressing

Capacity

Security

Site Survey

Floor plans are a useful aid to surveying a new site

Help with the determination of coverage needs – this will show where communications is needed and therefore where APs will be installed

Site Survey

Walk-through permits visual confirmation of the actual site (desks, office location, etc)

Check for building construction

- wall construction (concrete & steel vs partition walls)

- hallways, open areas etc

Site Survey

Optimum location of APs

Coverage of APs once installed

Actual bit and error rates in selected locations

Number of APs

Site Survey

Measurements may consist of

frame error rates

interfering signal strengths (noise)

received signal strength

multipath signal interference

Site Survey

Antenna choices for coverage, diversity

Signal amplifiers (if necessary – remember increasing signal

power may cause interference to others and may increase the potential number of clients using the access point)

Site Survey

Channel Layout

APs will often overlap in coverage

Selection of non-overlapping channels (1 6 11)

Coverage must be in 3 dimensions if inside a building

Mobility

DHCP

Addressing - private addresses

- NAT

Mobile-IP

IP Addressing

Many security plans require the use of private addresses

- class A 10.x.x.x

- class B 172.16.x.x – 172.31.x.x

- class C 192.168.x.x

IP Addressing

Once a private address has been assigned, the network cannot access the external Internet

To permit connection to outside world, Network Address Translation is necessary

DHCP (Dynamic Host Configuration Protocol)

IP addresses offered by a server to hosts

Static – MAC addresses in a table are mapped to a fixed IP address

Dynamic – IP addresses are allocated from a pool (range of addresses)

DHCP

Permits central point of control for management of IP addresses

Allows efficient allocation of IP addresses

DHCP

MAC addresses may be checked against a list of “approved” clients

DHCP server may be local to client or may be centralised

Mobility

Even with DHCP, addresses will tend to be static

Since wireless devices can move about, fixed addresses may be unsuitable

Even in a small network, use of subnet addressing will not suit static addressing

Mobility

Mobile IP offers a more dynamic way of implementing an IP solution that can be used with wireless networks

Mobile stations are allocated to a home network and have a static address in that network

When the station operates in another foreign network it must use an address from that network

Mobility

The mobile station registers with a foreign agent (commonly a router)

Communications from and for the mobile station are carried between the foreign agent and a home agent using a care-of address given from the foreign network

Mobility

Mobile IP is implemented using three basic functions:

Discovery

Router advertisement (ICMP) messages contain extensions that support their identification as a mobile agent

Mobility Registration

A UDP-based registration process permits the mobile node to register with an available foreign agent (if none available, then a mobile node may become its “own” foreign agent)

The process usually requires authentication

Mobility Tunneling

Agents must carry the mobile node’s IP packets between the home and foreign networks

The traffic between networks must be carried over the global Internet and so must be encapsulated

This traffic should be secured by authentication and encryption

Mobility Tunneling

Encapsulation can be:

IP-within-IP encapsulation

Minimal Encapsulation (specifically identified IP packets)

GRE (Generic Router Encapsulation)

Capacity

No. of clients depends upon the amount of traffic that users generate

The capacity of an Access Point operating at 11 Mbps will be shared at around 6 Mbps

Contemporary users using web, email, file accesses will generate around 100-300 kbps each

Connection Rate Number of Clients @ 6Mbps

100 kbps 60

200 kbps 30

300 kbps 20

Sustained Throughput Compared with Number of Clients

Security

WEP -> WPA -> 11i

SSL

VPN