WiFi Networking Architecture
Rob Flickenger
Types of links
Wireless networks can be arranged in one of these three logical configurations:
- Point-to-Point
- Point-to-Multipoint
- Multipoint-to-Multipoint
Point-to-Point
The simplest connection is the point-to-point link.
Point-to-Multipoint
When more than one connection communicates with a central point, this is a
point-to-multipoint network.
Multipoint-to-Multipoint
When any node of a network may communicate with any other, this is a
multipoint-to-multipoint network (also referred to as an
ad-hoc or mesh network)
Radio modes
802.11 cards can be operated in one of these modes:
- Master (AP)
- Managed (Client, a.k.a. station)
- Ad-hoc
- Monitor
Radios may only operate in one mode at a time*
Typical (intended) use
Master mode
Master mode (also called AP or infrastructure mode) is used to create
a service that looks like a traditional access point. The wireless card creates a network with a specified name (called the
SSID) and channel, and offers network services on it.
Wireless cards in master mode can only communicate with cards that are
associated with it in managed mode.
Managed mode
Managed mode is sometimes also referred to as client mode. Wireless
cards in managed mode will join a network created by a master, and will
automatically change their channel to match it.
Clients using a given AP are said to be associated with it. Managed mode
cards do not communicate with each other directly, and will only communicate with an associated master.
Ad-hoc mode
Ad-hoc mode creates a multipoint-to-multipoint network when there is
no master or AP available.
In ad-hoc mode, each wireless card communicates directly with its neighbors. Nodes must be in range of each
other to communicate, and must agree on a network name and channel.
802.11 Radio Modes in action
Montior mode
Monitor mode is used by some tools (such as Kismet) to
passively listen to all radio traffic on a given channel.
This is useful for analyzing problems on a wireless link or observing spectrum usage in the local
area.
Monitor mode is not used for normal communications.
The TCP/IP model
802.11 provides a link-local connection.
Bridged networking
In a simple local area wireless network, a bridged architecture is usually
adequate.
- Very simple configuration
- Roaming works very well
- Increasingly inefficient as nodes are added
- All broadcast traffic is repeated
- Virtually unusable on very large wide-area networks
The bridged AP
Routing
Large networks are built by applying routing between nodes.
- Static routing is often used on point-to-point links
- Dynamic routing (such as RIP or OSPF) can be used on larger networks,
although they are not designed to work with imperfect wireless links
- Mesh routing protocols (OLSR, HSLS, AODV) work very well with
wireless networks, particularly when using ad-hoc.
APs with routing
Relaying traffic
In ad-hoc mode, all radios can communicate with each other as long as they
are in range. They will not relay traffic for other nodes without an
additional routing protocol.
In infrastructure mode, clients must be within range of an access point.
The AP will relay traffic between all associated clients, but clients
cannot talk to each other directly.
AP vs. Ad-Hoc
Mesh = Ad-hoc + Routing
A mesh network (implemented with 802.11 equipment) is essentially a
group of radios operating in ad-hoc mode, with some kind of routing applied.
Many mesh routing protocols (such as OLSR) may be applied to any physical
network, including Master / Managed nodes, or even Ethernet.
Dynamic mesh
Repeaters
Problems with wide-area 802.11
802.11 networks were designed to operate at relatively short distances (up
to a couple of hundred meters). Range can be extended significantly by
using high gain antennas, but this is not a complete solution.
Over long distances, a number of problems become apparent that are not
handled well by the 802.11 protocol itself.
The "Hidden Node"
When two clients are in range of the same access point but not each other,
their transmissions can interfere with each other. This condition is called
a hidden node problem.
- Hidden node is alleviated somewhat by CTS/RTS (channel reservation)
- Specify a maximum packet size, above which CTS/RTS is used
- CTS/RTS is not perfect, but can help at a cost of overall throughput.
Timing issues
Due to the very fast timing of 802.11 frames, speed of light becomes an issue at long
distances. At approximately 15 km, standard timings are too short for
acknowledgements to be received.
Some cards and drivers (such as Atheros) allow timings to be adjusted,
permitting very long distance communications.
Credits
Portions of this talk were adapted from Wireless Networking in the
Developing World, http://wndw.net/
