1. Departamento de Tecnología Electrónica
Some of these slides are copyrighted by: Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.
Chapter 5. Wireless Local Area Network 1

2. Chapter 5: Wireless Local Area Networks
5.1 Introduction
5.2 Standard IEEE
5.2.1 Standards
5.2.2 IEEE network components
5.2.3 Standard IEEE
5.2.4 IEEE network topologies
Wireless Local Area Networks 2

3. Wireless Local Area Networks
Chapter 5: Wireless Local Area Networks
5.1 Introduction
5.2 Standard IEEE
5.2.1 Standards
5.2.2 IEEE network components
5.2.3 Standard IEEE
5.2.4 IEEE network topologies
Wireless Local Area Networks 3

4. Wireless Local Area Networks
Introduction
Introduction
No cables
They use the electromagnetic spectrum
Generally they are integrated within wired LANs
Why?
Allow mobility
Allow installation in places where cables can’t be installed (or are expensive)
Wireless Local Area Networks 4

5. Wireless Local Area Networks
Introduction
Introduction
Wireless Local Area Networks 5

6. Wireless Local Area Networks
Introduction
Advantages:
Allow the same features as wired LANs, but without cable limitations.
Mobility
Reduce installation time/cost
Flexibility
May work inside buildings or between buildings
Disadvantages:
Need a trasnsmission medium based on radio frequency (RF) -> Electromagnetic spectrum is limited
Transmission rates are slower than in wired LANs
Security problems
Wireless Local Area Networks

7. Wireless Local Area Networks
Electromagnetic spectrum
Use of electromagnetic spectrum
0-200 MHz: Radio, television, remote controls, wireless phones, etc.
200 MHz- 1GHz: alarms, medical implants, walkie-talkies, television, mobile phones.
1- 2 GHz: GPS, medical telemetry, phone mobiles
2.4 GHz: free band… satellite radio, satellite phones, microwave ovens, weather radars, WI-FI, BLUETOOTH.
GHz: satellite communications (e.g, TV)
5-50 GHz: Wi-fi, police radars
GHz: short-distance signals.
Wireless Local Area Networks

8. Wireless Local Area Networks
Electromagnetic spectrum
ISM frequency bands:
900 – 928 MHz
2,400 – 2,4835 GHz
5,725 – 5,850 GHz
Wireless Local Area Networks

9. Wireless Local Area Networks
Chapter 5: Wireless Local Area Networks
5.1 Introduction
5.2 Standard IEEE
5.2.1 Standards
5.2.2 IEEE network components
5.2.3 Standard IEEE
5.2.4 IEEE network topologies
Wireless Local Area Networks 9

10. Wireless Local Area Networks
Standards
Standars
Standardization of WLANs is in charge of IEEE & WIFI Alliance.
IEEE in standard is in charge of:
Define specifications of high performance WLANs
Ensures Interoperability
Security
Quality of Service.
WIFI Alliance is in charge of:
Certificates that a manufacturer’s product may interoperate with another manufacturer’s one.
Promote the use of WLANs
Wireless Local Area Networks

11. Wireless Local Area Networks
Standards
LAN/MAN Standards
Wireless Local Area Networks 11

12. Wireless Local Area Networks
Chapter 5: Wireless Local Area Networks
5.1 Introduction
5.2 Standard IEEE
5.2.1 Standards
5.2.2 IEEE network components
5.2.3 Standard IEEE
5.2.4 IEEE network topologies
Wireless Local Area Networks 12

13. Wireless Local Area Networks
Network components
IEEE Network components
Networks that follow standard are formed by four basic elements
Distribution system
Access Point
Station or client
Wireless medium
Wireless Local Area Networks

14. 802.11 Network components Wireless Network Adapter
Station or client: Device using a NIC that follows standard IEEE
PC, laptop, PDA,…
Wireless Network Adapter
Wireless Local Area Networks

15. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
Radio units
Several types of Wi-Fi adapters
PCI Cards
With internal antenna
With external antenna
USB Adapters
PCMCIA Adapters
Wireless Local Area Networks

16. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
PCI Cards
With internal antenna
More usual
Disadvantage: very sensitive to the host location
With external antenna
Allows locating the antenna in the best place for receiving a more powerful signal.
PCI cards in n have three antennas.
Wireless Local Area Networks

17. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
PCI Cards
Advantages
Reliable: once they are installed, there are usually no problems
Disadvantages
Need HW installation
Allow their use in an only host
Wireless Local Area Networks

18. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
USB Adapters
With internal antenna
More usual.
Less range, cheaper.
With external antenna
More gain: more quality of signal
Also in n there are three antennas.
Wireless Local Area Networks

19. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
USB Adapters
Advantages
Mobility: Allows locating them in the best place for receiving a more powerful signal.
May be used in any host. Only a USB port is needed.
If it is neccessary, they may pass from a host to another (only driver installation is needed)
Disadvantages
Much more unstable than Wi-Fi PCI Cards.
Units with interior antenna do not usually have high gain, so that they do not work properly in places where the quality of the signal is bad.
Wireless Local Area Networks

20. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
PCMCIA Cards
With internal antenna
More useful for laptops but less range than units with external antenna.
With external antenna
Higher range. Antenna is not usually very large, and it is generally folding
Also in n there are three antennas (internal, for practical reasons).
Wireless Local Area Networks

21. Wireless Local Area Networks
Network components
Station or client
Wireless Network Adapters
PCMCIA Cards
Adavantages
More quality of signal than USB Adapters and simillar to Wi-Fi PCI cards.
Disadvantages
May be used only in hosts with PCMCIA port.
They all need driver installation
Wireless Local Area Networks

22. Wireless Local Area Networks
Network components
Access Point (AP):
Device in charge of “Medium Control Access” for the clients of a WLAN and allows connection to a wired network (bridge)
An AP is a wireless hub.
Different from a wireless router (very common nowadays). A wireless router is a combination of an AP and a router and may execute more complex functions than a simple AP.
Wireless Local Area Networks

23. Wireless Local Area Networks
Network components
Access Point (AP):
Bridge: allows interconnecting different networks, regardless of the protocol they are using. Works in 1 & 2 OSI layers.
A router also allows interconnecting several networks, but the network layer protocol must be common (E.g, IP)
If we want to interconnect two networks that are using the same protocol, it is desirable to use a router.
Wireless Local Area Networks

24. Wireless Local Area Networks
Network components
Station & AP
Antennas
Directional antennas or beam antennas
Radiate signal in a very determined direction. Narrow beam with high range.
Range is determined by the combination of the gain of the antenna, emitting power in the transmitter AP & sensitivity in the receiver AP. No signal is captured out of range.
Very useful for long-distance point to point communication
Wireless Local Area Networks

25. Wireless Local Area Networks
Network components
Station & AP
Antennas
Onmidirectional antennas
Signal is radiated in all directions, but the signal’s range is short
Useful for radiating a signal in an extensive area.
Sectorial antennas
A combination of directional & omnidirectional antennas.
Expensive
When long distance and extensive area range are neccesary.
Wireless Local Area Networks

26. Wireless Local Area Networks
Network components
Wireless Medium: Use of RF for the transport of MAC_PDUs.
Frequency
Standard
Wavelength
2.4 Ghz
802.11b/g
12.5 cm
5.x GHz
802.11a
5-6 cm
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27. Wireless Local Area Networks
Network components
Distribution System: LAN or WLAN technology used to make WLAN range wider.
Wireless case:
Several AP.
WDS AP: Master AP.
The other APs are slaves and act as relay stations: WDS Stations.
All in the same channel
SSID may be the same one or different
Not standard. Not supported by all hosts and there may be incompatibilities.
Incompatible with some security mechanisms
Wireless Local Area Networks

28. Wireless Local Area Networks
Chapter 5: Wireless Local Area Networks
5.1 Introduction
5.2 Standard IEEE
5.2.1 Standards
5.2.2 IEEE network components
5.2.3 Standard IEEE
5.2.4 IEEE network topologies
Wireless Local Area Networks 28

29. Wireless Local Area Networks
Standard IEEE
Standard IEEE
LLC (802.2)
MAC
PHY
Wireless Local Area Networks

30. Standard IEEE 802.11. Physical Layer
Physical Layer (PHY)
Cell topology.
Half-duplex
Frequency bands: 2,4 GHz & 5 GHz
A license is not neccessary.
Every band has a set of channels
Wireless Local Area Networks

31. Standard IEEE 802.11. Physical Layer
Physical Layer (PHY)
Cell topology.
Physical layer options
Standard
Band
Tx Rate
802.11a
5 GHz
54 Mbps
802.11b
2,4 GHz
11 Mbps
802.11g
802.11n
Both
200 Mbps
Wireless Local Area Networks

32. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Medium Access Technique: CSMA/CA
Before transmitting info, a station must determine the medium state (free or busy)
If the channel is not busy, there is an additional wait: InterFrame Space (IFS)
If the channel is busy or it gets busy while IFS, the tx must wait until current transaction ends.
Wireless Local Area Networks

33. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Medium Access Technique: CSMA/CA
After current transaction ends: Backoff algorithm is executed
Additional and random wait, chosen in an interval called contention window (CW)
Measured in units of slot time (slots)
If during this wait, medium is not free for a time equal or higher than IFS, wait is suspended until the condition is fulfilled.
Wireless Local Area Networks

34. Standard IEEE 802.11. Data Link Layer
Data to tx arrival
Wait
MAC Sublevel
Medium Access Technique: CSMA/CA
IFS
IFS
IFS
IFS
Data CW
Station A
Data CW B
Data
Backoff C
Data
Backoff CW D
Data
Backoff CW E
Wireless Local Area Networks

35. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Medium Access Technique: CSMA/CA
Problems in WLAN:
Hidden nodes. Channel is busy by a station that is not heard by the correspondent node
Exposed nodes. A station thinks that the channel is busy though it is, in fact free, as other nodes does not intereferes in the communication.
Wireless Local Area Networks

36. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Medium Access Technique: MACA
Contention that allows reservations to avoid collisions (CSMA/CA, CA = Collision Avoidance)
RTS (Request to Send) / CTS (Clear to Send)
Not employed (overload) if:
There are a few stations
Very dense network: all the stations are in range of every station.
Small frames.
Wireless Local Area Networks

37. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Medium Access Technique: MACA
Wireless Local Area Networks

38. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Every MAC_PDU containing data is ACKed by the rcvr
Encryptation and authenticaction algorithms are implemented.
A MAC_PDU may contain up to 4 MAC addresses (source, destination, transmitter & receiver).
Wireless Local Area Networks

39. Standard IEEE 802.11. Data Link Layer
MAC Sublevel
Three types of MAC_PDUs:
Data
Control
Management
Wireless Local Area Networks

40. Standard IEEE 802.11. Data Link Layer
MAC level
Data MAC_PDUs:
Superior layer info (MAC_SDU) is carried
MAC_PCI is 34 bytes long.
MTU is 2312.
In case of the existance of many interferences MAC_SDU fragmentation is enabled
Wireless Local Area Networks

41. Standard IEEE 802.11. Data Link Layer
MAC sublevel
Control MAC_PDUs:
Used for the “reservation” of the medium and for ACKs
ACK: sent by MAC sublevel to ACK a Data MAC_PDU.
Does not indicate that MAC_PDU destination has received it.
RTS (Request to Send) / CTS (Clear to Send) for the reservation of the medium.
RTS is sent by MAC sublevel to request the use of the medium and indicate the duration of the reservation (time needed).
CTS is sent by MAC sublevel as a response to RTS. Indicates that the sender is able to transmit and the duration of the reservation (time left)
Wireless Local Area Networks

42. Standard IEEE 802.11. Data Link Layer
MAC sublevel
Management MAC_PDUs:
To manage wireless links.
Beacon. Sent periodically by MAC sublevel to inform about the existance of a wireless network
Interval: configurable parameter.
Probe request. Enable MAC sublevel to search for wireless networks within an area.
Informs about transmission rates.
Probe response. Response to Probe Request.
Association request. MAC sublevel requests the connection to a wireless network.
Association response. Connection confirmation.
Others.
Wireless Local Area Networks

43. Wireless Local Area Networks
Chapter 5: Wireless Local Area Networks
5.1 Introduction
5.2 Standard IEEE
5.2.1 Standards
5.2.2 IEEE network components
5.2.3 Standard IEEE
5.2.4 IEEE network topologies
Wireless Local Area Networks 43

44. Wireless Local Area Networks
Network topologies
The basic communication set in a network is the BSS (Basic Service Set) or cell.
Every BSS has a coverage area, so that all the stations belonging to the BSS can communicate to the others
A name known as SSID (Service Set Identifier) is assigned to them.
According to the number of BSSs and the kind of devices within a network, there are three types of networks:
Ad hoc networks or Independent BSS (IBSS).
There are only clients.
Infrastructure BSS.
There are clients and an AP.
EBSS (Extended BSS)
Multiple BSS to allow bigger coverage area.
Wireless Local Area Networks

45. Wireless Local Area Networks
Network topologies
Ad hoc Network
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46. Wireless Local Area Networks
Network topologies
Infrastructure BSS
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47. Wireless Local Area Networks
Network topologies
EBSS Network
BSS
BSS
EBSS
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48. Wireless Local Area Networks
Network topologies
Functioning in Infrastructure Mode / EBSS
Every AP has a BSSID (MAC of its wireless interface) and a SSID (configured by the network administrator).
In EBSS, every cell has the same SSID, but is distinguished by its AP’s BSSID.
does not limit the number of clients per AP.
To connect to a wireless network, a client must know BSSID & SSID in a cell.
APs send Beacon periodically, with BSSID and, optionally with SSID
The client sends Probe request with SSID, waiting a Probe response, from the AP, containing its BSSID.
Wireless Local Area Networks

49. Wireless Local Area Networks
Network topologies
Functioning in Infrastructure Mode / EBSS
A client who know BSSID & SSID of a cell request the association (connection) with an AP by means of the Association Request
If the AP accepts the client, it sends an Association Response with an Association Id.
AP registers client’s MAC in its Addressing Table.
An AP controls the communication in all its associated clients
Clients never communicate with each other directly
They only proccess MAC_PDUs coming from their associated AP.
Wireless Local Area Networks

50. Wireless Local Area Networks
Network topologies
Functioning in Infrastructure Mode / EBSS
APs keep Addressing tables, like bridges do.
They learn from the traffic that goes through it.
They forward info based on the dest MAC addr
An AP that is connected to a Distribution System acts as a bridge, but
Injects traffic to the wireless interface if the dest is one of its clients or if it is broadcast/multicast
Injects traffic in the Distribution System (like a bridge)
AP adapts logical addressing if it’s neccessary.
E.g: Distribution System based on 802.3
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51. Wireless Local Area Networks
Network topologies
Association
Wireless Local Area Networks

52. Wireless Local Area Networks
Network topologies
Sending of data MAC-PDU
Wireless Local Area Networks