Introduction to Wireless LAN

1. Wireless LAN

2. Mobile Communication Technology
according to IEEE (examples)
Local wireless networks
WLAN 802.11
802.11a
802.11b
802.11i/e/…/n/…/z/az
802.11g
WiFi
802.11h
Personal wireless nw
WPAN 802.15
802.15.4
802.15.1
802.15.2
Bluetooth
802.15.4a/b/c/d/e/f/g…q/r/s
ZigBee
802.15.3
Wireless distribution networks
WMAN 802.16 (Broadband Wireless Access)
[hist.: 802.20 (Mobile Broadband Wireless Access)]
802.16e (addition to .16 for mobile devices)
+ Mobility
WiMAX
802.15.3b/c…e
802.15.5, .6 (WBAN), …

3. Characteristics of wireless LANs
 Advantages
 very flexible within the reception area
 ad-hoc networks without previous planning possible
 (almost) no wiring difficulties (e.g. historic buildings,
firewalls)
 more robust against disasters like, e.g., earthquakes, fires
- or users pulling a plug...
 One time investment, cost effective

4. Characteristics of wireless LANs
 Disadvantages
 typically low bandwidth compared to wired networks (1-10
Mbit/s) due to shared medium, lots of interference
 different/proprietary solutions, especially for higher bit-
rates or low-power, standards take their time (e.g. IEEE
802.11n, ac), devices have to fall back to older/standard
solutions
 products have to follow many national restrictions if
working wireless, it takes a very long time to establish
global solutions like, e.g., IMT-2000

5. Design goals for wireless LANs
• global, seamless operation
• low power for battery use
• no special permissions or licenses needed to use the
LAN
• robust transmission technology
• simplified spontaneous cooperation at meetings
• easy to use for everyone, simple management
• protection of investment in wired networks
• security (no one should be able to read my data), privacy
(no one should be able to collect user profiles), safety
(low radiation)
• transparency concerning applications and higher layer
protocols, but also location awareness if necessary
• …

6. Basic Transmission Technologies
 Infra Red
 Radio Waves

7. Comparison: infrastructure vs. ad-hoc vs.
mesh networks
infrastructure
network
ad-hoc network
AP
AP
AP
wired network
AP: Access Point
mesh network

8. 802.11 – Classical architecture of an
infrastructure network
 Station (STA)
 terminal with access
mechanisms to the wireless
medium and radio contact to
the access point
 Basic Service Set (BSS)
 group of stations using the
same radio frequency
 Access Point
 station integrated into the
wireless LAN and the
distribution system
Distribution System
Portal
802.x LAN
Access
Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access
Point
STA1
STA2
STA3
ESS

9. 802.11 – Classical architecture of an
infrastructure network
 Portal
 bridge to other (wired)
networks
 Distribution System
 interconnection network to
form one logical network
(EES: Extended Service Set)
based on several BSS
Distribution System
Portal
802.x LAN
Access
Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access
Point
STA1
STA2
STA3
ESS

10. 802.11 - Architecture of an ad-hoc network
 Direct communication within a limited range
 Station (STA):
terminal with access mechanisms to the wireless
medium
 Independent Basic Service Set (IBSS):
group of stations using the same radio frequency
802.11 LAN
IBSS2
802.11 LAN
IBSS1
STA1
STA4
STA5
STA2
STA3

11. IEEE standard 802.11
mobile terminal
access point
fixed
terminal
infrastructure
network
LLC- Logical Link Control

12. 802.11 - Layers and functions
 MAC
 access mechanisms,
fragmentation, encryption
 MAC Management
 synchronization, roaming,
MIB, power management
 PLCP (Physical Layer
Convergence Protocol)
 clear channel assessment
signal (carrier sense)
 PMD (Physical Medium
Dependent)
 modulation, coding
 PHY Management
 channel selection, MIB
 Station Management
 coordination of all
management functions
PMD
PLCP
MAC
LLC
MAC
Management
PHY
Management
PHY
DLC
Station
Management
DLC: Data Link Control
MIB: Management Information Base

13. 802.11 - Physical layer (historical)
 3 versions: 2 radio (typ. 2.4 GHz), 1 IR
 data rates 1 or 2 Mbit/s
 FHSS (Frequency Hopping Spread Spectrum) –
obsolete
 DSSS (Direct Sequence Spread Spectrum)
 Infrared - obsolete

14. FHSS PHY packet format
 Frequency hopping spread spectrum (FHSS) is a
spread spectrum technique which allows for the
coexistence of multiple networks in the same area by
separating different networks using different hopping
sequences.
 The frame consists of two basic parts, the PLCP part
(preamble and header) and the pay-load part.
 While the PLCP part is always transmitted at 1 Mbit/s,
payload, i.e. MAC data, can use 1 or 2 Mbit/s.

15. FHSS PHY packet format
 Synchronization: The PLCP preamble starts with 80
bit synchronization, which is a 010101... bit pattern.
This pattern is used for synchronization of clock of
potential receivers.
 Start frame delimiter (SFD):The following 16 bits
indicate the start of the frame and provide frame
synchronization. The SFD pattern is
0000110010111101.

16. FHSS PHY packet format
 PLCP_PDU length word (PLW):This first field of the
PLCP header indicates the length of the payload. PLW
can range between 0 and 4,095.
 PLCP signalling field (PSF):This 4 bit field indicates the
data rate of the payload following. All bits set to zero
(0000) indicates the lowest data rate of 1 Mbit/s. The
granularity is 500 kbit/s, thus 2 Mbit/s is indicated by
0010 and the maximum is 8.5 Mbit/s (1111). This system
obviously does not accommodate today’s higher data
rates.

17. FHSS PHY packet format
 Header error check (HEC): Finally, the PLCP header
is protected by a 16 bit checksum with the standard
ITU-T generator polynomial.

18. DSSS PHY packet format
 Direct sequence spread spectrum (DSSS) is the
alternative spread spectrum method separating by
code and not by frequency.
 The frame consists of two basic parts, the PLCP part
(preamble and header) and the payload part.
 While the PLCP part is always transmitted at 1 Mbit/s,
payload, i.e., MAC data, can use 1 or 2 Mbit/s.

19. DSSS PHY packet format
 Synchronization: The first 128 bits are not only used
for synchronization, frequency offset compensation.
The synchronization field only consists of alternative
0s and 1’s.
 Start frame delimiter (SFD):This 16 bit field is used
for synchronization at the beginning of a frame and
consists of the pattern 1111001110100000.

20. DSSS PHY packet format
 Signal: Originally, only two values have been defined
for this field to indicate the data rate of the payload.
The value 0x0A indicates 1 Mbit/s, 0x14 indicates 2
Mbit/s, 0x6E (i.e. 01101110) indicates 11Mbits/s.
 How to get this value?
 Service: This field is reserved for future use;
however, 0x00 indicates an IEEE 802.11 compliant
frame.

21. DSSS PHY packet format
 Length: 16 bits are used in this case for length
indication of the payload in microseconds.
 Header error check (HEC): Signal, service, and
length fields are protected by this checksum using
the ITU-T CRC-16 standard polynomial.

22. 802.11 - MAC layer architecture
 The MAC layer has to fulfill several tasks.
 It has to control medium access,
 It can offer support for roaming, authentication, and power
conservation.
 Three basic access mechanisms
1. Basic method - based on a version of CSMA/CA,
2. Optional method avoiding the hidden terminal problem,
3. a contention-free polling method for time-bounded service.
 The first two methods are also summarized as
distributed coordination function (DCF), the third
method is called point coordination function (PCF).
 The MAC mechanisms are also called distributed
foundation wireless medium access control
(DFWMAC).

23. 802.11 - CSMA/CA access method

24. 802.11 - CSMA/CA access method
• If station wants to send
• starts sensing the medium (Carrier Sense based on CCA,
Clear Channel Assessment)
• If the medium is free for the duration of an Inter-
Frame Space (IFS), the station can start sending
(IFS depends on service type)

25. 802.11 - CSMA/CA access method
• If the medium is busy, the station has to wait for a
free IFS, then the station must additionally wait a
random back-off time (collision avoidance, multiple
of slot-time)
• if another station occupies the medium during the
back-off time of the station, the back-off timer stops
(fairness)

26. 802.11 – CSMA/CA broadcast

27. 802.11 – CSMA/CA Unicast
 Station has to wait for DIFS before sending data
 Receivers acknowledge at once (after waiting for
SIFS) if the packet was received correctly (FCS)
 Automatic retransmission of data packets in case of
transmission errors, but exponential increase of
contention window
t
SIFS
DIFS
data
ACK
waiting time
other
stations
receiver
sender
data
DIFS
contention

28. 802.11 – DCF with RTS/CTS
 Station can send RTS with reservation parameter
after waiting for DIFS (reservation determines
amount of time the data packet needs the medium)
 acknowledgement via CTS after SIFS by receiver (if
ready to receive)

29. 802.11 – DCF with RTS/CTS
 sender can now send data at once,
acknowledgement via ACK
 other stations store medium reservations distributed
via RTS and CTS

30. Fragmentation
 Fragmentation is used in case the size of the packets sent has
to be reduced (e.g., to diminish the probability of erroneous
frames).
 Each fragi (except the last one) also contains a duration (as
RTS does), which determines the duration of the NAV.
 By this mechanism, fragments are sent in a row.
 In this example, there are only 2 fragments.

31. 802.11 – MAC Frame format
 Types: control frames, management frames, data
frames
 Sequence numbers: important against duplicated
frames due to lost ACKs
 Addresses: receiver, transmitter (physical), BSS
identifier, sender (logical)
 Miscellaneous: sending time, checksum, frame
control, data
• Only the first
three and the
last field are
present in all
frames!
• 802.11ac allows
for a variable
frame body

32. Special Frames: ACK, RTS, CTS
 Acknowledgement
 Request To Send
 Clear To Send
Frame
Control
Duration
Receiver
Address
Transmitter
Address
FCS
2 2 6 6 4
bytes
Frame
Control
Duration
Receiver
Address
FCS
2 2 6 4
bytes
Frame
Control
Duration
Receiver
Address
FCS
2 2 6 4
bytes
ACK
RTS
CTS

33. Bluetooth
 Basic idea
 local area networks with a very limited coverage and
without the need for an infrastructure.
 Interconnecting computer and peripherals, handheld
devices, PDAs, cell phones
 Embedded in other devices
 Data transfer at a speed of about 720 Kbps within 50
meters (150 feet) of range or beyond through walls,
clothing and even luggage bags.

34. Bluetooth Protocol - Overview
 Uses the master and slave relationship
 Master and slaves together form a Piconet when
master allows slaves to talk
 Up to seven ‘slave’ devices can be set to
communicate with a ‘master’ in a Piconet – Why?
 Scatternet is formed when several of piconets are
linked together to form a larger network in an ad hoc
manner.
 Bluetooth operates on 79 channels in the 2.4 GHz
band with 1 MHz carrier spacing.
 2.402 GHz to 2.480 GHz

35. Piconet
 Collection of devices
connected in an ad hoc
fashion.
 One unit acts as master and
the others as slaves for the
lifetime of the piconet.
 Master determines hopping
pattern, slaves have to
synchronize.
 Each piconet has one
master and up to 7
simultaneous slaves (> 200
could be parked)

36. Forming a piconet
 All devices in a piconet hop together
 Master gives slaves its clock and device ID
 Hopping pattern: determined by device ID (48 bit, unique
worldwide)
 Phase in hopping pattern determined by clock
 Addressing
 Active Member Address (AMA, 3 bit)
 Parked Member Address (PMA, 8 bit)

37. Scatternet
 Linking of multiple co-located piconets through the
sharing of common master or slave devices
 Devices can be slave in one piconet and master of
another
M=Master
S=Slave
P=Parked
SB=Standby
M
S
P
SB
S
S
P
P
SB
M
S
S
P
SB
Piconets

38. Bluetooth protocol stack
Physical Radio Layer
 The radio layer moves the bits from master to slave, or
vice versa.
 It deals with Specification of the air interface, i.e.,
frequencies, modulation, and transmit power

39. Bluetooth protocol stack
Physical Radio Layer, cont…
 Bluetooth uses the license-free frequency band at
2.4 GHz allowing for worldwide operation with some
minor adaptations to national restrictions.
 Bluetooth operates on 79 channels in the 2.4 GHz
band with 1 MHz carrier spacing.
 2.402 GHz to 2.480 GHz
 Bluetooth transceivers use Gaussian FSK for
modulation.

40. Bluetooth protocol stack
Physical Radio Layer, cont…
 Three Power Classes
 Power class 1: Maximum power is 100 mW and minimum
is 1 mW (typ.100 m range without obstacles).
 Power class 2:Maximum power is 2.5 mW, nominal power
is 1 mW, and minimum power is 0.25 mW (typ. 10 m
range without obstacles).
 Power class 3:Maximum power is 1 mW.

41. Bluetooth protocol stack
Baseband Layer
 The baseband layer is somewhat analogous to the
MAC sublayer but also includes elements of the
physical layer.
 It deals with how the master controls time slots and
how these slots are grouped into frames.

42. Bluetooth protocol stack
Baseband Layer, cont…
 Description of basic connection establishment,
packet formats, timing, and basic QoS parameters.
 The master in each piconet defines a series of 625
µsec time slots,
 The master's transmissions starting in the even slots, and
 The slaves' transmissions starting in the odd ones.
 This is traditional time division multiplexing, with the
master getting half the slots and the slaves sharing the
other half.
 Frames can be 1, 3, or 5 slots long.

43. Bluetooth protocol stack
Baseband Layer, cont…
 Each frame is transmitted over a logical channel,
called a Link, between the master and a slave.
 ACL (Asynchronous Connection-Less) Link
 Used for packet-switched data available at irregular
intervals. These data come from the L2CAP layer (upper
layer) on the sending side and are delivered to the L2CAP
layer on the receiving side.
 ACL traffic is delivered on a best-efforts basis.
 No guarantees are given. Frames can be lost and may
have to be retransmitted.
 A slave may have only one ACL link to its master.

44. Bluetooth protocol stack
Baseband Layer, cont…
 Each frame is transmitted over a logical channel,
called a Link, between the master and a slave.
 SCO (Synchronous Connection Oriented) Link
 This type of channel is allocated a fixed slot in each
direction.
 Due to the time-critical nature of SCO links, frames sent
over them are never retransmitted.
 Forward error correction can be used to provide high
reliability.
 A slave may have up to three SCO links with its master.

45. Bluetooth protocol stack
Link Manager
 The link manager handles the establishment of
logical channels between devices, including
power management, authentication, and quality
of service.

46. Bluetooth protocol stack
Major baseband states of a Bluetooth device

47. Bluetooth protocol stack
L2CAP
 The logical link control and adaptation protocol
(L2CAP) is a data link control protocol on top of
the baseband layer offering logical channels
between Bluetooth devices.
 Three different types of logical channels that are
transported via the ACL between master and
slave:
 Connectionless
 Connection Oriented
 Signaling

48. References
 J. Schiller, “Mobile Communications”, 2nd Edition,
Pearson, 2009.
 Chapter 2: Sec 2.5, 2.8,
 Chapter 7 Introduction, Sec 7.1 – 7.3.4.2 , 7.3.4.4, 7.5.
(topics from Sec 7.5 are only those that covered in
lectures)
 Andrew S. Tanenbaum, Computer Networks, Fourth
Edition, Sec 4.6.