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1. Wireless Networks
Lecture
Dr .Ekhlas Kadhum Hamza
Dept. of Electrical Eng.
Baghdad University
Master course
Second semester
1.4.2015

2. 2
Lecture Outline
 IEEE 802.11 Data Link Layer
 IEEE 802.11 Medium Access Control,
 IEEE 802.11 MAC Sublayer,
 HIPERLAN Family of Standards,
 Performance of a Bluetooth Piconet in the
Presence of IEEE 802.11 WLANs
 Architecture of a Wireless Wide-Area Network
(WWAN)
 WWAN Subsystem Entities(User Equipment)

3. 3
1 - IEEE 802.11 Data Link Layer
 Like 802.3 (Ethernet),
 the 802.11 data l ink layer is made up of two sub-layers: the Logical
Link Control (LLC) sub- layer and the Media Access Control (MAC)
sub-layer.
 Both 802.3 and 802.11 use the same LLC, specified by 802.2, one
reason why integrati ng 802.11 and 802.3 networks is relatively
simple.
 The 802.11 MAC sub-layer is also similar but does different in the
way the shared radio carrier is accessed. While Ethernet uses Carrier
Sense Multiple Access with Collision Detection (CSMA/CD), 802.11
uses a variation called Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA)

4. 4
1 -IEEE 802.11 Data Link Layer
 In CSMA/CA a station that intends to transmit ‘listens’ for traffic on
the radio carrier frequency and sends if it is clear after a random delay
period. If the receiving station receives the packet intact it sends an
acknowledgement (ACK) to confirm the packet has been received. If
the transmitting station does not receive an ACK it assumes a collision
occurred and transmits again after a random delay period.
 Another aspect of the 802.11 data link layer that is different than
Ethernet is the use of a packet fragmentation and CRC error checking
with each packet. Ether net implements these functions at higher pr
otocol layers whereas 802.11 fragments packets and uses CRC at the
data link layer. This allows the WLAN to send smaller packets that are
less likely to be corrupted by interference, decreasing the need for re-
transmissions.

5. 5
IEEE 802.11 Medium Access
Control
 MAC layer covers three functional areas:
 Reliable data delivery
 Access control
 Security

6. 6
Reliable Data Delivery
 More efficient to deal with errors at the MAC
level than higher layer (such as TCP)
 Frame exchange protocol
 Source station transmits data
 Destination responds with acknowledgment (ACK)
 If source doesn’t receive ACK, it retransmits frame
 Four frame exchange
 Source issues request to send (RTS)
 Destination responds with clear to send (CTS)
 Source transmits data
 Destination responds with ACK

7. 7

8. 8
Comparison of MAC access schemes in
wireless networks.

9. 9
IEEE 802.11 Medium Access
Control
 Wireless local area networks operate using a shared, high bit rate
transmission medium to which all devices are attached and
information frames relating to all calls are transmitted. MAC sublayer
defi nes how a user obtains a channel when he or she needs one.
 The 802.11 MAC layer provides for two other robustness features:
cycle redundancy check (CRC) checksum and packet fragmentation.
Each packet has a CRC checksum calculated and attached to ensure
that the data was not corrupted in transmit. This is different from the
Ethernet, where higher-level protocols such as TCP handle error
checking

10. 10
IEEE 802.11 MAC Sublayer
 In IEEE 802.11, the MAC sublayer is responsible for asynchronous data
service (e.g., exchange of MAC service data units (MSDUs)), security service
(confidentiality, authentication, access control in conjunction with layer
management), and MSDU ordering
 The MAC frame contains addressing informa- tion, information to set the
network allocation vector (NAV), and a frame check sequence to verify the
integrity of the frame. The general IEEE 802.11 MAC frame format is shown
in Figure
 The MAC frame format contains four address fi elds. Any particular frame
type may contain one, two, three, or four address fi elds. The address format in
IEEE 802.11-1997 is a 48-bit address, used to identify the source and
destination of MAC addresses contained in a frame, as IEEE 802.3. In
addition to source address (SA) and destination address (DA), three additional
address types are defi ned: the transmitter address, the receiver address (RA),
and the basic service set identifi er (BSSID). The BSSID is a unique identifi er
for a particular basic ser- vice set of the IEEE 802.11 WLAN. In an
infrastructure basic service set, the BSSID is the MAC address of the AP

11. 11
The general IEEE 802.11 MAC frame format
is 35 to 50 meters. HIPERLAN/1
provides quality of service (QoS),
which lets critical traffi c be
prioritized.

12. 12
HIPERLAN Family of Standards
•HIPERLAN/1 is aligned with the IEEE 802 family of standards and is very much
like a modern wireless Ethernet. HIPERLAN/1, a standard completed and ratified
in 1996, defines the operation of the lower portion of the OSI reference model,
namely the data link layer and physical layer
•The HIPERLAN MAC layer defines the various protocols which provide the
HIPERLAN/1 features of power conservation, security, and multihop routing (i.e.,
support for forwarding), as well as the data transfer service to the upper layers of
protocols
•HIPERLAN/1 uses the same modulation technology that is used in GSM,
Gaussian minimum shift keying (GMSK). It has an over air data rate of 23.5 Mbps
and maximum user data rate (per channel) of over 18 Mbps. The range in a typical
indoor environment is 35 to 50 meters. HIPERLAN/1 provides quality of
service (QoS), which lets critical traffic be prioritized.

13. 13
Table : A comparison of HIPERLAN/2 and IEEE 802.11.

14. 14
Performance of a Bluetooth Piconet in the
Presence of IEEE 802.11 WLANs
 Due to its global availability, the 2.4 GHz ISM unlicensed band is a
popular frequency band to low-cost radios. Bluetooth and the IEEE
802.11 WLAN both operate in this band. Therefore, it is anticipated
that some interference will result from both these systems operating in
the same environment. Interference may lead to significant
performance degradation.
 The collision probability of Bluetooth to the IEEE 802.11 frequncy
hopping ( FH ) system is 1/79. In the IEEE 802.11 direct sequence
(DS), the data stream is converted into a symbol stream which spreads
over a relatively wide band channel of 22 MHz, so the interference on
a Bluetooth packet from IEEE 802.11 DS system is much higher than
that from the 802.11 FH system. It is because the bandwidth of a
channel in DS is 22 times as wide as Bluetooth one channel. The
collision probability of Bluetooth to the IEEE 802.11 DS system is
22/79.

15. 15
PER from M Neighboring IEEE 802.11 WLANs*
Under IEEE 802.11 FH, the probability of M IEEE 802.11 induced
collisions on a Bluetooth packet is given as
Under IEEE 802.11 DS, the probability of M IEEE 802.11
induced collisions on a Bluetooth packet is given as:

16. 16

17. 17
Architecture of a Wireless Wide-Area Network
(WWAN)
 A wireless network does not operate in isolation; it uses the services of public
switched telephone networks (PSTNs) to make or receive calls from wireline
users.
 A number of functions is required to support the services and facilities in a
wireless wide-area network (WWAN). The basic subsystems of the WWAN
are: radio station subsystem (RSS), networking and switching subsystem (NSS),
and operational and maintenance subsystem (OMSS)( see fig 7.1)
 The radio subsystem is responsible for providing and managing transmission
paths between the user equipment and the NSS. This includes management of
the radio interface between the user equipment and the rest of the WWAN
system. The NSS has the responsibility of managing communications and
connecting user equipment to the relevant networks or other users. The NSS is
not in direct contact with the user equipment, nor is the radio subsystem in direct
contact with external networks.

18. 18
Architecture of a Wireless Wide-Area Network
(WWAN)

19. 19
Architecture of a Wireless Wide-Area Network
(WWAN)
 The user equipment, radio subsystem, and NSS form the operational
part of the WWAN system. The OMSS provides the means for a
service provider to control them. Figure 7.1 shows the model for the
WWAN system. In the WWAN, interaction between the subsystems
can be grouped into two main parts :
 Operational part: External Networks NSS RSS UE User⇔ ⇔ ⇔ ⇔
 Control and maintenance part: OMSS Service Provider⇔

20. 20
WWAN Subsystem Entities
Figure 7.2 shows the functional entities of a WWAN and
their logical interaction. A brief description of these
functional entities is provided below
User Equipment

21. 21
WWAN Subsystem Entities

22. 22
Assignment
 Example 21.5
 Example 21.6
 Example 21.7