Chapter 4 Medium Access Control Sublayer PART 2

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4.4.4: The 802.11 Frame Structure
-The 802.11 standard defines three different classes of frames in the air: data, control, and management.
Each of these has a header with a variety of fields used within the MAC sublayer.
Data Frame: first comes the Frame control field which is made up of 11 subfields.
1) Frame Control Field(Bits): Protocol version which is 00 to allow future versions of 802.11 to operate.
Type (data, control, or management) and Subtype fields (RTS or CTS). The To DS and From DS bits are
set to indicate whether the frame is going to or coming from the network connected to the APs =
distribution system. More fragments bit means more fragments will follow. The Retry marks a
retransmission of a frame sent earlier. Power Management indicates that the sender is going into power
save made. More-data bit indicates that the sender has additional frames for the receiver. Protected
demonstrates that the frame body has been encrypted for security. Finally, Order tells the receiver that
the higher layer expects the sequence of frames to arrive strictly in order.
2) Duration Field: tells how long the frame and its acknowledgement will occupy the channel, measured
in microseconds.
3) Addresses: Data frames sent to or from an AP has three addresses all in standard IEEE 802 format.
First address is the receiver, the second is the transmitter, the third gives distant endpoint
4) Sequence: numbers frames so that duplicates can be detected.
5) Data Field: contains the payload, up to 2312 bytes. The first bytes of this payload are in a formal
known as Logical Link Control (LLC) which is the glue that identifies the higher layer protocol to which
the payloads should be passed
6) Frame Check Sequence: same 32 bit CRC
4.4.5: Services
Association Service: used by mobile stations to connect themselves to APs. It is used just after a station
moves within radio range of the AP. Upon arrival, the station learns the identity and capabilities of the
AP either from the beacon frames or by directly asking the AP.
Re-association: lets a station change its preferred AP. Useful for mobile stations moving from one AP to
another AP in the same extended 802.11 LAN (like a handover).
Disassociate: either the station or the AP can break their relationship, station uses this service
before shutting down or leaving the network, AP may use this when going for maintenance
Authenticate: for security reasons, before they can send frames via the AP
“Open” 802.11, anyone is allowed to use it
WPA2 (Wifi Protected Access 2): the AP can talk to an authentication server that has a
username and password database to determine if the station is allowed to access the network.
Several frames are exchanged between the station and the AP with a challenge and response
that lets the station prove it has the right credentials (this happens after association)
Distribution Service: once frames reach the AP, this determines how to route them. If the destination is
local to the AP, the frames can be sent out directly over the air. Otherwise, they will have to be
forwarded over the wired network.
Integration Service: handles any translation that is needed for a frame to be sent outside the 802.11
LAN
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Data Delivery Service: lets stations transmit and receive data using protocols, higher layers must deal
with detecting and correcting errors since transmission over 802.11 is not reliable
Privacy Service: manages the detail of encryption and decryption
QOS Traffic Scheduling Service: to handle traffic with different priorities, uses protocols to give voice
and video traffic preferential treatment
Transmit Power Control Service: gives station the information they need to meet regulatory limits on
transmit power that vary from region to region.
Dynamic Frequency Selection Service: gives station the information they need to avoid transmitting on
frequencies in the 5 GHz band that are being used for radar in the proximity
4.5: Broadband Wireless
WiMAX (Worldwide Interoperability for Microwave Access): 802.16
4.5.1: Comparison of 802.16 with 802.11 and 3G
-WiMAX combines aspects of both 802.11 and 3G, making it more like 4G technology
-802.11 and WiMAX are both about wirelessly connecting devices to the Internet at megabit/sec speed
instead of using cable or DSL. WiMAX is designed to carry IP packets over the air and to connect to an IP
based wired network. They both are based on OFDM and MIMO technology to ensure good
performance
-WiMAX is similar to 3G by achieving high capacity by the efficient use of spectrum. WiMAX base
stations are more powerful than 802.11 APs, they have better antennas, work on a licensed spectrum
around 2.5 GHz. The result is a managed and reliable service with goof support for quality service
4.5.2: The 802.16 Architecture and Protocol Stack
Architecture: Base stations connect directly to the provider’s backbone network, which in turn connect
to the internet. The base stations communication with stations over the wireless air interface. Two kind
of stations exist: Subscriber Stations remain in a fixed location and Mobile stations can receive service
while they are moving
Protocol Stack: The bottom layer deals with transmission: “Fixed and Mobile WiMAX”, there is a
different physical layer for each offering; both layers operated in licensed spectrum below 11 GHz and
use OFDM. Above the physical layer, the Data Link Layer consist of three sublayer: the Security Sublayer,
MAC Common Sublayer (the main protocols such as channel management are located), and Service
Specific Convergence Sublayer (provide interface to the network layer. Next, is IP; since IP is
connectionless and MAC sublayer is connection oriented, this layer must map between addresses and
connections.
4.5.3: The 802.16 Physical Layers
-WiMAX use licensed spectrum around 3.5 GHz or 2.5 GHz. Finding available spectrums is a problem,
802.16 allows operation from 2 GHz to 11 GHz. Example, 3.5 GHz for fixed WiMAX and 1.25-20 MHz for
mobile WiMAX
-Transmissions are sent over these channels with OFDM, the channel is divided into more subcarriers
with longer symbol duration to tolerate larger wireless signal degradations (WiMAX parameters are 20x
larger than 802.11 parameters)
-Designers of 802.16 did not like how GSM and DAMPS use equal frequency bands for upstream and
downstream traffic although for web surfing, there is often way more downstream therefore 3:1 ratio.
OFDMA (Orthogonal Frequency Division Multiple Access): different sets of subcarriers can be assigned
to different stations, so that more than one station can send or receive at once. This allows flexibility in
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how bandwidth is assigned which can increase performance because subcarriers can be assigned to
stations that can use them best
TDD (Time Division Duplex): stations usually alternate between sending and receiving (usually used in
WiMAX)
Downlink- Preamble to synchronize all stations, a downlink transmission from the base station
(base station sends map that tells all stations how the downlink and uplink subcarriers are
assigned over the frame). The base station controls the maps, so it can allocate different
amounts of bandwidth to stations from frame to frame depending on the need of each station.
Next, the base station sends bursts of traffic to different subscribers and mobile stations on the
subcarriers at the times given in the map. The downlink ends with a guard time for stations to
switch from receiving to transmitting.
Uplink-The subscriber and mobile stations send their bursts of traffic to the base station in the
uplink positions that were reserved in the map. Ranging: the process by which new stations
adjust their timing and request initial bandwidth to connect to the base station
FDD (Frequency Division Duplex): a station sends and receives a the same time on different subcarrier
frequencies
4.5.4: The 802.16 MAC Sublayer Protocol
Security Sublayer: encryption is used to keep secret all data transmitted. Only the frame payloads are
encrypted, the headers are not = you can see whose talking to who but not what their saying. When a
subscriber connects to a base station, they perform mutual authentication with RSA public-key
cryptography using X.509 certificates. The payloads are encrypted using a symmetric-key system either
AES (Rijindael) or DES with cipher blocking chaining.
MAC Common Sublayer: this is connection-oriented and point-to-multipoint, which means that one
base station communicates with multiple subscriber stations.
Downlink: the base station controls the physical layer bursts that are used to send information
to the different subscriber stations. The MAC sublayer packs its frame into this structure
Service Specific Convergence Sublayer:
Uplink: more complicated since there are competing subscribers
Four classes of Services (all connection oriented):
Constant Bit Rate Service: intended for transmitting uncompressed voice. This service needs to send a
predetermined amount of data at predetermined time intervals. It is accommodated by dedicating
certain bursts to each connection of this type. Once the bandwidth is allocated, the bursts are available
automatically, without the need to ask for each one.
Real Time Variable Bit Rate Service: is for compressed multimedia and other soft real time applications
in which the amount of bandwidth needed at each instant may way. It is accommodated by the base
station polling the subscribed at a fixed interval to ask how much bandwidth is needed this time.
Non-Real Time Variable Bit Rate Service: is for heavy transmissions that not real time such as large file
transfers. The base stations polls the subscriber often, but not at prescribed time intervals.
Best Effort Service: is for everything else. No polling is done and the subscriber must contend for
bandwidth with other best effort subscribers. Requests for bandwidths are sent in bursts marked in the
uplink map as available for contention. If request is successful, its success will be noted in the next
downlink map.
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