Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks
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#1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Southern Methodist University Fall 2003
EETS 8316/NTU CC745-NWireless Networks
Lecture 10: Wireless LAN
Instructor: Jila Serajemail: jseraj@engr.smu.edu
http://www.engr.smu.edu/~jseraj/tel: 214-505-6303
#2EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Session Outline
Wireless LAN
#3EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Wireless LAN
Wish List—High speed
—Low cost
—No use/minimal use of the mobile equipment battery
—Can work in the presence of other WLAN
—Easy to install and use
—Etc
#4EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Wireless LAN Architecture
Server
PDA Laptop
LaptopLaptop
Laptop
Access Point Access Point
Ad Hoc
Pager
DS
#5EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Wireless LAN Architecture, Cont…
Logical Link Control Layer
MAC Layer: Consist of two sub layer, physical Layer and physical convergence layer
Physical convergence layer, shields LLC from the specifics of the physical medium. Together with LLC it constitutes equivalent of Link Layer of OSI
#6EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
What Is Hidden Node?
A CB
A can hear BC can hear BA can not hear CC can not hear A sending data
#7EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
LBT MAC Protocol
LBT= Listen Before You Talk
—Based on CSMA-CA
—First send Ready To Send (RTS) to the receiving node
—Receiving node send a Continue To Send (CTS) message, takes care of hidden node.
—Data transmission starts after RTS/CTS.
—Data is acknowledged on the MAC level. Counteract error caused by RF environment.
#8EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Integrated CSMA/TDMA MAC Protocol
Supports guaranteed bandwidth traffic and random access traffic
The bandwidth is divided into a random part and a reserved part.
Random part is LBT, reserved part
During high traffic all bandwidth can be used for reserved traffic (like wireless telephony)
H1 Reserved-1 H2 Reserved-2 H3 LBT
#9EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Reservation/Polling MAC Protocol
Works only with AP
Fair and slow. First-in-First-Out
Wireless station send a request.
All requests are queued.
Wireless stations are polled in the same order that the requests have arrive.
All data reception is acknowledged.
#10EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Power Management
Battery life of mobile computers/PDAs are very short. Need to save
The additional usage for wireless should be minimal
Wireless stations have three states
—Sleep
—Awake
—Transmit
#11EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Power Management, Cont…
AP knows the power management of each node
AP buffers packets to the sleeping nodes
AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when.
The node is awake only when it is sending data, receiving data or listening to TDIM.
#12EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Access Point Functions
Access point has three components
—Wireless LAN interface to communicate with nodes in its service area
—Wireline interface card to connect to the backbone network
—MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently
#13EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Bridge Functions
Speed conversion between different devices, results in buffering.
Frame format adaptation between different incompatible LANs
Adding or deleting fields in the frame to convert between different LAN standards
#14EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Routing
Building routing tables can be done as
—Source tree, keeps track where other nodes are and the best way of reaching them. When sending a packet the route is also determined. It must be done in each node and is heavy.
—Spanning tree, is built iteratively, each bridge advertises it identity and all other bridges it knows and how many hops it takes to get there. Then each bridge follows a specific algorithm to calculate how get to each bridge with least hop.
#15EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Bridge Functions, Routing
Create a routing table for sending packets
Listen to all packets being sent.
Find out which nodes are in which sub-network by analyzing the source address. Store that data in a routing table.
If a packet is addressed to a known node, only repeat the data on that sub-network, otherwise repeat it on all networks.
#16EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Bridge Functions, Routing, Cont…
Age the entries after a timer value has expired since the last communication
If the timer is too long, we might send data to a node that might have left the sub-network or is turned off or even gone to coverage area of another access point.
If the timer is too short, we remove the user too early and repeat the packet destined to it in all sub-networks.
Other functions of a bridge, buffering for speed conversion, changing frame format between LANs.
#17EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobility Management
AP has three components—WLAN interface
—Backbone LAN interface
—MAC layer bridge functionBackbone Network
Access PointAccess Point
Access Point
#18EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobility Management, Cont..
A node can associate when it enters the coverage area of an AP
A node can disassociate when power down or leaving the service area
It shall re-associate when it handoffs to another AP.
AP bridge function keeps track of all nodes associated with it.
#19EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
WLAN Addressing
In wireline LAN, each node has an IP address that is associated with its physical location
When a device can move from one location to another, the association between the physical location and IP address no longer holds
The solution is presented in mobile IP
#20EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobile IP Principals
Internet is a large network and introducing a new function, e. g. Mobile IP can not be disruptive.
Constraints of mobile IP are
—Mobility should be at network layer
—No impact on higher levels
—No impact on the nodes not directly involved in the mobile IP function
—Uninterrupted operation for mobile devices
#21EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobile IP Principals, Cont…
The principle is very simple, use c/o addressing
For each mobile device, we associate a Home IP address associated with a Home Network.
The new LAN is called the Visiting Network
The software that takes care of mobility in each server (router) is called agent.
#22EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobile IP Principals, Cont…
Two types of agents, Home Agent and Visiting Agent.
Whenever the mobile device connects to a new network, a c/o address is given to it by the Visiting Agent.
This c/o address is reported to the Home Agent.
All packets addressed to the mobile device are addressed to its Home Address, and thus sent to its Home Network.
#23EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobile IP Principals, Cont…
Upon reception of the packet, the Home Agent recognized the address belonging to a mobile device.
Home Agents looks up the c/o address in its table.
The packet is then wrapped in a new packet with the c/o address on it, called encapsulation
C/o address causes the packet to be forwarded to the Visiting Agent.
#24EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobile IP Principals, Cont…
Visiting Agent recognizes the received address as the c/o address, unwrap the packet; called de-capsulation; and send it to its intended receiver.
This activity is called tunneling, referring to the idea creating a tunnel between the Home Network and Visiting Network and sending all data to that mobile device on that tunnel.
Several tunnels can be created between two networks
#25EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Mobility Management in WLAN
Mobile IP principles are used to take care of mobility in the wireless LAN.
Every wireless device has an address in its Home LAN, and gets a c/o address in the Visiting LAN.
#26EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
IEEE 802.11 WLAN, History
1997 IEEE 802.11 working group developed standard for inter-working wireless LAN products for 1 and 2 Mbps data rates in 2.4 GHz ISM (industrial, scientific, and medical) band (2400-2483 MHz)
Required that mobile station should communicate with any wired or mobile station transparently (802.11 should appear like any other 802 LAN above MAC layer), so 802.11 MAC layer attempts to hide nature of wireless layer (eg, responsible for data retransmission)
#27EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
802.11 WLAN History, Cont..
1999 IEEE 802.11a amendment for 5 GHz band operation and 802.11b amendment to support up to 11 Mbps data rate at 24 GHz
MAC sub layer uses CSMA/CA (carrier sense multiple access with collision avoidance)
#28EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
802.11 Architecture
MAC Layer
Physical Layer ConvergenceProcedure (PLCP)
Physical Medium Dependent(PMD) sub layer
MAC provides asynchronous, connectionless service
#29EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Frame type and subtypes
Three type of frames—Management
—Control
—Asynchronous data
Each type has subtypes
Control—RTS
—CTS
—ACK
#30EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Frame type and subtypes, Cont..
Management
—Association request/ response
—Re-association request/ response
—Probe request/ response
—privacy request/ response
—Beacon (Time stamp, beacon interval, TDIM period, TDIM count, channels sync info, ESS ID, TIM broadcast indicator)
#31EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Frame type and subtypes, Cont..
Management…
—TIM (Traffic Indication Map) indicates traffic to a dozing node
—dissociation
—Authentication
#32EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Authentication
Three levels of authentication
—Open: AP does not challenge the identity of the node.
—Password: upon association, the AP demands a password from the node.
—Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key.
#33EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
802.11 MAC Frame Format
FrameControl
Duration Addr 1
ProtocolVersion
Type Sub type To DS
FromDS
RetryLastFragment
RSVDEPPower Mgt
CRCSequenceControl
User Data
Address 4Addr 2 Addr 3
MAC Header
#34EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
802.11 MAC Frame Format
Address Fields contains
—Source address
—Destination address
—AP address
—Transmitting station address
DS = Distribution System
User Data, up to 2304 bytes long
#35EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
IEEE 802.11 LLC Layer
Provides three type of service for exchanging data between (mobile) devices connected to the same LAN
—Acknowledged connectionless
—Un-acknowledged connectionless, useful for broadcasting or multicasting.
—Connection oriented
Higher layers expect error free transmission
#36EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
IEEE 802.11 LLC Layer, Cont..
Each SAP (Service Access Point) address is 7 bits. One bit is added to it to indicate whether it is order or response.
Control has three values—Information, carry user data—Supervisory, for error control and flow
control—Unnumbered, other type of control packet
Destination SAP
Source SAP
DataControl
#37EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
IEEE 802.11 LLC <-> MAC Primitives
Four types of primitives are exchanged between LLC and MAC Layer
Request: order to perform a function
Confirm: response to Request
Indication: inform an event
Response: inform completion of process began by Indication
#38EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Reception of packets
AP Buffer traffic to sleeping nodes
Sleeping nodes wake up to listen to TIM (Traffic Indication Map) in the Beacon
AP send a DTIM (Delivery TIM) followed by the data for that station.
Beacon contains, time stamp, beacon interval, DTIM period, DTIM count, sync info, TIM broadcast indicator
#39EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN
1995 ETSI technical group RES 10 (Radio Equipment and Systems) developed HIPERLAN/1 wireless LAN standards using 5 channels in 5.15-5.3 GHz frequency range—Technical group BRAN (Broadband Radio
Access Network) is standardizing HIPERLAN/2 for wireless ATM
—ETSI URL for Hiperlan information http://www.etsi.org/frameset/home.htm?/technicalactiv/Hiperlan/hiperlan2.htm
#40EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Characteristics
HIPERLANs with same radio frequencies might overlap
—Stations have unique node identifiers (NID)
—Stations belonging to same HIPERLAN share a common HIPERLAN identifier (HID)
—Stations of different HIPERLANs using same frequencies cause interference and reduce data transmission capacity of each HIPERLAN
—Packets with different HIDs are rejected to avoid confusion of data
#41EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Protocol Layers
Data link layer = logical link control (LLC) sub layer + MAC sub layer + channel access control (CAC) sub layer
data link
physical
LLC
MAC
network
CAC
#42EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Protocol Layers, Cont..
MAC sub layer:—Keeps track of HIPERLAN addresses (HID
+ NID) in overlapping HIPERLANs—Provides lookup service between network
names and HIDs—Converts IEEE-style MAC addresses to
HIPERLAN addresses—Provides encryption of data for security
#43EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Protocol Layers, Cont..
MAC sub layer:—Provides “multi hop routing” – certain
stations can perform store-and-forwarding of frames
—Recognizes user priority indication (for time-sensitive frames)
#44EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Protocol Layers, Cont..
CAC sub layer:—Non-preemptive priority multiple access
(NPMA) gives high priority traffic preference over low priority
—Stations gain access to channel through channel access cycles consisting of 4 phases:
#45EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Protocol Layers, Cont…
CAC is designed to give each station (of same priority) equal chance to access the channel —First stations with highest priority data are
chosen. The rest will back off until all higher priority data is transmitted.
—Stations with the same priority level data, compete according to a given rule to choose “survivors”
—Survivors wait a random number of time slots and then listen to see if the channel is idle
#46EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
HIPERLAN Protocol Layers, Cont…
—If the channel is idle then it starts transmitting.
—Those who could not transmit wait until next period
#47EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
Reading assignment
Mobile Data and Wireless LAN technologies, Riffat Dayem, Chapters 4, 6 and 8.
Wirless LAN, Jim Geier, Part I chapter 3, Part II chapter 4
#48EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU
3G
http://www.3gpp.org/
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