Transcript Document

Lecture 13 and 14: WLAN
Anders Västberg
[email protected]
08-790 44 55
Slides are a selection from the slides from
chapter 12,13, and 14 from:
http://williamstallings.com/Wireless/Wireless2e.html
Mobile IP Uses
• Enable computers to maintain Internet connectivity
while moving from one Internet attachment point to
another
• Mobile – user's point of attachment changes
dynamically and all connections are automatically
maintained despite the change
• Nomadic - user's Internet connection is terminated
each time the user moves and a new connection is
initiated when the user dials back in
– New, temporary IP address is assigned
Operation of Mobile IP
• Mobil node is assigned to a particular network –
home network
• IP address on home network is static – home address
• Mobile node can move to another network – foreign
network
• Mobile node registers with network node on foreign
network – foreign agent
• Mobile node gives care-of address to agent on home
network – home agent
Capabilities of Mobile IP
• Discovery – mobile node uses discovery
procedure to identify prospective home and
foreign agents
• Registration – mobile node uses an
authenticated registration procedure to inform
home agent of its care-of address
• Tunneling – used to forward IP datagrams
from a home address to a care-of address
Discovery
• Mobile node is responsible for ongoing discovery
process
– Must determine if it is attached to its home network or a
foreign network
• Transition from home network to foreign network can
occur at any time without notification to the network
layer
• Mobile node listens for agent advertisement messages
– Compares network portion of the router's IP address with
the network portion of home address
Agent Solicitation
• Foreign agents are expected to issue agent
advertisement messages periodically
• If a mobile node needs agent information
immediately, it can issue ICMP router
solicitation message
– Any agent receiving this message will then issue
an agent advertisement
Move Detection
• Mobile node may move from one network to another
due to some handoff mechanism without IP level
being aware
– Agent discovery process is intended to enable the agent to
detect such a move
• Algorithms to detect move:
– Use of lifetime field – mobile node uses lifetime field as a
timer for agent advertisements
– Use of network prefix – mobile node checks if any newly
received agent advertisement messages are on the same
network as the node's current care-of address
Co-Located Addresses
• If mobile node moves to a network that has no
foreign agents, or all foreign agents are busy, it can
act as its own foreign agent
• Mobile agent uses co-located care-of address
– IP address obtained by mobile node associated with mobile
node's current network interface
• Means to acquire co-located address:
– Temporary IP address through an Internet service, such as
DHCP
– May be owned by the mobile node as a long-term address
for use while visiting a given foreign network
Registration Process
• Mobile node sends registration request to
foreign agent requesting forwarding service
• Foreign agent relays request to home agent
• Home agent accepts or denies request and
sends registration reply to foreign agent
• Foreign agent relays reply to mobile node
Registration Operation
Messages
• Registration request message
– Fields = type, S, B, D, M, V, G, lifetime, home
address, home agent, care-of-address,
identification, extensions
• Registration reply message
– Fields = type, code, lifetime, home address, home
agent, identification, extensions
Registration Procedure
Security
• Mobile IP designed to resist attacks
– Node pretending to be a foreign agent sends registration
request to a home agent to divert mobile node traffic to
itself
– Agent replays old registration messages to cut mobile node
from network
• For message authentication, registration request and
reply contain authentication extension
– Fields = type, length, security parameter index (SPI),
authenticator
Types of Authentication
Extensions
• Mobile-home – provides for authentication of
registration messages between mobile node and home
agent; must be present
• Mobile-foreign – may be present when a security
association exists between mobile node and foreign
agent
• Foreign-home – may be present when a security
association exists between foreign agent and home
agent
Tunneling
• Home agent intercepts IP datagrams sent to
mobile node's home address
– Home agent informs other nodes on home network
that datagrams to mobile node should be delivered
to home agent
• Datagrams forwarded to care-of address via
tunneling
– Datagram encapsulated in outer IP datagram
Mobile IP Encapsulation
Options
• IP-within-IP – entire IP datagram becomes
payload in new IP datagram
– Original, inner IP header unchanged except TTL
decremented by 1
– Outer header is a full IP header
• Minimal encapsulation – new header is inserted
between original IP header and original IP payload
– Original IP header modified to form new outer IP
header
• Generic routing encapsulation (GRE) – developed
prior to development of Mobile IP
Wireless LAN Applications
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LAN Extension
Cross-building interconnect
Nomadic Access
Ad hoc networking
Wireless LAN
Configurations
LAN Extension
• Wireless LAN linked into a wired LAN on
same premises
– Wired LAN
• Backbone
• Support servers and stationary workstations
– Wireless LAN
• Stations in large open areas
• Manufacturing plants, stock exchange trading floors,
and warehouses
Cross-Building Interconnect
• Connect LANs in nearby buildings
– Wired or wireless LANs
• Point-to-point wireless link is used
• Devices connected are typically bridges or
routers
Nomadic Access
• Wireless link between LAN hub and mobile
data terminal equipped with antenna
– Laptop computer or notepad computer
• Uses:
– Transfer data from portable computer to office
server
– Extended environment such as campus
Ad Hoc Networking
• Temporary peer-to-peer network set up to meet
immediate need
• Example:
– Group of employees with laptops convene for a
meeting; employees link computers in a temporary
network for duration of meeting
Wireless LAN Requirements
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Throughput
Number of nodes
Connection to backbone LAN
Service area
Battery power consumption
Transmission robustness and security
Collocated network operation
License-free operation
Handoff/roaming
Dynamic configuration
Wireless LAN Categories
• Infrared (IR) LANs
• Spread spectrum LANs
• Narrowband microwave
Strengths of Infrared Over
Microwave Radio
• Spectrum for infrared virtually unlimited
– Possibility of high data rates
• Infrared spectrum unregulated
• Equipment inexpensive and simple
• Reflected by light-colored objects
– Ceiling reflection for entire room coverage
• Doesn’t penetrate walls
– More easily secured against eavesdropping
– Less interference between different rooms
Drawbacks of Infrared
Medium
• Indoor environments experience infrared
background radiation
– Sunlight and indoor lighting
– Ambient radiation appears as noise in an infrared
receiver
– Transmitters of higher power required
• Limited by concerns of eye safety and excessive power
consumption
– Limits range
Spread Spectrum LAN
Configuration
• Multiple-cell arrangement
• Within a cell, either peer-to-peer or hub
• Peer-to-peer topology
– No hub
– Access controlled with MAC algorithm
• CSMA
– Appropriate for ad hoc LANs
IEEE 802 Protocol Layers
Protocol Architecture
• Functions of physical layer:
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Encoding/decoding of signals
Preamble generation/removal (for synchronization)
Bit transmission/reception
Includes specification of the transmission medium
Protocol Architecture
• Functions of medium access control (MAC) layer:
– On transmission, assemble data into a frame with address
and error detection fields
– On reception, disassemble frame and perform address
recognition and error detection
– Govern access to the LAN transmission medium
• Functions of logical link control (LLC) Layer:
– Provide an interface to higher layers and perform flow and
error control
Separation of LLC and MAC
• The logic required to manage access to a
shared-access medium not found in traditional
layer 2 data link control
• For the same LLC, several MAC options may
be provided
MAC Frame Format
• MAC control
– Contains Mac protocol information
• Destination MAC address
– Destination physical attachment point
• Source MAC address
– Source physical attachment point
• CRC
– Cyclic redundancy check
Logical Link Control
• Characteristics of LLC not shared by other
control protocols:
– Must support multiaccess, shared-medium nature
of the link
– Relieved of some details of link access by MAC
layer
LLC Services
• Unacknowledged connectionless service
– No flow- and error-control mechanisms
– Data delivery not guaranteed
• Connection-mode service
– Logical connection set up between two users
– Flow- and error-control provided
• Acknowledged connectionless service
– Cross between previous two
– Datagrams acknowledged
– No prior logical setup
Differences between LLC
and HDLC
• LLC uses asynchronous balanced mode of
operation of HDLC (type 2 operation)
• LLC supports unacknowledged connectionless
service (type 1 operation)
• LLC supports acknowledged connectionless
service (type 3 operation)
• LLC permits multiplexing by the use of LLC
service access points (LSAPs)
IEEE 802.11 Architecture
• Distribution system (DS)
• Access point (AP)
• Basic service set (BSS)
– Stations competing for access to shared wireless medium
– Isolated or connected to backbone DS through AP
• Extended service set (ESS)
– Two or more basic service sets interconnected by DS
IEEE 802.11 Services
Distribution of Messages
Within a DS
• Distribution service
– Used to exchange MAC frames from station in one
BSS to station in another BSS
• Integration service
– Transfer of data between station on IEEE 802.11
LAN and station on integrated IEEE 802.x LAN
Transition Types Based On
Mobility
• No transition
– Stationary or moves only within BSS
• BSS transition
– Station moving from one BSS to another BSS in
same ESS
• ESS transition
– Station moving from BSS in one ESS to BSS
within another ESS
Association-Related Services
• Association
– Establishes initial association between station and AP
• Reassociation
– Enables transfer of association from one AP to another,
allowing station to move from one BSS to another
• Disassociation
– Association termination notice from station or AP
Access and Privacy Services
• Authentication
– Establishes identity of stations to each other
• Deathentication
– Invoked when existing authentication is terminated
• Privacy
– Prevents message contents from being read by
unintended recipient
IEEE 802.11 Medium Access
Control
• MAC layer covers three functional areas:
– Reliable data delivery
– Access control
– Security
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
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Source issues request to send (RTS)
Destination responds with clear to send (CTS)
Source transmits data
Destination responds with ACK
Access Control
Medium Access Control
Logic
Interframe Space (IFS)
Values
• Short IFS (SIFS)
– Shortest IFS
– Used for immediate response actions
• Point coordination function IFS (PIFS)
– Midlength IFS
– Used by centralized controller in PCF scheme when using
polls
• Distributed coordination function IFS (DIFS)
– Longest IFS
– Used as minimum delay of asynchronous frames
contending for access
IFS Usage
• SIFS
– Acknowledgment (ACK)
– Clear to send (CTS)
– Poll response
• PIFS
– Used by centralized controller in issuing polls
– Takes precedence over normal contention traffic
• DIFS
– Used for all ordinary asynchronous traffic
MAC Frame Format
MAC Frame Fields
• Frame Control – frame type, control information
• Duration/connection ID – channel allocation time
• Addresses – context dependant, types include source
and destination
• Sequence control – numbering and reassembly
• Frame body – MSDU or fragment of MSDU
• Frame check sequence – 32-bit CRC
Frame Control Fields
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Protocol version – 802.11 version
Type – control, management, or data
Subtype – identifies function of frame
To DS – 1 if destined for DS
From DS – 1 if leaving DS
More fragments – 1 if fragments follow
Retry – 1 if retransmission of previous frame
Frame Control Fields
• Power management – 1 if transmitting station is in
sleep mode
• More data – Indicates that station has more data to
send
• WEP – 1 if wired equivalent protocol is implemented
• Order – 1 if any data frame is sent using the Strictly
Ordered service
Control Frame Subtypes
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Power save – poll (PS-Poll)
Request to send (RTS)
Clear to send (CTS)
Acknowledgment
Contention-free (CF)-end
CF-end + CF-ack
Data Frame Subtypes
• Data-carrying frames
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Data
Data + CF-Ack
Data + CF-Poll
Data + CF-Ack + CF-Poll
• Other subtypes (don’t carry user data)
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Null Function
CF-Ack
CF-Poll
CF-Ack + CF-Poll
Management Frame
Subtypes
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Association request
Association response
Reassociation request
Reassociation response
Probe request
Probe response
Beacon
Management Frame
Subtypes
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Announcement traffic indication message
Dissociation
Authentication
Deauthentication
Wired Equivalent Privacy
Authentication
• Open system authentication
– Exchange of identities, no security benefits
• Shared Key authentication
– Shared Key assures authentication
Physical Media Defined by
Original 802.11 Standard
• Direct-sequence spread spectrum
– Operating in 2.4 GHz ISM band
– Data rates of 1 and 2 Mbps
• Frequency-hopping spread spectrum
– Operating in 2.4 GHz ISM band
– Data rates of 1 and 2 Mbps
• Infrared
– 1 and 2 Mbps
– Wavelength between 850 and 950 nm
IEEE 802.11a and IEEE
802.11b
• IEEE 802.11a
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Makes use of 5-GHz band
Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
Uses orthogonal frequency division multiplexing (OFDM)
Subcarrier modulated using BPSK, QPSK, 16-QAM or 64QAM
• IEEE 802.11b
– Provides data rates of 5.5 and 11 Mbps
– Complementary code keying (CCK) modulation scheme
IEEE 802.11g and IEEE
802.11n
• IEEE 802.11g: Provides data rates up to 108
Mbps and is compatible with 802.11b
• IEEE 802.11n: Even higher data rates.