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Transcript of Lecture 13 and 14: WLAN Anders Västberg [email protected] 08-790 44 55 Slides are a selection from...
Lecture 13 and 14: WLAN
Anders Västberg
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
• LAN Extension
• Cross-building interconnect
• Nomadic Access
• Ad hoc networking
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
• 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
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
Protocol Architecture
• Functions of physical layer:– 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
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– Source issues request to send (RTS)– Destination responds with clear to send (CTS)– Source transmits data– Destination responds with ACK
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 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
• 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
• 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– Data– Data + CF-Ack– Data + CF-Poll– Data + CF-Ack + CF-Poll
• Other subtypes (don’t carry user data)– Null Function– CF-Ack– CF-Poll– CF-Ack + CF-Poll
Management Frame Subtypes
• Association request
• Association response
• Reassociation request
• Reassociation response
• Probe request
• Probe response
• Beacon
Management Frame Subtypes
• Announcement traffic indication message
• Dissociation
• Authentication
• Deauthentication
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– 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 64-
QAM
• IEEE 802.11b– Provides data rates of 5.5 and 11 Mbps– Complementary code keying (CCK) modulation scheme