Download Basic Service Set (BSS)

Wireless Networking
WAN Design
Module-06
Jerry Bernardini
Community College of Rhode Island
5/25/2017
Wireless Networking J. Bernardini
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Presentation Reference Material
• CWNA Certified Wireless Network
Administration Official Study Guide, Fourth
Edition, Tom Carpenter, Joel Barrett
– Chapter-5 Pages 204-231
• The California Regional Consortium for
Engineering Advances in Technological
Education (CREATE) project
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IEEE 802.11 Service Sets
• Basic Service Sets – 3 types
– Independent Basic Service Set (IBSS)
– Infrastructure Basic Service Set (BSS)
– Extended Basic Service Set (ESS)
Note: The BSS is the fundamental building block for all 802.11 wireless
networks.
– Note:
– for IEEE 802.3 each device on the network is a node or host
– for IEEE 802.11 each device on the network is a Station (STA)
Basic Service Set
BSS - The Basic Service Set is a term used to describe the
collection of Stations which may communicate together within an
802.11 WLAN.
•
•
•
•
Basic Service Set (BSS): Group of
wireless devices served by single AP
– infrastructure mode
BSS must be assigned unique identifier
– Service Set Identifier (SSID)
• Serves as “network name” for
BSS
Basic Service Area (BSA): Geographical
area of a BSS
– Max BSA for a WLAN depends on
many factors
Dynamic rate shifting: As mobile
devices move away from AP,
transmission speed decreases
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Extended Service Set
• ESS - is comprised of a number BSS’s
• ESS stations must have the same SSID
• The BSSID is the “name” of the BSS (not same as SSID)
• APs can be positioned so that cells overlap to facilitate roaming
– Wireless devices choose AP based on signal strength
– Stations going from one BSS to another will deal with Handoff
Wired LAN
ESS
SSID
BSS2
(BSSID2)
BSS1
(BSSID1)
BSS3
(BSSID3)
IEEE Wireless LAN Configurations:
Independent Basic Service Set
•
•
•
Independent Basic Service Set (IBSS):
Wireless network that does not use an
AP
– Wireless devices communicate
between themselves
– Peer-to-peer or ad hoc mode
BSS more flexible than IBSS in being
able to connect to other wired or
wireless networks
IBSS useful for quickly and easily
setting up wireless network
– When no connection to Internet or
external network needed
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Service Set Identifiers - SSID and BSSID
• SSID -Service Set Identifier is a 1-32 byte alphanumeric
sequence that uniquely names an ESS (the network name).
• Any SSID or Null SSID is a blank SSID used to associate with
anyone.
• BSSID- Basic Service Set Identifier is a 48-bits that uniquely
identifies a BSS
Wired LAN
AP
ESS
Id = SSID
BSS
Id = BSSID
Reassociation
Wired LAN
Access Points
Link Fading
1
2
1
Wireless Clients
Reassociation
Request
Load Balancing or Sharing
Wired LAN
Access Points
1
2
2
2
1
1
Wireless Clients
2
1
WLAN IP Addressing
• In standard networking, IP protocol responsible for
moving frames between computers
– Network layer protocol
• TCP/IP works on principle that each network host has
unique IP address
– Used to locate path to specific host
– Routers use IP address to forward packets
– Prohibits mobile users from switching to another network and using same IP
number
• Users who want to roam need new IP address on every network
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Mobile IP
•
Provides mechanism within TCP/IP protocol to support mobile computing
– Computers given home address,
• Static IP number on home network
– Home agent: Forwarding mechanism that keeps track of where mobile
computer located
– When computer moves to foreign network, a foreign agent provides routing
services
• Assigns computer a care-of address
• Computer registers care-of address with home agent
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Mobile IP Components
MN – Mobile Node (VMD)
FA – Foreign Agent (MAP)
HA – Home Agent (MAP)
AAP – Authoritative Access Point
MAP – Mobile IP Access Point (MN)
VMD – Visiting Mobile Device (MN)
COA – Care-of-Address
HAN – Home Agent Network
FAN – Foreign Agent Network
CN – Correspondent Node
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Subnets and Mobile IP
• Subnets provide many advantages:
–
–
–
–
make network management easier
Subnets reduce broadcasts
Subnets provide security
Subnets use subnet masks (255.255.255.0)
• WLAN subnets creates problems for roaming devices
• Example:
– 192.146.118.20 /24 can seamlessly roam within 192.146.118.x /24
– Note /24 is a 255.255.255.0 subnet mask
– 192.146.118.20 /24 can not seamlessly roam within 202.165.200.x /24
without changing the IP address
• Mobile IP offers a solution
– 192.146.118.20 can roam as a “guest” on the 202.165.200.x network
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Mobile IP Devices
• Visiting Device
– Some APs support Mobile IP and Visiting devices
• Mobile IP Access Point
– Performs all Mobile IP services
• Home Agent
– Visiting devices home router
– Tunnels packets from “visited” network to “visiting” device
• Authoritative Access Point (AAP)
– Uses a subnet map to track all home “visiting” devices
• Foreign Agent
– A router the is the attachment point for “visiting” devices
– Tunnels packets to the Home Agent for the “visiting” device
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Mobile IP Process - Discovery
Mobile IP
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3-Step Process
1-Discovery
2-Registration
MN – Mobile Node
FA – Foreign Agent
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1
4
HA – Home Agent
3-Tunneling
1- Mobile Node (MN) gets address from Home Agent (HA)
2- MN moves to Foreign Agent (FA ) network
3- MN discovers FA or solicits for an agent.
4- MN gets Care-of-Address (COA) from FA
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Mobile IP Process - Registration
Mobile IP
3-Step Process
1-Discovery
12
10
2-Registration
5
7
6
9
8
11
3-Tunneling
MN – Mobile Node
FA – Foreign Agent
HA – Home Agent
CN – Correspondent Node
5- MN send registration request to FA
6- FA checks request, adds MN to pending list
10- FA checks reply and adds MN to visitor list and
relays reply to MN
7- FA relays request to HA
11-FA creates a routing entry and tunnel to HA
8- HA checks request and creates a mobile binding
12- MN check reply and creates a tunnel to the HA
9- HA sends registration reply to FA
Through the FA
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Mobile IP- Tunneling and Packet
Forwarding
MN – Mobile Node
FA – Foreign Agent
HA – Home Agent
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CN – Correspondent Node
COA- Care-of-Address
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15
13- Correspondent Node (CN) sends packet to MN and is routed to HA
14- HA intercepts packets and tunnels to the FA using the COA
15- FA relays the packets to the MN
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Infrastructure Mode
CWNA Guide to Wireless LANs,
Second Edition
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Channel reuse
CWNA Guide to Wireless LANs,
Second Edition
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Flip flop between access points
CWNA Guide to Wireless LANs,
Second Edition
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WLAN Design Models
•Point-to-Point (PtP)
•Point-to-Multipoint (PtMP)
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WLAN Modes
• Single MAC Model
– Edge, Autonomous, Stand-Alone, Fat-AP
• Split MAC Model
– Centralized, Thin-AP
• Mesh Network
– Distributed, Multipath, IEEE 802.11s
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Single MAC Model
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Split MAC Model
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WLAN Model Evolution
• Intelligent Edge(Distribution)
– Quick to setup but for small-medium networks
– Difficult to mage for large networks
• WLAN Network Management Systems
– Centralized Management Distribution Processing
– For large networks
• Centralized WLAN Architecture (Split MAC)
– For large networks with centralized controller
– Large amount of wiring needed
• Distributed Data Forwarding (DDF) WLAN
– Similar to Split MAC but uses Fat-AP
• Unified WLAN Architecture
– Wireless built in to every thing including switches
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WLAN Power Management Features
• Active Mode
– No power saving but improved station and AP performance
– For desktops and line powered laptops
• Power Save Mode
– Dozing and Wake modes
– Switches to wake to check for frames
• WMM Power Save
– U-APSD Unscheduled Automatic Power-Save Delivery
– This is an industry certification
– IEEE 802.11e-2005
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Power Management
•
•
A WLAN laptop must remain “awake” in order to receive network transmissions
– Original IEEE 802 standard assumes stations always ready to receive network
messages
Power management: Allows mobile devices to conserve battery life without
missing transmissions
– Transparent to all protocols
– Differs based on WLAN configuration
– AP records which stations awake and sleeping
– Buffering: If sleeping, AP temporarily stores frames
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Power Management
• At set times AP send out beacon to all stations
– Contains traffic indication map (TIM)
– At same time, all sleeping stations switch into active listening mode
• Power management in ad hoc mode:
– Ad hoc traffic indication message (ATIM) window: Time at which all stations
must be awake
• Wireless device sends beacon to all other devices
– Devices that previously attempted to send a frame to a sleeping device will
send ATIM frame indicating that receiving device has data to receive and must
remain awake
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Continuous Aware Mode
• Constantly Awake Mode provides the best
performance allowing the client a strong connection
between the wireless card and the AP; however, it
also rapidly drains the client’s battery, resulting in
shorter battery life.
Power Management with TIM/DTIM/ATIM
• Traffic Indication Map (TIM)
– A table stored on the AP of all STA’s in Power Save mode
– TIM is used to determine which STA’s require frame buffering
– Every Beacon contains a TIM
• Delivery Traffic Indication Message (DTIM)
– Used to manage STAs and to program wakeup
– Sent on every few (third or some interval)Beacon
• Ad Hoc Traffic Indication Message (ATIM)
– Use to power manage IBSS
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