Wireless Networking

WAN DesignModule-06

Jerry BernardiniCommunity College of Rhode Island

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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

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• 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

BSS - The Basic Service Set is a term used to describe the collection of Stations which may communicate together within an 802.11 WLAN.

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

BSS1(BSSID1)

ESSSSID

BSS2(BSSID2)

BSS3(BSSID3)

IEEE Wireless LAN Configurations: Independent Basic Service Set

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• 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

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

ESSId = SSID

BSSId = BSSID

Reassociation

Wireless Clients

Access Points

Wired LAN

1

1 2

Load Balancing or Sharing

Wireless Clients

Access Points

Wired LAN

2

1 2

1

1

1

2

2

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

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• 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

Mobile IP-Computer relocated

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CWNA Guide to Wireless LANs, Second Edition 13

Infrastructure Mode

CWNA Guide to Wireless LANs, Second Edition 14

Channel reuse

CWNA Guide to Wireless LANs, Second Edition 15

Flip flop between access points

WLAN Design Models

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•Point-to-Point (PtP)•Point-to-Multipoint (PtMP)

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

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• 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

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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