CWNA guide to wireless LANs 2nd ch05

CWNA Guide to Wireless LANs, Second Edition Chapter Five IEEE 802.11 Media Access Control and Network Layer Standards Objectives • List and define the three types of WLAN configurations • Tell the function of the MAC frame formats • Explain the MAC procedures for joining, transmitting, and remaining connected to a WLAN • Describe the functions of mobile IP CWNA Guide to Wireless LANs, Second Edit IEEE Wireless LAN Configurations: Basic Service Set • 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 CWNA Guide to Wireless LANs, Second Edit IEEE Wireless LAN Configurations: Basic Service Set (continued) Figure 5-1: Basic Service Set (BSS) CWNA Guide to Wireless LANs, Second Edit IEEE Wireless LAN Configurations: Extended Service Set • Extended Service Set (ESS): Comprised of two or more BSS networks connected via a common distribution system • APs can be positioned so that cells overlap to facilitate roaming – Wireless devices choose AP based on signal strength – Handoff CWNA Guide to Wireless LANs, Second Edit IEEE Wireless LAN Configurations: Extended Service Set (continued) Figure 5-2: Extended Service Set (ESS) CWNA Guide to Wireless LANs, Second Edit 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 CWNA Guide to Wireless LANs, Second Edit IEEE Wireless LAN Configurations: Independent Basic Service Set (continued) Figure 5-3: Independent Basic Service Set (IBSS) CWNA Guide to Wireless LANs, Second Edit IEEE 802.11 Media Access Control (MAC) Layer Standards • Media Access Control (MAC) layer performs several vital functions in a WLAN – – – – Discovering WLAN signal Joining WLAN Transmitting on WLAN Remaining connected to WLAN • Mechanics of how functions performed center around frames sent and received in WLANs CWNA Guide to Wireless LANs, Second Edit MAC Frame Formats • Packet: Smaller segments of a digital data transmission – Strictly speaking, other terms used to describe these smaller segments • Frames: Packet at MAC layer – Or Data Link layer in OSI model – IEEE MAC frames different from 802.3 Ethernet frames in format and function – Used by wireless NICs and APs for communications and managing/controlling wireless network CWNA Guide to Wireless LANs, Second Edit 10 Transmitting on the WLAN: Quality of Service and 802.11e (continued) Table 5-1: Wi-Fi Multimedia (WMM) CWNA Guide to Wireless LANs, Second Edit 44 Transmitting on the WLAN: Quality of Service and 802.11e (continued) • 802.11e draft (continued): – Enhanced Distributed Channel Access (EDCA): Contention-based but supports different types of traffic • Four access categories (AC) • Provides “relative” QoS but cannot guarantee service – Hybrid Coordination Function Controlled Channel Access (HCCA): New form of PCF based upon polling • Serves as a centralized scheduling mechanism CWNA Guide to Wireless LANs, Second Edit 45 Remaining Connected to the WLAN: Reassociation • Reassociation: Device drops connection with one AP and establish connection with another – Several reason why reassociation may occur: • Roaming • Weakened signal – When device determines link to current AP is poor, begins scanning to find another AP • Can use information from previous scans CWNA Guide to Wireless LANs, Second Edit 46 Remaining Connected to the WLAN: Power Management • When laptop is part of a WLAN, 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 CWNA Guide to Wireless LANs, Second Edit 47 Remaining Connected to the WLAN: Power Management (continued) Figure 5-19: Power management in infrastructure mode CWNA Guide to Wireless LANs, Second Edit 48 Remaining Connected to the WLAN: Power Management (continued) • 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 CWNA Guide to Wireless LANs, Second Edit 49 WLAN Network Layer Standards: 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 CWNA Guide to Wireless LANs, Second Edit 50 WLAN Network Layer Standards: 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 CWNA Guide to Wireless LANs, Second Edit 51 WLAN Network Layer Standards: Mobile IP (continued) Figure 5-20: Mobile IP components CWNA Guide to Wireless LANs, Second Edit 52 WLAN Network Layer Standards: Mobile IP (continued) Figure 5-21: Computer relocated in Mobile IP CWNA Guide to Wireless LANs, Second Edit 53 WLAN Network Layer Standards: Mobile IP (continued) Figure 5-22: Encapsulated Mobile IP frame CWNA Guide to Wireless LANs, Second Edit 54 Summary • A Basic Service Set (BSS) is defined as a group of wireless devices that is served by a single access point (AP) • An Extended Service Set (ESS) is comprised of two or more BSS networks that are connected through a common distribution system • An Independent Basic Service Set (IBSS) is a wireless network that does not use an access point • Frames are used by both wireless NICs and access points for communication and for managing and controlling the wireless network CWNA Guide to Wireless LANs, Second Edit 55 Summary (continued) • The MAC layer provides four major functions in WLANs: discovering the WLAN signal, joining the WLAN, transmitting on the WLAN, and remaining connected to the WLAN • Discovery is a twofold process: the AP or other wireless devices must transmit an appropriate frame (beaconing), and the wireless device must be looking for those frames (scanning) • Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association CWNA Guide to Wireless LANs, Second Edit 56 Summary (continued) • The IEEE 802.11 standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF) • The 802.11 standard provides for an optional polling function known as Point Coordination Function (PCF) • The 802.11e draft defines a superset of features that is intended to provide QoS over WLANs CWNA Guide to Wireless LANs, Second Edit 57 Summary (continued) • Power management allows mobile devices to be off as much as possible to conserve battery life but not miss data transmissions • Mobile IP provides a mechanism within the TCP/IP protocol to support mobile computing CWNA Guide to Wireless LANs, Second Edit 58 ... Destination address always set to all ones CWNA Guide to Wireless LANs, Second Edit 16 Discovering the WLAN: Beaconing (continued) Figure 5-7: Beaconing CWNA Guide to Wireless LANs, Second Edit 17 Discovering... (CSMA/CD) CWNA Guide to Wireless LANs, Second Edit 29 Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-12: Hidden node problem CWNA Guide to Wireless LANs, Second... without being detected CWNA Guide to Wireless LANs, Second Edit 24 Joining the WLAN: Association • Association: Accepting a wireless device into a wireless network – Final step to join WLAN • After