Madge Networks 802.11b Manual do Utilizador Página 1

Link Layer Standards and Protocols

Other Features of Token Ring• A user can designate a priority value so that only those users with this value or higher can use the network• An active

11FDDI -Fiber Distributed Data Interface (ANSI X3T12)• Similar to token ring, except that– FDDI runs on fiber-optic (token ring runs on twisted pair)–

12ThroughputToken ringToken busCSMA/CD bus24.020.04.0Non-contention based, deterministic access gives token ring/bus technologies better throughput

Elements of a Wireless NetworkNetwork Infrastructure(Optional)Wireless hosts• Laptop, PDA, IP phone• Run applications• May be stationary (non-mobile)

Elements of a Wireless NetworkNetwork infrastructureBase station• Typically connected to wired network• Relay - responsible for sending packets betwee

Elements of a Wireless NetworkNetwork InfrastructureWireless link• Typically used to connect mobile(s) to base station• Also used as backbone link • M

Elements of a wireless networkNetwork InfrastructureInfrastructure mode• base station connects mobiles into wired network• handoff: mobile changes bas

Elements of a wireless networkAd hoc mode• No base stations• Nodes can only transmit to other nodes within link coverage• Nodes organize themselves in

Wireless Link Characteristics• Differences from wired link ….– Decreased signal strength: radio signal attenuates as it propagates through matter (pat

Wireless LANs and CSMA• Before transmitting, a sending station needs to know if there is activity around the receiver– Is CSMA/CD effective here?• CSM

Link Layer StandardsLLC IEEE 802.2MAC IEEE 802.XIP Layer (Network Layer)Physical Layer

Hidden Terminal Problem• Node B can communicate with both A and C• A and C cannot hear each other• When A transmits to B, C cannot detect the transmis

Exposed Station Problem• Station B is transmitting to station A• Station C wanting to transmit to station D, would do the following– Sense the channel

General Techniques for Medium Access in Wireless• Use CSMA/CA : – sense channel for traffic– wait a random amount of time and start transmitting– Use

IEEE 802.11 Wireless LAN• 802.11b– 2.4-5 GHz unlicensed radio spectrum– up to 11 Mbps– direct sequence spread spectrum (DSSS) in physical layer• all h

802.11 LAN architecture• Wireless host communicates with base station– base station = access point (AP)• Basic Service Set (BSS)(aka “cell”) in infras

802.11: Channels, association• 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies– AP admin chooses frequency for AP–

802.11 Modes of Operation• Most wireless devices cannot transmit and listen at the same time on a single frequency, thus CSMA/CD is not used• The supp

DCF• DCF, Distributed Coordination Function– No central controller; just like Ethernet– Protocols used are:• CSMA/CA (CSMA with Collision Avoidance)–

IEEE 802.11: Medium Access• Avoid collisions: 2+nodes transmitting at same time• 802.11: CSMA - sense before transmitting– don’t collide with ongoing

IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS thentransmit entire frame (no CD)2 if sense channel busy thenstart rand

Medium Access Control Layer Services• Concerned with the following– Channel allocation– Random backoff during collision– Prioritizing– Error detection

Avoiding Collisions (more)Idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames• se

Collision Avoidance: RTS-CTS exchangeAPABtimeRTS(A)RTS(B)RTS(A)CTS(A)CTS(A)DATA (A)ACK(A)ACK(A)reservation collisiondefer

The 802.11 MAC Protocol…NAV=NetworkAllocationVector,aninternalremindertokeepquietforacertainperiodoftime• The use of virtual channel s

The 802.11 MAC Protocol…• NAV for a particular node depends on which frame it is able to hear– Nodes in sender’s range have longer NAV compared to nod

PCF: Point Coordination Function• The base station polls the other stations asking if they have any frames to transmit, and co-ordinates transmissions

Extending Wireless Network• Some nodes are allowed to roam (e.g., your laptop) • Access points (AP) or base stations, are connected to a wired network

Extending Wireless Network…• Each roaming station must be associated with a single AP. This can be done:– Via active scanning, • The node sends a Pro

framecontroldurationaddress1address2address4address3payload CRC22 66 6 260 - 23124seqcontrol802.11 Frame: AddressingAddress 2: MAC addressof wireless

InternetrouterAPH1R1AP MAC addr H1 MAC addr R1 MAC addraddress 1address 2address 3802.11 frameR1 MAC addr AP MAC addr dest. address source address 8

framecontroldurationaddress1address2address4address3payload CRC22 66 6 260 - 23124seqcontrolTypeFromAPSubtypeToAPMore fragWEPMoredataPowermgtRetry Rsv

Logical Link Control Services• MAC layer is best-effort. Additional features can be introduced via LLC. IEEE 802.2 committee defines three types of LL

802.11 Services• Distribution Services (inter-cell)– Association• When a station enters the range of a base station– Disassociation• Breaking connecti

802.11 Services…• Station services (Intra-cell)– Authentication• After association, a challenge/response interaction is done– De-authentication–Privac

Wi-MAX (802.16)• Worldwide Interoperability for Microwave Access• Originally conceived as the “last-mile”technology• Client systems, called subscriber

The 802.16 Physical Layer• The 802.16 transmission environment.

802.16 Physical Layer• Operates in the 10-to-66 GHz frequency range.– 802.16a will operate in the 2-to-11 GHz band– 802.16b will operate in the 5 GHz

Other Popular Wireless• Bluetooth : Aimed to eliminate wires between personal networked devices– E.g., data transfer between a PDA and a cell phone• U

Standard Link Layer Protocols• Ethernet (IEEE 802.3)• Token Ring (IEEE 802.5/IBM)• FDDI (ANSI X3T12)• Wireless– Wi-fi (802.11x)– Wi-Max (IEEE 802.16)–

IEEE 802.5/IBM Token Ring• The stations form a logical ring (star-wired-ring network)– Small frame (token) circulates when idle– Only the possession o

Token Ring Operation

Token Ring MAC FrameToken Specifier