Wireless LANs. Wireless Network

Transcription

1 Wireless LANs Gruppo Reti TLC COMPUTER NETWORKS Standard for LANs 26 Wireless Network Needs for wireless networks: Widespread usage of mobile terminals (notebooks, tablets, smartphones) Users want to have the same kind of services when in a mobile and in a fixed environment Network cabling sometime is impossible or hard (e.g., historical buildings) COMPUTER NETWORKS Standard for LANs 27 Pag. 1

2 Wireless Networks Europa: ETSI HIPERLAN Project Uses frequency bands at 5.2, 17.1 GHz 3 different specifications, known as HIPERLAN, HIPERACCESS, and HIPERLINK (the latest are partially related to ATM) USA: Integrated in the IEEE 802 project and compatible with the other LAN technologies Uses unlicensed frequencies in the ISM (industrial, scientific and medical) bands: 2.4, 5 GHz Different technical features but similar philosophy COMPUTER NETWORKS Standard for LANs 28 Common features Support either centralized or ad-hoc networks Random access MAC Capacity comparable to cabled LANs (10-50 Mbit/s), but smaller aggregated throughput LLC layer identical to wired LANs COMPUTER NETWORKS Standard for LANs 29 Pag. 2

3 Scenarios Ad-Hoc Configuration: No base station and direct communication between terminals Extended LAN configuration AP = Access Point o Base Station BSS = Basic Service Set COMPUTER NETWORKS Standard for LANs 30 The Standard Define the MAC and physical layer protocols Bitrate: from 1 to 54 Mbit/s (on the physical channel, depending on the frequency) Basic Service Set (BSS): is a group of terminals (PC NIC) in an Ad-Hoc network Access Point (AP): bridge to other IEEE 802.x LANs BSS+AP = Distribution System or Extended Service Set (ESS) Limited mobility support COMPUTER NETWORKS Standard for LANs 31 Pag. 3

4 MAC protocol CSMA-CA: CSMA, with Collition Avoidance Listen the channel for DISF (Distributed Inter Frame Space) seconds Transmit the frame The receiver sends an explicit ACK after SIFS (Short Inter Frame Space) seconds If channel is busy, will retry using a binary backoff NAV: Network Allocation Vector (minimal time before trying again the access procedure) COMPUTER NETWORKS Standard for LANs 32 MAC Protocol COMPUTER NETWORKS Standard for LANs 33 Pag. 4

5 Hidden Terminal Problem Plain CSMA/CA might be inefficient when there are hidden terminals : A and C cannot directly talk due to obstacles or attenuation, but they can talk with B, and their communications toward B may overlap and collide COMPUTER NETWORKS Standard for LANs 34 Hidden Terminal Problem Transmission of two short request and confirmation messages: RTS: Request-to-Send CTS: Clear-to-Send CTS freezes all the terminals near the receiver (but possibly hidden to the transmitter): avoids collitions over the data frames RTS/CTS are short, so collitions over them will have limited impact defines also a centralized polling protocol COMPUTER NETWORKS Standard for LANs 35 Pag. 5

6 RTS/CTS COMPUTER NETWORKS Standard for LANs 36 Pag. 6