Switching (2013) (796251), страница 2

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It operates at data link layer.802.11 standards differences:• Band (5.7 or 2.4 GHz): smaller frequency signals have better range and less absorbed by obstacles,but larger antennas.• Modulation techniques: direct sequence spread spectrum (DSSS) is worse than orthogonal frequency division multiplexing (OFDM).Carrier Sense Multiple Access with Collision Avoidance (CSMA/CD): devices must sense the mediumfor energy and wait until the medium is free before sending.In small business and homes wireless router act as AP, switch and router.Shared service set identifier (SSID) - unique identifier used by clients to distinguish between WLANs inthe same vicinity.The band is broken into several channels with a separator between center frequences of successive channels.

No overlapped channels are preferred.Service sets:• ad hoc networks (without access point)• basic service sets (single AP)• extended service setsThe common distribution system allows multiple APs to be in single BSS.Primary components of 802.11:• beacon (frame to advertise the presence)• probe (used by clients to find their networks)• authentication• association (establishing a data link)Process before sending data:1. Client sends SSID and supported rates, AP responses the same fields and security implementation.2. Authentication is based on wired equivalency protection (WEP).(a) authentication request by client(b) response a text(c) client encrypts the text using shared key and sends to AP(d) decrypting and answering3.

Associate stage: client learns BSSID which is AP MAC, AP maps a logical port (AID) to the client.AAA - authentication (client identification), authorization (checking special credentials) and accounting(holding logs).Threads to wireless security:10• war driving (simply exploiting open networks)• hackers (exploit weak privacy measures)• using client's data in open networks• man-in-the-middle• denial of serviceSteps to secure WLAN:1. SSID (open, not secure)2. WEP (static, breakable keys, not scalable)3.

WPA (strong user-based authentication)4. WPA2 (AES, dynamix key management)Extensible authentication protocol (EAP) is a framework for authenticating network access.1. identify request (from AP)2. identify response3. EAP request (from authentication server)4. EAP response5. successfulTKIP is the encryption method certified as WPA.AES has the same functions as TKIP, but uses additional data from MAC header to recognize tamperingand also adds sequence number to header.Methods of access controlling:• disabling SSID broadcasts• MAC filtering• WPA2Standard troubleshooting practice:1.

Eliminate a client device as problem source.2. Confirm the physical status of devices.3. Inspect wired links.Channel overlapping may cause problems.Other devices can "hog" some channels.Orientation of antenna can reduce coverage in some places.11.

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