Switched and Wireless LANs

2 Shared media LANs u Limits to Shared Media LANs –FDDI, 100Base-X, 100VG-AnyLAN all shared media LANs v Only one station can transmit at a time, causing latency v Every station hears every message, so as the number of stations grow, the LAN saturates –100 Mbps speed only delays saturation

3 Shared media LANs u Shared Media Networks with Hubs (such as 10Base-T) –Incoming frame arrives through a single port –Hub broadcasts frames out all ports –Congestion on output ports Hub

4 Switched LANs u In a switched network –Incoming frame arrives on a single port –Frame sent out again only on a single port--the one leading to the receiver –No congestion on other ports Switch

5 Station A Station B Station C Station D Connection 1 A-C Connection 1 A-C Connection 2 B-D Connection 2 B-D With a switch, multiple stations may transmit simultaneously: no congestion as traffic grows.

6 Switching in Perspective u Switching is the wave of the future for LANs –Congestion does not increase as the number of stations grows u However, –Today, however, switches are still more expensive than 10Base-T or 100Base-X hubs u Read CISCO white paperwhite paper –discount the sales talk –see 3COM images of switches.images

7 Switch connections u paths called connections must be pre-defined between stations u a fixed logical data link (logical connection) is established between stations before transmission even begins u during the transmission, all traffic between the stations must pass over that data link u unless a data link has been pre-established, two stations may not communicate at all u only OSI Layer 2 (Data Link Layer) protocols are needed

8 Ethernet Switches u Ethernet Hubs are Half Duplex u Most Ethernet Switches are Full Duplex –No collisions are possible –So two stations can both transmit to each other at the same time (full duplex operation) –Requires full duplex switches –Requires full duplex NICs u Lowest-cost LAN switches u Not standardized, so buyers tend to get locked into a single vendor

9 ATM Switches u Asynchronous Transfer Mode u Will allow much higher speeds –155 Mbps to a few Gbps u Can also be used for long-distance networking –A single solution for both needs u Quality of service guaranteed u Far more expensive than Ethernet LAN switches

10 ATM Switches u standardized (others not yet) u scalable: as low as 1 Mbps to 2.4 Gbps –can start with relative slow speeds (cheaper) –increase the speed as needs arise –without changing protocol

11 ATM and Ethernet u 100Mbps and Gigabit Ethernet are outselling ATM for LAN usage u High-speed Ethernet is less expensive u Staff does not have to learn ATM technology u Sales of NICs - Ethernet, Token Ring and ATM. Sales of NICs

12 Wireless LAN Broadcast Signal Transceiver Transmitting Transceiver Receiving Cluster Transceiver Receiving Antenna Hub Controller Wireless LAN

13 Typical Wireless LAN Operation with Access Points Switch Client PC Server Large Wired LAN Access Point A Access Point B UTPRadio Link Handoff If mobile computer moves to another access point, it switches service to that access point Notebook CSMA/CA+ACK UTP

14 Typical Wireless LAN Operation with Access Points Wireless Notebook NIC Access Point Industry Standard Coffee Cup To Ethernet Switch Antenna (Fan) PC Card Connector

15 Typical Wireless LAN Operation with Access Points D-Link Wireless Access Point Using Two Antennas Reduces Multipath Interference (See Ch. 3)

16 Linksys Switch With Built-In Wireless Access Point Using Two Antennas Reduces Multipath Interference (See Ch. 3) Typical Wireless LAN Operation with Access Points

17 Typical Wireless LAN Operation with Access Points u The Wireless Station sends an frame to a server via the access point u The access point is a bridge that converts the frame into an Ethernet frame and sends the frame to the server Mobile Station Access Point Ethernet Switch Server Frame Frame

18 Typical Wireless LAN Operation with Access Points u The server responds, sending an frame to the access point u The access point converts the frame into an frame and sends the frame to the mobile station. Mobile Station Access Point Ethernet Switch Server Frame Frame

Wireless LAN Speeds u Mbps (rare) 2.4 GHz band (limited in bandwidth) u b11 Mbps, 2.4 GHz 3 channels/access point u a54 Mbps, 5 GHz (> bandwidth than 2.4 GHz) 11 channels/access point u g54 Mbps, 2.4 GHz limited bandwidth

Broadcast Operation u The Wireless Stations and Access Points Broadcast their Signals. –Only one access point or wireless station may transmit at any moment or signals will become scrambled. Collision About to Occur Access Point Wireless Station Wireless Station

21 CSMA/CA + ACK in Wireless LANs u CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) –Station or access point sender listens for traffic v If there is no traffic, can send if there has been no traffic for a specified amount of time v If the specified amount of time has not been met, must wait for the specified amount of time. Can then send if the line is still clear

22 CSMA/CA + ACK in Wireless LANs u CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) –Station or access point sender listens for traffic v If there is traffic, the sender must wait until traffic stops v The sender must then set a random timer and must wait while the timer is running v If there is no traffic when the station or access point finishes the wait, it may send

23 CSMA/CA + ACK in Wireless LANs u ACK (Acknowledgement) –Receiver immediately sends back an acknowledgement; no waiting because ACKs have highest priority –If sender does not receive the acknowledgement, retransmits using CSMA/CA

24 Who Implements CSMA/CA+ACK? u Stations (when they send) u Access Points (when they send) Mobile Station Access Point Frame CSMA/CA+ACK

25 Request to Send (RTS) / Clear to Send (CTS) u There is a widely used option we should cover. –After a station may send, its first message may be a Request-to-Send (RTS) message instead of a data message –Only if the other party sends a Clear-to-Send (CTS) message does the sender begin sending data Mobile Station Access Point RTS CTS

26 Ad Hoc Networks u Ad Hoc Mode –There is no access point. –Stations broadcast to one another directly –Not scalable but can be useful for SOHO use –NICs automatically come up in ad hoc mode

Security u Attackers can lurk outside your premises –In “war driving,” drive around sniffing out unprotected wireless LANs –In “drive by hacking,” eavesdrop on conversations or mount active attacks. Site with WLAN Outside Attacker

Security u By default, security on WLAN NICs and access points is turned off, making external attacks trivial u WLAN vendors offer Wired Equivalent Privacy (WEP), but this is weak and easily broken. u The Working Group is working on a temporary replacement (TKIP) and longer-term security replacement, i u Even if corporate access points can be secured, many departments create unauthorized rogue access points that are seldom secured.