Switched and Wireless LANs

2Shared media LANs

Limits to Shared Media LANs

– FDDI, 100Base-X, 100VG-AnyLAN all shared media LANs

Only one station can transmit at a time, causing latency

Every station hears every message, so as the number of stations grow, the LAN saturates

– 100 Mbps speed only delays saturation

3Shared media LANs

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

4Switched LANs

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

5Switch

Switch

StationA

StationB

StationC

StationD

Connection 1A-C

Connection 1A-C

Connection 2B-D

Connection 2B-D

With a switch, multiple stations may transmit simultaneously: no congestion as traffic grows.

6Switching in Perspective

Switching is the wave of the future for LANs

– Congestion does not increase as the number of stations grows

However,

– Today, however, switches are still more expensive than 10Base-T or 100Base-X hubs

Read CISCO white paper

– discount the sales talk

– see 3COM images of switches.

7Switch connections paths called connections must be pre-defined

between stations

a fixed logical data link (logical connection) is established between stations before transmission even begins

during the transmission, all traffic between the stations must pass over that data link

unless a data link has been pre-established, two stations may not communicate at all

only OSI Layer 2 (Data Link Layer) protocols are needed

8Ethernet Switches Ethernet Hubs are Half Duplex

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

Lowest-cost LAN switches

Not standardized, so buyers tend to get locked into a single vendor

9ATM Switches

Asynchronous Transfer Mode

Will allow much higher speeds– 155 Mbps to a few Gbps

Can also be used for long-distance networking– A single solution for both needs

Quality of service guaranteed

Far more expensive than Ethernet LAN switches

10ATM Switches

standardized (others not yet)

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

11ATM and Ethernet

100Mbps and Gigabit Ethernet are outselling ATM for LAN usage

High-speed Ethernet is less expensive

Staff does not have to learn ATM technology

Sales of NICs - Ethernet, Token Ring and ATM.

12Wireless LAN

BroadcastSignal

TransceiverTransmitting

TransceiverReceiving

ClusterTransceiver

Receiving

Antenna

Hub Controller

Wireless LAN

13Typical 802.11 Wireless LAN Operation with Access Points

Switch

Client PCServer

Large Wired LAN

AccessPoint A

AccessPoint B

UTP Radio Link

HandoffIf mobile computermoves to another

access point,it switches serviceto that access point

Notebook

CSMA/CA+ACK

UTP

14 Typical 802.11 Wireless LAN Operation with Access Points

WirelessNotebook

NIC

Access Point

IndustryStandard

CoffeeCup

To EthernetSwitch

Antenna(Fan) PC Card

Connector

15Typical 802.11 Wireless LAN Operation with Access Points

D-LinkWirelessAccessPoint

Using Two Antennas Reduces Multipath Interference (See Ch. 3)

16

LinksysSwitchWith

Built-InWirelessAccess Point

Using Two Antennas Reduces Multipath Interference (See Ch. 3)

Typical 802.11 Wireless LAN Operation with Access Points

17Typical 802.11 Wireless LAN Operation with Access Points

The Wireless Station sends an 802.11 frame to a server via the access point

The access point is a bridge that converts the 802.11 frame into an 802.3 Ethernet frame and sends the frame to the server

MobileStation

AccessPoint

EthernetSwitch

Server

802.11Frame

802.3Frame

18Typical 802.11 Wireless LAN Operation with Access Points

The server responds, sending an 802.3 frame to the access point

The access point converts the 802.3 frame into an 802.11 frame and sends the frame to the mobile station.

MobileStation

AccessPoint

EthernetSwitch

Server

802.11Frame

802.3Frame

19802.11 Wireless LAN Speeds

802.11 2 Mbps (rare)2.4 GHz band (limited in

bandwidth)

802.11b 11 Mbps, 2.4 GHz3 channels/access point

802.11a 54 Mbps, 5 GHz (> bandwidth than 2.4 GHz)11 channels/access point

802.11g 54 Mbps, 2.4 GHzlimited bandwidth

20802.11 Broadcast Operation

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.

CollisionAbout toOccurAccess

Point

WirelessStation

WirelessStation

21CSMA/CA + ACK in 802.11 Wireless LANs

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)– Station or access point sender listens for traffic

If there is no traffic, can send if there has been no traffic for a specified amount of time

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

22CSMA/CA + ACK in 802.11 Wireless LANs

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)– Station or access point sender listens for traffic

If there is traffic, the sender must wait until traffic stops

The sender must then set a random timer and must wait while the timer is running

If there is no traffic when the station or access point finishes the wait, it may send

23CSMA/CA + ACK in 802.11 Wireless LANs

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

24Who Implements CSMA/CA+ACK?

Stations (when they send)

Access Points (when they send)

MobileStation

AccessPoint

802.11Frame

CSMA/CA+ACK

25Request to Send (RTS) / Clear to Send (CTS)

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

MobileStation

AccessPoint

RTS

CTS

26Ad Hoc 802.11 Networks

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

27802.11 Security

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 802.11 WLAN

OutsideAttacker

28802.11 Security

By default, security on 802.11 WLAN NICs and access points is turned off, making external attacks trivial

WLAN vendors offer Wired Equivalent Privacy (WEP), but this is weak and easily broken.

The 802.11 Working Group is working on a temporary replacement (TKIP) and longer-term security replacement, 802.11i

Even if corporate access points can be secured, many departments create unauthorized rogue access points that are seldom secured.