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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.