Chapter 11 LAN Systems

1
Chapter 11 LAN Systems

• Fall 2007
• SCSC 412

2
Objectives

• Exam the different LAN systems
• Ethernet LANs
• Token Ring LANs (covered in slide 6)
• FDDI

3
History of Ethernet

• History of Ethernet
• Based on a 1973 Ph.D. dissertation by Bob
  Metcalfe
• Xerox PARC further developed Ethernet
• Ethernet is based on the IEEE 802.3 standard
• The oldest LAN technology remains popular today,
  because
• High-speed
• Low cost
• Multiple vendors support

4
Ethernet Topology

• Ethernet topology
• In basic form, Ethernet operates on a bus
  topology at 10 Mbps using baseband transmission
  with Manchester data coding
• The layout of Ethernet LAN is normally a physical
  star (fig 11.2)
• Equipment room
• The choice of medium is used to connect the nodes
  (fig 11.3 pros and cons of each type)
• UTP
• STP
• Coaxial cable
• Fiber optic cable

5
Ethernet Media

• 10Base-5 thick Ethernet
• 10Base-2 thin Ethernet
• 10Base-T
• 10Base-F
• 100Base-T fast Ethernet
• 100Base-TX
• 100Base-FX
• 100Base-T4
• Gigabit Ethernet
• 1000Base-CX
• 1000Base-SX
• 1000Base-LX
• 1000Base-T
• 10-Gigabit Ethernet (standard 802.3ae is being
  developed)

6
Gigabit Ethernet

• Issues on Gigabit Ethernet
• Is used as backbone for server connections or as
  the backbone in enterprise networks
• Switching hub vs. shared medium hub
• Carrier extension bits
• Frame bursting

7
Token Ring

• History
• Originally developed by IBM
• IEEE 802.5 standard
• IBM token ring star topology (physical)
• Multistation access unit (MAU)
• Logic topology is ring
• Normally operate at 4 Mbps on UTP wire or at 16
  Mbps on STP or CAT 5 UTP
• 100 Mbps high-speed token ring (HSTR)
• Major advantage of token ring is fast media fault
  correction.
• Remove faulty segment from token ring

8
Fiber Distributed Data Interface (FDDI)

• FDDI is 100 Mbps fiber optic LAN
• Developed in 1980s
• FDDI has also been implemented over twisted
  copper wire copper distributed data interface
  (CDDI)
• The standard defines two types SMF, MMF
• SMF can deliver connectivity over longer
  distances
• SMF has higher performance than MMF.
• SMF is used to connect buildings or over greater
  geographical areas
• MMF is used to connect devices within a building

9
FDDI Topology

• An FDDI network has primary and secondary token
  rings
• Why two rings?
• Message passing in FDDI
• Piggyback
• FDDI-II
• a new standard provides additional capability
• Can handle circuit switched traffic
• Constant data rate connection between two
  stations
• Required for voice and video applications

10
Wireless LAN

• Wireless Technology
• IEEE 802.11 Standards
• 802.11 Security issues

11
Wireless Technologies

• Wireless communication involves transmitting
  signals via radio waves through air and space
• television transmissions, cellular phones,
  satellite transmissions, spying, surveillance,
  and garage door openers,
• frequency and amplitudes
• Signals are measured in frequency and amplitudes.
  
• The frequency dictates how much data can be
  carried and how far.
• The higher the frequency, the more data the
  signal can carry
• The higher the frequency, the more susceptible
  the signal is to atmospheric interference. (more
  like light, which will be blocked by obstacles)
• High frequency equipments are more expensive

12
Wireless Technologies

• In wireless technologies, each device must share
  the allotted radio frequency spectrum with all
  other wireless devices that need to communicate.
• only one computer can send data at any given
  time, otherwise a collision can take place.
• Ethernet LAN employs the CSMA/CD (collision
  detection) technology. Wireless technology is
  very similar to Ethernet but it uses CSMA/CA
• Two different types of spread spectrum
  techniques
• frequency hopping
• Direct sequence

13
Frequency Hopping Spread Spectrum

• Frequency Hopping Spread Spectrum (FHSS) takes
  the total amount of bandwidth (spectrum) and
  splits it into smaller sub-channels.
• The sender and receiver work at one of these
  channels for a specific amount of time and then
  move to another channel.
• Hop sequence the FHSS algorithm determines the
  individual frequencies that will be used and in
  what order
• reduce the probability of interference, difficult
  for eavesdropping

14
Direct Sequence Spread Spectrum

• Direct Sequence Spread Spectrum (DSSS)
• the sender combines the data with the chipping
  sequence, the new form of the information is
  modulated with a radio carrier signal, and it is
  shifted to the necessary frequency and
  transmitted.
• The receiver has to know the correct chipping
  sequence to change the received data into its
  original format.
• the sender and receiver must be properly
  synchronized.
• the sequence of how the chips are applied is
  referred to as the chipping code.

15
Wireless Technologies
16
WLAN Media

• Radiated (unguided) media
• Radio
• IR
• IEEE802.11
• CSMA / CA MAC protocol
• Work in two modes
• Infrastructure mode
• Ad hoc mode

17
WLAN Topologies

• Ad hoc mdoe two computers can communicate
  directly with one another sometimes called a
  peer-to-peer topology
• Infrastructure mode Use a central device, called
  an access point (AP), to control communications
• Star physical topology
• all the signals travel through one central device
• Logical bus topology
• BSS and ESS

18
IEEE 802.11 Standards

• 802.11 standards contains a list of standards.
  Each defines a different physical layer
• 802.11b (Wi-Fi)
• 1 11 Mbps, Operates at 2.4 GHz band
• Use Direct sequence spread spectrum (DSSS)
  modulation
• DSSS sends data over several different
  frequencies at the same time
• Special pseudo noise codes (chipping codes) are
  modulated with the data stream
• Receivers recognize a spread signal and return
  the message to its original form.
• Benefit of DSSS
• Resists narrowband interference, noise jamming
  and unauthorized detection

19
IEEE 802.11 Standards

• 802.11a
• 6 54 Mbps, Operate at 5 GHz band
• Use orthogonal frequency division multiplexing
  (OFDM) modulation
• Not backward compatible with 802.11b
• 802.11g
• Up to 54 Mbps
• Use OFDM modulation
• Operates at 2.4 GHz band, backward compatible
  with 802.11b

20
IEEE 802.11 Standards

• IEEE created several task groups to work on
  specific areas within wireless communications.
• IEEE 802.11 project
• The first WLAN standard, 802.11 was developed in
  1997
• Uses FHSS
• works in the 2.4-GHz (unlicensed) frequency range
• provides 12 Mbps transfer rate

21
IEEE 802.11 Standards

• 802.11b
• Uses DSSS
• works in the 2.4-GHz (unlicensed) frequency range
• provides up to 11 Mbps transfer rate
• The most common standard used today
• 802.11a
• Uses use the orthogonal frequency division
  multiplexing (OFDM) modulation scheme
• works in the 5-GHz frequency range
• provides up to 54 Mbps transfer rate, but covers
  smaller range

22
IEEE 802.11 Standards

• 802.11g
• A speed extension for 802.11b, backward
  compatible with 802.11b
• provides up to 54 Mbps transfer rate
• works in the 2.4-GHz (unlicensed) frequency range

23
IEEE 802.11 Standards

• 802.11e
• provided QoS and proper support of multimedia
  traffic.
• 802.11f
• outlines how authentication and other necessary
  information can be properly shared among
  different APs during roaming
• Roaming as the user moves out of the range of the
  first AP, another AP needs to pick up and
  maintain her signal
• And others

24
Security in WLAN

• Security in WLAN
• Wired Equivalent Privacy (WEP)
• Extensible Authentication Protocol (EAP)

25
WEP

• Wired Equivalent Privacy (WEP) protocol
• The wireless device is authenticated to the AP by
  proving that it has the necessary encryption key.
  
• The AP sends a random value to the wireless
  device.
• The device encrypts this value with its
  cryptographic key and returns it.
• The AP decrypts and extracts the response, and if
  it is the same as the original value, the device
  is authenticated.
• Encrypted data

26
WPA

• Security flaws in WEP
• Use static keys
• Software can break key after listening to only
  100 to 1000 MB of data
• Wi-Fi protected access (WPA)
• Provide stronger security than WEP
• The password entered only initializes the
  encryption process. The actual key changes
  constantly
• Upgrade WEP with WEP via firmware supplied by
  vendors of the wireless equipments
• If any device in a WLAN still using WEP, the
  entire network will use WEP ? weak security

27
802.11x and EAP

• 802.1x is the standard for Ethernet security
• Require use authentication before granting
  network access
• Combined with Extensible Authentication Protocol
  (EAP) standard, it handles authentication and key
  management

28
EAP

• Extensible Authentication Protocol (EAP) exchange
  message among the client, AC and the
  authentication server.
• Fig 11.4 (p305)
• IEEE802.11i standard incorporates 802.1X/EAP into
  wireless networking
• Include Temporal Key Integrity Protocol (TKIP)
• TKIP generates random values that are used in the
  encryption process
• Include CCM Protocol (CCMP)
• use AES algorithm
• The Advanced Encryption Standard (AES)

29
War Driving for WLANs

• War driving attackers who drive around looking
  for wireless LANs to intercept

30
War Driving for WLANs

• Tools for war driving
• Sniffer Kismet and NetStumbler
• WEP craker Airsnarf, AirSnort, and WEPCrack
• How to prevent war driving?
• the best practices pertaining to WLAN
  implementations