wireless lan - introduction omer ben-shalom. lecture brief this lecture will touch briefly on the...
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Wireless LAN - Introduction
Omer Ben-shalom
Lecture brief This lecture will touch briefly on the following
items: WLAN as a disruptive technology Advantages and disadvantages of WLAN 802.11 (WiFi) technology and main standards Some WLAN myth and reality Mobility definitions (Time permitting) Challenges in WLAN deployment and how to
meet them The impact of WLAN on client, network and apps.
WLAN as a disruptive technology Like previous disruptive technologies (IP, the Internet) WLAN
is changing the way people work and live WLAN is taking a strong hold in the home. People are very
happy with being ‘Unwired’ and the concept is pushed by industry looking for more growth engines.
IT is pressured by users to deliver WLAN If IT does not deliver people tend to start installing their own
WLAN The technology is problematic for the corporate and many
would like to avoid implementing it due to various concerns IT organizations are starting to see the real benefits in WLAN
as well in both cost savings and (more importantly) productivity gains
As in the past the simpler technology wins over better ones
The advantages of WLAN Its ‘sexy’ and hyped, People like new toys It saves carrying a dongle and/or patch cord (more seriously) Enhances user productivity
No clear ROI. Most quote few more hours of work a week 11 minutes more a week will cover costs according to study Provides improved efficiency/productivity
As a primary network the total cost is markedly lower then the wired LAN Need to be able to do Wireless Voice for that
Allows flexible service provisioning With the right gear multiple networks can be provided for
different users in the same location opaquely
Disadvantages of WLAN Unless used as primary means of connectivity
presents a second infrastructure Security – signal leakage outside the buildings.
Physical perimeter security is gone Performance/Reliability
Back to shared media with relative low speed Operates in the non-licensed band and therefore open to
interference Increases the number of managed entities by an
order of magnitude At least in the Naïve implementation options
The state of WLAN WLAN is starting to mature Both standard and (many) proprietary solutions
Few of the new startups will likely survive Getting to become the main connectivity method
Has to include voice over WLAN Requires SLA (uptime, performance) much closer to wired
The drive for implementing primary WLAN is two fold: Improved user productivity Lower TCO
Lots of new architectures and options are popping up
802.11b – Physical and MAC layers Two WLAN stations conversing on shared
WLAN infrastructure. WLAN is only concerned with the physical and MAC layers
application
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
application
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLCLLC LLC
Basic 802.11 terminology AP – Access Point. A central controller that can
extend the range of the service set stations in the BSS talk through a central controller (AP) The AP sets configurable parameters that all must match Those are carried in special packets called beacons
BSS – Basic service set Group of stations using a single media and coordination
function in a Basic Set Area (BSA) All stations can communicate with each other directly If no central controller exist this is an iBSS
Membership in a BSS is defined by the Service Set Identifier (SSID) and the BSSID (Normally controlled by the AP) Multiple APs per SSID. Potentially multiple SSID per AP
Basic 802.11 terminology Different APs connect through a distribution
system (DS). Normally a wired backbone All the APs connected on the DS and their
BSS form the ESS - Extended service set The ESS is a single L2 environment
/broadcast domain Stations send packets other stations in the same
ESS ‘directly’ Stations can freely move within the ESS
An EBSS environment
DS
Hidden node problem not everyone hears everyone
Distance Physical barriers (walls etc) A traffic to B can collide with C traffic to B without
A or C being in the know
CBA
Radio standards Wireless LAN (WiFi) is a layer 1-2 technology based on Ethernet Uses CSMA (Collision sense multiple access) but unlike
Ethernet attempts Collision Avoidance (CA) rather than detection (CD) so considered CSMA/CA
Tailored for the noisy radio band Supports client to client Ad-Hoc networking and base station
(AP) based connectivity called infrastructure mode Operates in two main bands:
The ISM 2.4 Ghz band (about 80 Mhz, 3 non overlapping channels). Each channel uses some 20mhz.
The 5 Ghz licensed band. Channels are also 20Mhz Can use RTS/CTS mechanism to deal with access rights and
solve the ‘hidden node’ problem. Incurs a lot of overhead due to collision avoidance scheme and
error correction in the noisy medium
The 802.11 main working groups 802.11 is the IEEE committee working on the WLAN standards
IEEE deals with the lower levels protocols only Focuses on short range, high throughput, relatively low power
PAN focuses on high throughput in lower power MAN/WAN such as WiMAX will focus on range with higher power
involved It includes a lot of working groups. Main ones are Radio standards (802.11, 802.11b, 802.11a, 802.11g, 802.11n) Other supporting functions (partial list)
802.11e - MAC Enhancements for QoS (Expected Sep’ 05) 802.11f - Inter Access Point Protocol 802.11i - MAC Enhancements for Enhanced Security 802.11R – Fast roaming
Radio standards in the 802.11 802.11 – The old FH/DSSS WLAN standard @1-
2mbps in the 2.4 GHz range, 3 channels 802.11b – improved modulation (CCK) @up to
11mbps at the 2.4 GHz range, 3 channels 802.11g – The newest 2.4GHz modulation using
OFDM and able to provide 54Mbps, 3 channels 802.11a – OFDM modulation in the 5GHz licensed
band, not available everywhere. OFDM, up to 54 Mbps and 8-12 usable channels. Uses 802.11h for transmit power control and channel selection
802.11n – OFDM modulation using multiple antenna (MIMO) provides >100mbps, not rectified
Different Standards for Different Needs
Home
SOHO
Auditorium
Campus
Hotspots
Business
802.11b
802.11g
802.11a
• Mature, globally deployed standardMature, globally deployed standard• Good wall penetration and rangeGood wall penetration and range
• High performance and scalabilityHigh performance and scalability
• Faster speeds than 802.11b, backward Faster speeds than 802.11b, backward compatible to 802.11b compatible to 802.11b• Good wall penetration and rangeGood wall penetration and range• Early standard and solutionsEarly standard and solutions
802.11a/b
• High performance, scalability and High performance, scalability and interoperability interoperability
802.11a/b/g
• Best overall solution for freedom, Best overall solution for freedom, flexibility and interoperability flexibility and interoperability
11
11
11
802.11b
54
54
54
2.4 GHz 3 non-overlapping
channels
802.11g 802.11a
5 GHz 8+ non-overlapping
channels§
5454
54
54
54 54
54
54
Feature 11a 11b 11g
Higher throughput Higher network capacity Better wall penetration Low wireless interference Existing Infrastructure
§Exact number of 11a channels depends on individual country restrictions.
WLAN myth - performance WLAN BW quoted is client association speed This is the biggest myth in WLAN because in
actuality the performance that one can get from the network is way lower than the one quoted 802.11b supports 11mbps but stops at 6 mbps
even for optimal size packets 802.11g and 802.11a are supposed to support 54
mbps but in reality support about 22mbps/15 mbps respectively with optimal packet sizes
With small packets this drops significantly
Performance 802.11b/a
0
5
10
15
20
25
30
0 50 100 150 200 250
802.11a802.11b
~4.5x
~2.5x
0
5
10
15
20
25
30
0 50 100 150 200 250
802.11a802.11b
~4.5x
~2.5x
~4.5x
~2.5x
Real life throughput of 802.11b/g/a
Throughput dependency on packet sizes802.11b TPT vs. Packet Size
(Using Long Preamble)
0
1,000,000
2,000,000
3,000,000
4,000,000
5,000,000
6,000,000
7,000,000
64 128 192 256 320 384 448 512 576 640 704 768 832 896 960 1024 1088 1152 1216 1280 1344 1408 1472 1500
11
5.5
2
1
WLAN myth – BW requirements Users do not consume as much BW as expected
Move from switched 100 mbps to shared ’10’ looks horrible But – users are pretty happy with their home ADSL
@750kbps down/96kbps up How much does a user really need?
Today the LAN is ‘non blocking’ and ‘free’ Bottleneck is usually servers Clients cannot really use 100mbps normally Changes with the move to WLAN Need solutions
WLAN Mobility - basics Mobility includes two different usage models
often mixed ‘Nomadic’ user –able to take laptop/PDA from one
place to another and work there (but not continue sessions)
‘Roaming’ user – ability to (seamlessly) continue working while moving. Harder by far
Achieving nomadic status is mainly about having coverage But each time all sessions need to be re-
established
WLAN Mobility - roaming Layer 2 roaming happens when a client changes AP
Challenge – hand over user fast enough to not drop packets or eve degrade voice quality.
Main problem – 802.1X re-authentication Solution – fast secure roaming (802.11r in work)
Layer 3 roaming happens when a client moves to a new EBSS There is a limit to how far a single BSS can be stretched Challenge – keep sessions open. IP change will tear down
sessions Solution – allow client to keep their IP between BSS. Usually with
mobile IP or L2 overlay network Same or worse timing problem
Seamless mobility The newest hype is about ‘Seamless mobility’ The ability to switch between transports
WiFi WiMax Cellular/3G LAN?
While not loosing the sessions Requires solutions in the network layer
(mobileIP) or the application layer
Main challenges in WLAN implementation Security, security, security (your standard FUD)
Wireless is easily tapped. WEP can be broken Rogue APs
Capacity/Performance LAN is switched 100 mbps. ‘shared 11mbps’ seems problematic No real way to stop malicious or innocent interference Requires applications to consider limited BW
Mobility – Roam without impacting quality too much Cost
Creating additional infrastructure for the ‘Wireless’ TCO for the infrastructure and clients
Management Number of managed entities grows significantly Users are on the move and difficult to track
WLAN security concerns Using no security allows everyone in range to tap in
to your network Simple security solutions (mac filtering ,hidden SSID
etc) do not work Everyone knows that WEP can be broken (but
apparently not how hard it really is) Most people still do not use encryption
Even businesses are often found unprotected Rogue APs are a real security threat
Allow anyone access to your LAN from outside Not implementing your own WLAN increases risk
WLAN Security - solutions WLAN security flaws have been the focus of (too)
many articles and discussions The hard fact is that the currently available solutions
are quite good (and will be discussed extensively later in the course) WEP is broken but it takes much more than what is
commonly perceived to break it Existing WPA has never been shown to be broken The full 802.11i with AES encryption is even stronger L2 and L3 VPN technologies can easily secure the WLAN
The single worst security threat is rogue APs and those are just worse if no WLAN is supplied
We will review the security solutions in depth in a future lecture
Capacity concerns and solutions WLAN capacity is much lower than the switched 100mbps
Also – the limited number of channels is a big consideration It is better than the old shared 10mbps unless packets are
extremely small since CA works better than CD with high utilization How much do users actually need?
Most users are happy enough on ADSL with 750K max. 300 Kbps/user for secondary service For Primary use is 1 mbps the magic number ? High impact of VOIP and other real time protocols or ones that use
small packets Move to higher throughput with 802.11a/g and 802.11n later on Tailor applications to recognize network conditions In the future smart antenna solutions will really help
Reliability concerns WLAN is wide open to interference
Unless you use a Faraday cage you are open to interference
Even a normal cordless phone can bring down a WLAN channel
A microwave can pretty much block the whole range Any cable tester for 2.4 with directional antenna will do
The common protocols of 802.11b and 802.11g use the unlicensed 2.4GHz band Very hard to persecute offenders
The security required adds more points of failure 802.11e not rectified yet (QOS)
Dealing with reliability Engineer the network for no single point of failure
Redundant coverage for AP and L2 switches Dual L3 devices Multiple authentication servers or VPN gateways
Implement interference detection/avoidance Use equipment that knows to automatically switch away
from channels blocked by interference Implement location services to find interference sources
Move to 802.11a Less interference than the ISM band 8-12 channels makes it difficult to block the service - The
wider the band the more difficult it is to block 802.11h allows channel agility and power control
WLAN management concerns With WLAN the number of network elements is
vastly larger than normal LAN Part due to area coverage and part to the low throughout of
each AP Managing such a large number of devices is very
problematic How do you set them all up How do you change configuration when needed
dynamically Updating software and firmware on all the APs is a big
problem
WLAN management solutions Smart management servers (available from a
few vendors) that can centrally manage large number of APs
Removing as much of the managed entities from the AP to a central location Different AP types: ‘Smart’, ‘thin’, ‘hybrid’ Each of the last two moves some of the MAC
layer to a central controller More on this in later lectures
Cost concerns WLAN infrastructure has many more entities
to manage/control/upgrade which contributes to high TCO
Clients need to be brought to new standards to gain benefit of better security and radio protocols
Every new technology is harder to support and means training the support personnel. It may also be less stable = more calls
Cost benefits APs are very cheap compared to LAN
switches so the cost of the infrastructure goes way down
No more Add/Move/Change cost for clients Our observation is that WLAN reduces the
number of support calls rather then increase them
Move to primary use allows real cost benefit
WLAN Impact on infrastructure The ‘one network serves all’ paradigm is broken
We got used to the LAN delivering any requirent for 95% of users
This is no longer the case Different users have different BW requirements and with
WLAN that has to be taken into account VOIP introduces different requirements and so does Video.
VOIP traffic clogs the network very easily ‘standard’ WLAN does not support multiple networks
Multiple AP on same location is expensive and causes co-habitation problems
WLAN has no real QoS today
WLAN impact on user Users can become mobile
Nomadic or Roaming user Users can use the network in places not
available before More usable work time Different use models, especially with roaming User productivity is much enhanced
WLAN impact on applications Applications writers used to consider the
network a ‘non issue’ Which often proved very wrong on the WLAN but
right for the LAN With WLAN and mobility that is not true
Users may lose the network temporarily and/or their BW may be limited/fluctuating
Applications need to be written to address Good example – outlook 2003 Bad example – net meeting