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