Lecture Notes #3
IEEE 802.11 Wireless LAN Standard
Part #1

Basic Concepts in Protocol
Architectures
2



What is a protocol?

An agreed-upon format for transmitting data between two devices
Key Features


Concerns the format of the data blocks

Answer:
Includes control information for coordination and error handling

Answer:

Includes speed matching and sequencing

Answer:
3

TCP/IP Architecture Dominance


TCP/IP protocols matured quicker than similar OSI protocols

When the need for interoperability across networks was recognized, only TCP/IP was available and ready to go
OSI model is unnecessarily complex

Accomplishes in seven layers what TCP/IP does with fewer layers
4

5




Communication network

Facility that provides a data transfer service among devices attached to the network
Internet


Collection of communication networks, interconnected by bridges/routers
Different from the WWW
Intranet


Internet used by an organization for internal purposes
Provides key Internet applications

Can exist as an isolated, self-contained internet
6

(cont.)


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End System (ES)

Device used to support end-user applications or services
Intermediate System (IS)

Device used to connect two networks
Bridge

IS used to connect two LANs that use similar LAN protocols
Router

IS used to connect two networks that may or may not be similar
7




Provide a link between networks
Provide for the routing and delivery of data between processes on end systems attached to different networks
Provide these functions in such a way as not to require modifications of the networking architecture of any of the attached subnetworks
8

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Addressing schemes
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Different schemes for assigning addresses
Maximum packet sizes

Different maximum packet sizes requires segmentation
Interfaces

Differing hardware and software interfaces
Reliability

Network may provide unreliable service
9



Internet has changed dramatically since the 1980s
Major scaling issues


Eventual exhaustion of the IPv4 address space
Ability to route traffic between ever increasing number of networks
10

(cont.)

Dotted Decimal Notation

IP addresses expressed as four 8-bit binary numbers, each separated by a dot

Binary numbers are then converted to decimal numbers
10000000 . 11000001 . 00110100 . 10010000
11

(cont.)
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
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32-bit global internet address
IPv4 address space  2 32 = 4,294,967,296
Two parts
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
Network identifier
Host identifier
Three types



Class A - supports over 16 million hosts on each of
127 networks
Class B - supports over 65,000 hosts on each of
16,000 networks
Class C - supports 254 hosts on each of 2 million networks
12


Classful networking
13


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Referred to as “/8s”
Start with binary 0
00000000 – reserved for default route
Range 1.x.x.x to 126.x.x.x

2 7 – 1 = 127 possible class A networks

2 24 – 2 = 16,777,214 possible class A hosts
All allocated

50% of the total IPv4 unicast address space
14

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Referred to as “/16s”
Start with 10
Range 128.0.x.x to 191.255.x.x
Second octet also included in network address
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2 14 = 16,384 possible class B networks
2 16 -2 = 65,534 possible class B hosts
All allocated
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25% of the total IPv4 unicast address space
15

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Referred to as “/24s”
Start with 110
Range 192.0.0.x to 223.255.255.x
Second and third octet also part of network address
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
2 21 = 2,097,152 possible class C networks
2 8 -2 = 254 possible class C hosts
Nearly all allocated
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12.5% of the total IPv4 unicast address space
16

Subnets and Subnet Masks
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
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Allow arbitrary complexity of internetworked
LANs within organization
Insulate overall internet from growth of network numbers and routing complexity

Subnet structure of a network is never visible outside of the organization’s private network
Site looks to rest of internet like single network

Each LAN assigned a subnet number
17

Subnets and Subnet Masks
(cont.)


The route from the Internet to any subnet of a given
IP address is the same, no matter which subnet the destination host is on

This is because all subnets of a given network number use the same network-prefix but different subnet numbers
The routers within the private organization need to differentiate between the individual subnets

However, as far as the Internet routers are concerned, all of the subnets in the private organization are collected into a single routing table entry
18

Subnets and Subnet Masks
(cont.)
BEFORE
Router
Rest of IP
Internetwork
All IP traffic to
139.12.0.0
AFTER
Router
Rest of IP
Internetwork
All IP traffic to
139.12.0.0
19

Subnets and Subnet Masks
(cont.)


Host portion of address partitioned into subnet number and host number
Default subnet masks

Class A  255.0.0.0


Class B  255.255.0.0
Class C  255.255.255.0
Network-prefix Host-Number
Network-prefix Subnet-Number Host-Number
20


Design issues

How many total subnets are needed today?
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
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How many total subnets will be needed in the future?
How many hosts are there on the largest subnet today?
How many hosts will there be on the largest subnet in the future?
21

An organization has been assigned the network number 193.1.1.0/24 and it needs to define six subnets. The largest subnet is required to support 25 hosts
Source: Understanding IP Addressing: Everything You Ever Wanted to Know by Chuck Semeria 22

23

The IEEE 802 Protocol
Architecture
24

25

Protocol Architecture - PHY

Physical Layer (PHY) Functions:

Encoding/decoding of signals

PSK, QAM

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Preamble generation and removal

For synchronization
Bit transmission/reception
Includes specification of the transmission medium and topology
26

Protocol Architecture – PHY
(cont.)

In some IEEE 802 standards, the physical layer is further subdivided into two sublayers

Physical layer convergence procedure (PLCP)
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Defines a method of mapping 802.11 MAC layer protocol data units (MPDUs) into a framing format suitable for sending and receiving user data and management information between two or more stations using the associated PMD sublayer
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Physical medium dependent (PMD)
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Defines the characteristics of, and method of transmitting and receiving, user data through a wireless medium between two or more stations
27

Protocol Architecture - MAC
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Medium Access Control (MAC) Layer
Functions:
28

Protocol Architecture – MAC
(cont.)
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MAC Frame Format
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MAC control
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Contains MAC protocol information

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Destination MAC address
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Destination physical attachment point
Source MAC address
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Source physical attachment point
Data
CRC
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Cyclic redundancy check
29

Protocol Architecture – MAC
(cont.)
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Generic MAC Frame Format
30

Protocol Architecture – LLC
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Logical Link Control (LLC) Layer Functions:
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Characteristics of LLC not shared by other control protocols:
31

Protocol Architecture – LLC
(cont.)
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Unlike many other link layer protocols, 802.11 incorporates positive ACKs

All transmitted frames must be ACK
LLC Services
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Unacknowledged connectionless service
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No flow and error control mechanisms
Data delivery not guaranteed
Connection-mode service
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Logical connection set up between two users
Flow and error control provided
Acknowledged connectionless service
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Cross between previous two
Datagrams acknowledged
No prior logical setup
32


WHY?

33

802.3
802.5
802.3
MAC
802.5
MAC
802.3
PHY
802.2 LLC
802.11
802.11 MAC
802.5
PHY
802.11
FHSS
PHY
802.11
DSSS
PHY
802.11a
OFDM
PHY
802.11b
HR/DSSS
PHY
LLC
Layer
MAC
Layer
PHY
Layer
34


802.11 networks consist of four major physical components
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Distribution System
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Access Points
Wireless Medium
Stations
Hand held computer
Stations
Laptop computer
Distribution
System
Wireless
Medium
Access
Point
35

(cont.)
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Distribution System (DS)
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Logical component of 802.11 used to forward frames to their destination
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Combination of bridging engine and DS medium
(e.g., backbone network)
802.11 does not specify any particular technology for the DS
In most commercial applications, Ethernet is used as the DS medium
36

(cont.)

Distribution System (DS)

In the language of 802.11, the
Ethernet is the backbone distribution system medium

However, it is not the entire DS!

To find the rest of the DS, we need to look at the access points (APs)
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Most commercial APs act as bridges
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They have at least one wireless network interface and at least one Ethernet network interface
37

(cont.)
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Access Points (APs)
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Frames on a 802.11 network must be converted to another type of frame for delivery

APs perform the wireless-to-wired bridging function
Cisco Motorola
38

(cont.)
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Wireless Medium
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Used to move frames from station to station
Several different physical layers are defined to support the 802.11 MAC
Originally, two RF PHY layers and one
IR PHY layer were defined
39

(cont.)

Stations
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Computing devices with wireless network interfaces
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Battery-operated mobile devices such as laptops or handheld computers

Stations can also be “static” devices
40

(cont.)

Types of Networks
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Basic building block of an 802.11 network is the basic service set (BSS)
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Basic Service Area
BSSs come in two flavors
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Independent BSS network (IBSS)
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Infrastructure BSS network
41

(cont.)
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IBSS network vs. Infrastructure BSS network
Laptop computer
42

(cont.)
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Types of Networks
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To provide wireless coverage to larger areas, an needed
Extended Service Set (ESS) is
An ESS is created by chaining several
BSSs together with a backbone network
ESSs are the highest-level abstraction supported by 802.11 networks
43


802.11 provides nine services


Three are used for moving data
Six services are management operations
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Keep track of mobile nodes
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Deliver frames accordingly
44

(cont.)
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Distribution Level
Services
Distribution
Integration
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Association
Reassociation
Disassociation
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Station Level
Services
Authentication
Deauthentication
Privacy
MSDU Delivery
45

Distribution Level Services
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Distribution

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Used by mobile stations in an infrastructure network every time they send data
Once frame is accepted by the AP, it uses this service to deliver frame to destination
Integration

Service provided by the DS


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Allows connection of the DS to a non-IEEE 802.11 network
Specific to DS used
Not specified by 802.11 standard except in terms of the services it must offer
46

Distribution Level Services
(cont.)


Association

Delivery of frames to mobile stations is made possible because mobile stations register (i.e., associate) with an
AP


DS then uses registration information to deliver frames to a MU
Unassociated units are not on the network , much like workstations with unplugged Ethernet cables
Reassociation

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Always initiated by mobile units
Occurs when mobile stations move b/w BSSs within a single ESS
47

Distribution Level Services
(cont.)
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Disassociation

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To terminate an existing association

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“Polite” task to perform during the station’s shutdown process
MAC is designed to accommodate stations that leave the network without formally disassociating
Any mobility data stored in the DS is removed when a station invokes the disassociation service
48


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Authentication


Necessary prerequisite to association
In practice, many APs are configured for “open-system” authentication
Deauthentication

Terminates an authenticated relationship

Because authentication is needed before network use is authorized, a side effect of deauthentication is termination of any current association
Example
Wired
Network
MU AP
49

(cont.)


Privacy


Wired Equivalent Privacy (WEP) service
Purpose is to provide roughly equivalent privacy to a wired network by encrypting frames as they travel across the 802.11 air interface
MSDU Delivery

Stations provide the MAC Service Data Unit delivery service

Responsible for getting the data to the actual endpoint
50

IEEE 802.11 Mobility Support



Mobility is the major motivation for deploying an
802.11 network
Stations can move while connected to the network and transmit frames while in motion
802.11 provides data link layer mobility within an
ESS but only if the backbone network is a single layer domain

Remember that APs act as bridges

Wireless medium must also act like a single link layer connection
51

IEEE 802.11 Mobility Support
(cont.)


No Transition

When stations do not move out of their current AP’s service area
BSS Transition

Requires cooperation of APs
52

IEEE 802.11 Mobility Support
(cont.)

BSS Transition (cont’d)

Stations with the same ESS ID may communicate with each other

Stations may be in different BSS areas and may be moving between BSSs
ESS 1
BSS 1
AP 1
BSS 3
BSS 2 BSS 4
AP 2
Router
AP 3 AP 4
53

IEEE 802.11 Mobility Support
(cont.)

ESS Transition
DS
BSS 1 BSS 2 BSS 3 BSS 4
ESS 1 ESS 2
54