Transcript Lecture #3: IEEE 802.11 Wireless Standard

IE 419/519
Wireless Networks
Lecture Notes #3
IEEE 802.11 Wireless LAN Standard
Part #1
Basic Concepts in Protocol
Architectures
2
Introduction

What is a protocol?


An agreed-upon format for transmitting data
between two devices
Key Features

Concerns the format of the data blocks


Includes control information for coordination and
error handling


Answer:
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
Comparison of OSI and TCP/IP
5
Internetworking Terms

Communication network


Internet



Facility that provides a data transfer service
among devices attached to the network
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
Internetworking Terms

End System (ES)


Device used to connect two networks
Bridge


Device used to support end-user applications or
services
Intermediate System (IS)


(cont.)
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
Functions of a Router



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
Router Functions

Addressing schemes


Maximum packet sizes


Different maximum packet sizes requires
segmentation
Interfaces


Different schemes for assigning addresses
Differing hardware and software interfaces
Reliability

Network may provide unreliable service
9
IP Addressing


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
IP Addressing

(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
IP Addressing



32-bit global internet address
IPv4 address space  232 = 4,294,967,296
Two parts



(cont.)
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
IP Addresses

Classful networking
13
IP Addresses - Class A




Referred to as “/8s”
Start with binary 0
00000000 – reserved for default route
Range 1.x.x.x to 126.x.x.x



27 – 1 = 127 possible class A networks
224 – 2 = 16,777,214 possible class A hosts
All allocated

50% of the total IPv4 unicast address space
14
IP Addresses - Class B




Referred to as “/16s”
Start with 10
Range 128.0.x.x to 191.255.x.x
Second octet also included in network
address



214 = 16,384 possible class B networks
216-2 = 65,534 possible class B hosts
All allocated

25% of the total IPv4 unicast address space
15
IP Addresses - Class C




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



221 = 2,097,152 possible class C networks
28-2 = 254 possible class C hosts
Nearly all allocated

12.5% of the total IPv4 unicast address space
16
Subnets and Subnet Masks


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

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


(cont.)
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
Network-prefix
Host-Number
Subnet-Number
Host-Number
20
Subnetting

Design issues




How many total subnets are needed today?
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
Example
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
Routing Using Subnets
23
The IEEE 802 Protocol
Architecture
24
IEEE 802 Reference Model
25
Protocol Architecture - PHY

Physical Layer (PHY) Functions:

Encoding/decoding of signals


Preamble generation and removal



PSK, QAM
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)


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
Physical medium dependent (PMD)

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

Medium Access Control (MAC) Layer
Functions:
28
Protocol Architecture – MAC

(cont.)
MAC Frame Format

MAC control


Destination MAC address



Destination physical attachment point
Source MAC address


Contains MAC protocol information
Source physical attachment point
Data
CRC

Cyclic redundancy check
29
Protocol Architecture – MAC

(cont.)
Generic MAC Frame Format
30
Protocol Architecture – LLC


Logical Link Control (LLC) Layer Functions:
Characteristics of LLC not shared by other
control protocols:
31
Protocol Architecture – LLC

Unlike many other link layer protocols, 802.11
incorporates positive ACKs


(cont.)
All transmitted frames must be ACK
LLC Services

Unacknowledged connectionless service


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Connection-mode service

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No flow and error control mechanisms
Data delivery not guaranteed
Logical connection set up between two users
Flow and error control provided
Acknowledged connectionless service



Cross between previous two
Datagrams acknowledged
No prior logical setup
32
Separation of LLC and MAC

WHY?

33
IEEE 802 Standard
LLC
Layer
802.2 LLC
802.3
802.5
802.3
MAC
802.5
MAC
802.3
PHY
802.5
PHY
802.11
MAC
Layer
802.11 MAC
802.11
FHSS
PHY
802.11
DSSS
PHY
802.11a
OFDM
PHY
802.11b
HR/DSSS
PHY
PHY
Layer
34
IEEE 802.11 Architecture

802.11 networks consist of four major
physical components




Distribution System
Access Points
Wireless Medium
Stations
Hand held computer
Stations
Laptop computer
Distribution
System
Access
Point
Wireless
Medium
35
IEEE 802.11 Architecture

(cont.)
Distribution System (DS)

Logical component of 802.11 used to forward
frames to their destination



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
IEEE 802.11 Architecture

(cont.)
Distribution System (DS)

In the language of 802.11, the backbone
Ethernet is the 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)


Most commercial APs act as bridges
They have at least one wireless network interface
and at least one Ethernet network interface
37
IEEE 802.11 Architecture

(cont.)
Access Points (APs)


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
IEEE 802.11 Architecture

(cont.)
Wireless Medium



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
IEEE 802.11 Architecture

(cont.)
Stations

Computing devices with wireless
network interfaces


Battery-operated mobile devices such as
laptops or handheld computers
Stations can also be “static” devices
40
IEEE 802.11 Architecture

(cont.)
Types of Networks

Basic building block of an 802.11
network is the basic service set (BSS)


Basic Service Area
BSSs come in two flavors


Independent BSS network (IBSS)
Infrastructure BSS network
41
IEEE 802.11 Architecture

(cont.)
IBSS network vs. Infrastructure BSS
network
Laptop computer
42
IEEE 802.11 Architecture

(cont.)
Types of Networks



To provide wireless coverage to larger
areas, an Extended Service Set (ESS) is
needed
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
IEEE 802.11 Services

802.11 provides nine services


Three are used for moving data
Six services are management
operations


Keep track of mobile nodes
Deliver frames accordingly
44
IEEE 802.11 Services
Distribution Level
Services

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Distribution
Integration
Association
Reassociation
Disassociation
(cont.)
Station Level
Services




Authentication
Deauthentication
Privacy
MSDU Delivery
45
Distribution Level Services

Distribution



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



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

Association

Delivery of frames to mobile stations is made possible
because mobile stations register (i.e., associate) with an
AP



(cont.)
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


Always initiated by mobile units
Occurs when mobile stations move b/w BSSs within a
single ESS
47
Distribution Level Services

(cont.)
Disassociation

To terminate an existing association



“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
Station Level Services

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
Station Level Services

Privacy



(cont.)
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

No Transition


(cont.)
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
AP 3
AP 4
Router
53
IEEE 802.11 Mobility Support
(cont.)
ESS Transition

BSS 1
ESS 1
BSS 2
DS
BSS 3
BSS 4
ESS 2
54