IE 419/519Wireless Networks

Lecture Notes #3IEEE 802.11 Wireless LAN Standard

Part #1

Basic Concepts in Protocol Architectures

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Introduction 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:

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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

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Comparison of OSI and TCP/IP

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Internetworking Terms 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

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Internetworking Terms (cont.)

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

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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

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Router Functions Addressing schemes

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

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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

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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

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IP Addressing (cont.)

32-bit global internet address IPv4 address space 232 = 4,294,967,296 Two parts

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

IP Addresses Classful networking

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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

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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

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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

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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

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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

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Subnets and Subnet Masks (cont.)

Rest of IP Internetwork

All IP traffic to 139.12.0.0

Rest of IP Internetwork

All IP traffic to 139.12.0.0

BEFORE

AFTER

Router

Router

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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 Host-NumberSubnet-Number

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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?

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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

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Routing Using Subnets

The IEEE 802 Protocol Architecture

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IEEE 802 Reference Model

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Protocol Architecture - PHY

Physical Layer (PHY) Functions: Encoding/decoding of signals

PSK, QAM Preamble generation and removal

For synchronization Bit transmission/reception Includes specification of the

transmission medium and topology

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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

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Protocol Architecture - MAC Medium Access Control (MAC) Layer

Functions:

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Protocol Architecture – MAC (cont.)

MAC Frame Format MAC control

Contains MAC protocol information Destination MAC address

Destination physical attachment point Source MAC address

Source physical attachment point Data CRC

Cyclic redundancy check

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Protocol Architecture – MAC (cont.)

Generic MAC Frame Format

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Protocol Architecture – LLC Logical Link Control (LLC) Layer

Functions:

Characteristics of LLC not shared by other control protocols:

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Protocol Architecture – LLC (cont.)

Unlike many other link layer protocols, 802.11 incorporates positive ACKs All transmitted frames must be ACK

LLC Services Unacknowledged connectionless service

No flow and error control mechanisms Data delivery not guaranteed

Connection-mode service 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

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Separation of LLC and MAC WHY?

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IEEE 802 Standard

802.2 LLC

802.11 MAC802.5MAC

802.5PHY

802.3MAC

802.3PHY

802.11FHSSPHY

802.11DSSSPHY

802.11aOFDMPHY

802.11bHR/DSSS

PHY

PHYLayer

LLCLayer

MACLayer

802.3 802.5 802.11

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IEEE 802.11 Architecture 802.11 networks consist of four

major physical components Distribution System Access Points Wireless Medium Stations

DistributionSystem

AccessPoint

WirelessMedium

Stations

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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

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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

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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

MotorolaCisco

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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

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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

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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

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IEEE 802.11 Architecture (cont.)

IBSS network vs. Infrastructure BSS network

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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

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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

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IEEE 802.11 Services (cont.)

Authentication Deauthentication Privacy MSDU Delivery

Distribution Integration Association Reassociation Disassociation

Station LevelServices

Distribution LevelServices

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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

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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 Always initiated by mobile units Occurs when mobile stations move b/w BSSs

within a single ESS

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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

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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

APMU

Wired Network

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Station Level Services (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

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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

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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

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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

BSS 1

BSS 2 BSS 4BSS 3

AP 1

AP 2

AP 3 AP 4

ESS 1

Router

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IEEE 802.11 Mobility Support (cont.)

ESS TransitionDS

ESS 1 ESS 2

BSS 1 BSS 2 BSS 3 BSS 4