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IE 419/519 Wireless Networks

IE 419/519 Wireless Networks. Lecture Notes #3 IEEE 802.11 Wireless LAN Standard Part #1. Basic Concepts in Protocol Architectures. Introduction. What is a protocol? An agreed-upon format for transmitting data between two devices Key Features Concerns the format of the data blocks

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IE 419/519 Wireless Networks

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  1. IE 419/519Wireless Networks Lecture Notes #3 IEEE 802.11 Wireless LAN Standard Part #1

  2. Basic Concepts in Protocol Architectures

  3. 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:

  4. 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

  5. Comparison of OSI and TCP/IP

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. IP Addresses • Classful networking

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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?

  22. 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

  23. Routing Using Subnets

  24. The IEEE 802 Protocol Architecture

  25. IEEE 802 Reference Model

  26. 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

  27. 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

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

  29. 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

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

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

  32. 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

  33. Separation of LLC and MAC • WHY?

  34. 802.2 LLC LLC Layer MAC Layer 802.11 MAC 802.5 MAC 802.3 MAC 802.11 FHSS PHY 802.11 DSSS PHY 802.11a OFDM PHY 802.11b HR/DSSS PHY PHY Layer 802.5 PHY 802.3 PHY IEEE 802 Standard 802.3 802.5 802.11

  35. IEEE 802.11 Architecture • 802.11 networks consist of four major physical components • Distribution System • Access Points • Wireless Medium • Stations Stations Distribution System Wireless Medium Access Point

  36. 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

  37. 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

  38. 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 Motorola Cisco

  39. 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

  40. 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

  41. 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

  42. IEEE 802.11 Architecture (cont.) • IBSS network vs. Infrastructure BSS network

  43. 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

  44. 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

  45. IEEE 802.11 Services (cont.) Distribution Level Services Station Level Services • Distribution • Integration • Association • Reassociation • Disassociation • Authentication • Deauthentication • Privacy • MSDU Delivery

  46. 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

  47. 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

  48. 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

  49. 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

  50. 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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