How are Wireless Networks different from Wired Networks

1. How are Wireless Networks different from Wired Networks

2. Destination address does not equal a physical location • In the design of wired networks, destination address is implicitly assumed to be equivalent to a physical location. • In wireless networks, we are required to handle both portable stations and mobile stations • Portable Station: a station that is moved from location to location, but is only used at a fixed location. • Mobile Station: a station that accesses the network while in motion.

3. Wireless Medium has different characteristics than wired media • Propagation properties: • strong attenuation • environment dependent • time-varying • asymmetric • Interference prone: • other signals sharing the medium • other networks operating in overlapping areas.

4. Wireless Medium (WM) impact on design and performance • WM is significantly less reliable • WM does not have absolute and readily observable boundaries • Lack of full connectivity • stations may be hidden from each other • Wireless networks have dynamic topologies

5. Special Requirements • Limitations on WM ranges raises the need to define basic coverage building blocks • Mobile stations are battery operated and require power management features • Unprotected nature of the wireless medium raises the need to provide special security features

6. IEEE 802.11 Architecture An overview

7. Basic Service Set (BSS)

8. Independent BSS (IBSS) • Most basic type of IEEE 802.11 LAN • A minimum LAN may consist of only two Stations • Possible when IEEE 802.11 STAs are able to communicate directly • Often formed without pre-planning, for only as long as the LAN is needed IBSS = ad hoc network

9. Distribution System (DS) • PHY limitations determine the direct station-to-station distance that may be supported • Increased coverage is obtained by creating an extended form of network that is built with multiple BSSs • The interconnection of the BSSs is by means of a Distribution System • Provides seamless integration of multiple BSSs • enables mobile device support (handles address to location mapping)

10. Infrastructure BSSs and Access Points (APs) • A BSS that is a component of an extended network is referred to as Infrastructure BSS • Data move between a BSS and the DS via an Access Point (AP) • An AP is an entity with station functionality that enables stations in the BSS to access the DS via the wireless medium (WM)

11. DS and Access Points (APs)

12. Station membership in a BSS • Dynamic: stations turn on, turn off, come within range and go out of range • To become a member of a BSS, a station joins the BSS using a synchronization procedure • To access all services of an infrastructure BSS, a station shall become “associated” • Associations involve the use of DS Services

13. Extended Service Set (ESS) • An ESS is the union of the BSSs connected by a DS • An ESS network appears the same to an LLC layer as an IBSS network • STAs within an ESS may communicate • Mobile STAs may move from one BSS to another within the same ESS transparently to LLC

14. ESS

15. BSSs in an ESS • BSSs may partially overlap • Providing contiguous coverage • BSSs may be physically collocated • Providing redundancy • BSSs could be physically disjoint • No limit on the distance between BSSs • One or more IBSS and/or ESS networks may be physically present in the same space

16. Area Concepts • Propagation characteristics are dynamic and unpredictable • Small changes in position or direction result in dramatic differences in signal strength • (See Figure 5-4 where the difference between the greatest and least signal strengths is 50 dB) • Difficulty in representing BSSs • Sets of stations, areas, volumes

17. Signal Intensity Map

18. Collocated Coverage Areas

19. Integration with Wired LANs • The integration of a wireless network with a traditional wired LAN is by means of a logical component called a Portal • Integration Service is responsible for any addressing or frame format changes • Both portal and AP functionalities may be offered in the same device

20. Integration with Wired LANs

21. Wireless LAN Mesh

22. Example of Non-mesh IEEE 802.11 Deployment Model

24. Non-mesh Deployments: Observations • End STAs must associate with an AP in order to gain access to the network • STAs are dependent on the AP with which they are associated to communicate

25. Mesh Networks: Motivation • Support for more flexible interoperable wireless connectivity • Implementing the DS with interoperable wireless links or multihop paths between the APs • Giving end-user devices the ability to establish interoperable peer-to-peer wireless links with neighboring end-user devices and APs

26. Example Mesh Network

27. Mesh Network Components • Mesh Point (MP) • QoS STAs that support Mesh Services: • formation of the mesh network • Operation: path selection and forwarding • Mesh Access Points (MAP) • Configuration of an MP collocated with an AP: • Single entity providing mesh and AP functionalities • Mesh Portals (MPP)

28. Mesh Network Model

29. Mesh Network Model • Mesh Network is an IEEE 802 LAN with: • IEEE 802.11 links • Control elements that forward frames among the network members • From the perspective of other networks and higher layer protocols: • Functionally equivalent to a broadcast Ethernet • MPs connected at link layer

30. Logical Service Interfaces

31. Overview of Services • 13 Services are used to: • Support delivery of MSDUs between STAs (6) • Control network access and confidentiality (3) • Provide spectrum management (2) • Support applications with QoS requirements (1) • Support higher layer timer synchronization (1) • Divided into two categories: • STA Service and Distribution System Service

32. STA Service (SS) • Present in every STA • Authentication • De-authentication • Data confidentiality • MSDU Delivery • Dynamic Frequency Selection (DFS) • Transmit Power Control (TPC) • Higher layer timer synchronization • QoS Traffic Scheduling

33. Distribution System Service (DSS) • DSS comprises: • Distribution • Integration • Association • Disassociation • Reassociation • QoS traffic scheduling

34. IEEE 802.11 Architecture

35. Distribution System Implementation Considerations • IEEE Std 802.11 explicitly does not specify the details of DS implementations. • DS may be either data link or network layer based. It may be created from many different technologies, including 802.x • IEEE Std 802.11recognizes and supports the use of WM as the DSM. • IEEE Std 802.11 specifies DS services

36. Distribution of messages within a DS • Primary service used by STAs (invoked by every data message to and from a STA operating in an ESS) • Distribution is via the DSS • IEEE Std 802.11 does not specify how the message is distributed within the DS • It is required to provide the DS with information for the DS to be able to determine the output point • This information is provided by the three association related services

37. Integration • Invoked if the DSS determines that the intended recipient of a message is a member of an integrated LAN • The output point is a portal • Integration function is responsible for media or address space translation • Details of an integration function is outside the scope of IEEE 802.11

38. Services that Support the Distribution Service The primary purpose of the MAC sublayer is to transfer MSDUs between MAC sublayer entities. The information required for the distribution service to operate is provided by the association services. Before a data message can be handled by the distribution service, a STA shall be “associated.”

39. Mobility Types • No-transition: • Static – no motion • Local movement – movement within the PHY range of the communicating STAs (the basic service area) • BSS-transition: • STA movement is from one BSS to another BSS in the same ESS • ESS-transition: • STA movement is from one BSS in one ESS to a BSS in another ESS. (Disruption of service is likely.)

40. Association Service • A DSS that provides the DS information as to which AP to access to reach a STA • Sufficient to support no-transition mobility • Necessary but not sufficient to support BSS-transition mobility • How the information is stored and managed within the DS is not specified

41. Association Service • At any given instant, a STA may be associated with no more than one AP • This insures that the DS may determine uniquely which AP is serving a STA. • Association is always initiated by the mobile STA • STA first learns what APs are present and what operational capabilities are available

42. Reassociation • A DSS needed to support BSS-transition mobility • Invoked by a STA to move a current association from one AP to another - keeps the DS informed of current AP-to-STA mapping • Also enables a STA to change association attributes of an established association while the STA remains associated with the same AP • Always initiated by the STA

43. Disassociation • Invoked when an existing association is to be terminated • May be invoked by either party to an association (AP or non-AP STA) • Diassociation is a notification, not a request – cannot be refused • AP may need to disassociate STAs to enable the AP to be removed for service…

44. Access Control and Data Confidentiality Services • Required for wireless networks to provide functionality equivalent to wired LANs • Two services: • Authentication • Equivalent to wired media physical connection • Data confidentiality • Equivalent to the confidential aspects of the closed wired media • RSNA (Robust Security Network Association)

45. Authentication • An SS that may be used by all STAs to establish their identity to STAs with which they wish to communicate (in ESS and IBSS networks) • Operates at the link level between STAs • A mutually acceptable level of authentication is necessary for an association to be established • A STA may be authenticated with many other STAs Note: IEEE 802.11 does not provide neither end-to-end (message origin to message destination) nor user-to-user authentication

46. Preauthentication • Typically done by a STA while it is already associated with an AP • Authentication process is time-consuming • Yet, Authentication is a prerequisite to association • Preauthentication helps speedup reassociation • IEEE 802.11 does not require STAs to preauthenticate with APs

47. Deauthentication • Invoked when an existing authentication is to be terminated • In an ESS, deauthentication causes the STA to be disassociated • It is a notification (not a request) • Shall not be refused by either party

48. Data Confidentiality • An SS that provides the ability to protect the contents of messages • The default for all STAs is “in the clear”

49. Spectrum Management Services • To satisfy requirements in some regulatory domains for operation in the 5 GHz band • Two Services are specified: • Transmit Power control (TPC) • To reduce interference with satellite services • Dynamic Frequency Selection (DFS) • To avoid co-channel operation with radar systems • To ensure uniform utilization of available channels

50. Transmit Power Control (TPC)Service • Association of a STA with an AP in a BSS based on the STA’s TP capability • Specification of regulatory and local maximum TP levels for the current channel • Selection of a TP for each transmission in a channel with constraints imposed by regulatory requirements • Adaptation of TP based on information (e.g., path loss, link margin estimates)