Chapter 99 - Question Revision

Spread Spectrum

• Spread-spectrum is designed to trade bandwidth efficiency for reliability, integrity, and security
• FHSS
  • Frequency Hopping Spread Spectrum
  • Spectrum is divided into many subchannels
  • two communicating systems hop on same frequency
• DSSS
  • Direct Sequence spread spectrum
    • stations are assigned orthogonal codes
    • use these code for transmission
    • other stations transmissions appears an noise

ARQ

• Automatic Repeat reQuest
• Reactive protocol
• control protocols for transmission of data over noisy or unreliable communication network.
• Use ACK and timeouts to acheive reliable data transmission
• Stop-and-wait ARQ, Go-Back-N ARQ

Aloha VS slotted Aloha

• Aloha
  • a terminal transmits whenever the user data is ready
  • if the sender finds that the packet get collided
    • it waits for a random period of time
    • send the packet again
  • Throughput : $S_{pure} = Ge^{-2G}$
• Slot-Aloha
  • time is slotted
  • length on the slot is the time to transmit a packet
  • node starts transmission in the beginning of the slots only
  • if collision occurs
    • sender waits for a random number of slots
    • transmit Packets agains
  • Throughput: $S_{slotted} = Ge^{-G}$

Why does IEEE standardize only two lower layers for WLANs

• only standardize PHY and MAC lyaer
• Interfaces to higher layer is the same as those in iEEE 80.2x standards
• The upper layers is still the same as normal LAN
• what the different is the medium for tranmission , it is in wireless now.

BEB works in IEEE 802.11 WLAN.

• Binary Exponential Backoff
• When multiple entities attempt to gain access to a shared resource, only one of them will succeed.
• Those who fail wait til the resoure becomes available then retry.
• Binary Exponential Backoff (BEB) is an algorithm to determine how long entities should backoff before they retry.
• With every unsuccessful attempt, the maximum backoff interval is doubled.
• BEB prevents congestion and reduces the probability of entities requesting access at the same time,

Why RTS-CTS needed

​ - to solve hidden terminal problem

• hidden terminal problem occurs when a node can communicate with a wireless access point (AP), but cannot directly communicate with other nodes that are communicating with that AP.

• RTS-CTS

  • the senders A sends RTS to receiver B
  • once the receiver B get RTS, it starts to send CTS to all other terminals, indicating that sender A is going to communicate with receiver B. After the this handshake, they can start send data.

Network allocation vector

• Shared Medium Access
  • Carrier Sensing
    • Virtual Carrier Sensing
      • provided by the Network Allocation Vector (NAV)
      • NAV Indicates how long the medium is reserved
      • NAV is set acccrdoing to fields (Duration ) indicated in most frames
• It is a virtual carrier sensing mechanism.
• The station listens to duration field to set their NAV
• It is a indicator for a station on how long it must defer from accessing the medium

ESS and Infrastruture BSS difference

Basic Service Set (BSS) : a set of stations communicating with each other

Extended Service Set (ESS): linking BSS using backbone network

​

Infrastructure BSS (Infracstruture mode)

• Terminals communicate via AP
• centralized

Extended Service Set (ESS):

• provide larger service areas
• does not specify pariticular technology
• requires backbone to provide a specific set of service

Bluetooth topology

• Piconet
  • master and slaves architecture
  • maximum up to 7 slaves
• scatternet
  • a group of piconet

Main Appilcation of ZigBee

• Low Data-rate radio services

  • Remote control,

  • Joystick

  • Personal Health care

  • sensors

    

ZigBee Mesh Topology

MIssion of 3gpp

• The 3rd Generation Partnership Project (3GPP)

• developing globally acceptable specifications for third generation (3G) mobile systems.

shortcomings of CSMA/CD in wireless networks

• collisions in wirerless channels are harder to detect
• colisions leads to usage of bandwidth which is scarce

Which layers IEEE 802.11 specify in their standards? Why?

• only standardize PHY and MAC lyaer
• Interfaces to higher layer is the same as those in iEEE 802.x standards
• The upper layers is still the same as normal LAN
• what the different is the medium for tranmission , it is in wireless now.

What are two principle operation modes of 802.11a/b/g/n systems?

• ad-hoc mode
• infrastructure mode

Briefly explain evolution of air interface technology from GSM towards LTE.

GSM

• TDMA and CDMA
• FSK digital modulation

GPRS

EDGE

1. Why is ad-hoc networking useful in militaristic applications?