9.1 Wireless Concepts - 9.2 Wireless standards Flashcards
Describe the Signaling method Frequency Hopping Spread spectrum (FHSS)
FHSS uses a narrow frequency band and hops data signals in a predictable sequence from frequency to frequency over a wide band of frequencies.
Describe the Signaling method Direct-Sequence Spread Spectrum (DSSS)
the transmitter breaks data into pieces and sends the pieces across multiple frequencies in a defined range. DSSS is more susceptible to interference and less secure then FHSS.
Describe the signaling method Orthogonal Frequency-Division Multiplexing (OFDM)
breaks data into very small data streams to send the information across long distances where environmental obstacles may be an issue. OFDM:
Describe the topology method Ad Hoc
An ad hoc network works in peer-to-peer mode without an access point. The wireless NICs in each host communicate directly with one another. An ad hoc network:
Describe the Topology method Infrastructure
An infrastructure wireless network uses an access point (AP) that functions like a hub on an Ethernet network. Infrastructure networks have the following characteristics:
The network uses a physical star topology with a logical bus topology.
You can easily add hosts without increasing administrative efforts.
The AP can be connected to a wired network easily, allowing clients to access both wired and wireless hosts.
The placement and configuration of APs require planning to implement effectively.
Describe Media access
Wireless networks use carrier sense multiple access/collision avoidance (CSMA/CA) to control media access and avoid (rather than detect) collisions. Collision avoidance uses the following process:
The sending device listens to make sure that no other device is transmitting. If another device is transmitting, the device waits a random period of time (called a backoff period) before attempting to send again.
If no other device is transmitting, the sending device broadcasts a request to send (RTS) message to the receiver or AP. The RTS includes the source and destination, as well as information on the duration of the requested communication.
The receiving device responds with a clear to send (CTS) message. The CTS also includes the communication duration period. Other devices use the information in the RTS and CTS to delay send attempts until the communication duration period (and subsequent acknowledgement) has passed.
The sending device transmits the data. The receiving device responds with an acknowledgement (ACK). If an acknowledgement is not received, the sending device assumes a collision occurred and retransmits the affected packet.
After the time interval specified in the RTS and CTS has passed, other devices can start the process again to attempt to transmit.
Name the devices that are used on wireless networks
A wireless NIC sends and receives signals.
A wireless AP is the equivalent of an Ethernet hub. The wireless NICs connect to the AP, and the AP manages network communication.
A wireless bridge connects two wireless APs into a single network or connects a wireless AP to a wired network. Most APs include bridging features.
What is a station (STA) in a wireless network
An STA is a wireless NIC in an end device such as a laptop or wireless PDA. STA often refers to the device itself, not just the NIC.
What is a (AP) Access Point
sometimes called a wireless AP (WAP), is the device that coordinates all communications between wireless devices, as well as the connection to the wired network. It acts as a hub on the wireless side and a bridge on the wired side. It also synchronizes the stations within a network to minimize collisions.
What is a Basic Service Set (BSS)
also called a cell, is the smallest unit of a wireless network. All devices in the BSS can communicate with each other. The devices in the BSS depend on the operating mode.
In an ad hoc implementation, each BSS contains two devices that communicate directly with each other.
In an infrastructure implementation, the BSS consists of one AP and all its associated STAs.
All devices within the BSS use the same radio frequency channel to communicate.
What is an independent Basic Service Set (IBSS)
is a set of STAs configured in ad hoc mode.
What is an Extended Service Set (ESS)
ESS consists of multiple BSSs with a distribution system (DS). The graphic above is an example of an ESS. In an ESS, BSSs that have an overlapping transmission range use different frequencies.
What is a Distribution System (DS)
The DS is the backbone or LAN that connects multiple APs (and BSSs) together. The DS allows wireless clients to communicate with the wired network and with wireless clients in other cells.
What is a SSID
The SSID, also called the network name, groups wireless devices together into the same logical network.
All devices on the same network (within the BSS and ESS) must have the same SSID.
The SSID is a 32-character value that is inserted into each frame. The SSID is case sensitive.
The SSID is sometimes called the ESSID (extended service set ID) or the BSSID (basic service set ID). In practice, each term means the same thing; however, they are technically different.
What is a Basic Service Set Identifier (BSSID)
The BSSID is a 48-bit value that identifies an AP in an infrastructure network or an STA in an ad hoc network. The BSSID allows devices to find a specific AP within an ESS that has multiple access points. STAs use it to keep track of APs as they roam between BSSs. The BSSID is the MAC address of the AP and is set automatically.
