In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another, or otherwise cause overlapping transmissions. These transmissions may belong to different users (as in TDMA) or to the same user (as in OFDM).
- Propagation Delay In Networking
- 802.11 Slot Time Propagation Delayed
- Propagation Delays
- 802.11 Slot Time Propagation Delays
Propagation delay influence in IEEE 802.11 outdoor networks. We analyze the influence of increasing propagation delay in the IEEE 802.11 MAC protocol. Packet delay, packet drop time. Packet Delay Modeling of IEEE 802.11 Wireless LANs P. The backoff time counter is decreased in terms of slot time as long as the channel is sensed idle. And δ is the propagation delay. Average Packet Delay.
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- Access Delay and Throughput Evaluation of Block ACK under 802.11 WLAN Yean-Fu Wen, Frank Yeong-Sung Lin, Kun-Wei Lai. Evaluate throughput but also block access delay time to choose the appropriate number of packets per block. Tpro Propagation delay for all packets. Tslot The slot time. TRTS.
- Σ to as a STA (station) as done in the 802.11 standard. Slot time is a unit of time equal to the sum of the RxTx turnaround time (time for a station to switch from receive to transmit mode), the channel sensing time, the channel propagation delay, and the MAC processing time. S IMPLIFIED IEEE 802.11 E H YBRID MAC M ODEL.
![Propagation Propagation](/uploads/1/2/6/2/126288777/558440668.jpg)
Download free sims 4. The purpose of the guard interval is to introduce immunity to propagation delays, echoes and reflections, to which digital data is normally very sensitive.
![Delay Delay](/uploads/1/2/6/2/126288777/675834267.jpg)
Use in digital communications systems[edit]
In OFDM, the beginning of each symbol is preceded by a guard interval. As long as the echoes fall within this interval, they will not affect the receiver's ability to safely decode the actual data, as data is only interpreted outside the guard interval.
In TDMA, each user's timeslot ends with a guard period, to avoid data loss and to reduce interference to the following user, caused by propagation delay. Thus a user's timeslot is protected from interference from the preceding user, by the guard interval (guard period) at the end of that preceding user's timeslot. It is a common misconception that each TDMA timeslot begins with a guard interval, however the ITU Technical Specifications (such as GSM 05.05) clearly define the guard period as being at the end of each timeslot, thus providing protection against data loss within that timeslot, and protection against interference to the following timeslot.
Longer guard periods allow more distant echoes to be tolerated. However, longer guard intervals reduce the channel efficiency. For example, in DVB-T, four guard intervals are available (given as fractions of a symbol period):
- 1/32 ; 1/16 ; 1/8 ; 1/4
Propagation Delay In Networking
Hence, 1/32 gives the lowest protection and the highest data rate; 1/4 results in the best protection but the lowest data rate.
Radio waves propagate at the speed of light, 3 μs per 1000 meter (5 μs/mile). Ideally, the guard interval is just longer than the delay spread of the channel.
802.11 guard interval[edit]
The standard symbol guard interval used in 802.11 OFDM is 0.8 μs. To increase data rate, 802.11n added optional support for a 0.4 μs guard interval. This provides an 11% increase in data rate.
802.11 Slot Time Propagation Delayed
The shorter guard interval results in a higher packet error rate when the delay spread of the channel exceed the guard interval and/or if timing synchronization between the transmitter and receiver is not precise. A scheme could be developed to work out whether a short guard interval would be of benefit to a particular link. To reduce complexity, manufacturers typically only implement a short guard interval as a final rate adaptation step when the device is running at its highest data rate.[1]
See also[edit]
References[edit]
- ^Perahia and Stacey, Next Generation Wireless LANs, Cambridge University Press, 2008
External links[edit]
Propagation Delays
- Technical Standard GSM 05.05 Radio Transmission and Reception. Contains descriptions and diagrams of the GSM use of TDMA timeslots, bursts, and guard periods.
- Guard interval and ISI-free OFDM transmission. Online experiment illustrates ISI-free OFDM transmission if guard time is longer or equal to the channel's maximum delay spread.
802.11 Slot Time Propagation Delays
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