Each transmission burst in 802.11 requires a guard interval that makes it so transmissions do not interfere with one another. Normally this is an 800ns timer that allows for time between bursts. This gives a safe time between transmissions. 802.11n outlined the idea of a short guard interval, decreasing the timer to 400ns. This gives less time between transmissions, which could potentially cause collisions between transmissions from the same device. In tests done in a closed environment, there was a definite increase in packet error rate. Without SGI, there was little to no packet error. With SGI, we saw a 30% increase in packet error rate. This is a significant increase in error, meaning that far more collisions are occurring. However, the throughput of a BSS using SGI was significantly better than a BSS that does not. While the error increased as predicted, the error did not affect the much higher rates that SGI predicted. Is this true in an uncontrolled environment?
IOL Blog: Mobile and Wireless
Wireless Reliability - Channel Bounding
By Jackson Corson - June 4, 2014
Channel bounding has drastically increased the bandwidth of a BSS (Basic Service Set). With 128 Data subcarriers, transmissions can carry far more coded bits per burst. The issue is that this bounded channel increases the number of channels with noise on them. This means more overlap can occur between WLANs operating within the spectra. As a result of this, channel bounding is not advised in the 2.4 GHz because it will make non overlapping channels no longer independent due to the signal spilling into overlapping channels.
Wireless Reliability - Modulation
By Jackson Corson - May 22, 2014
Modulation schemes have not changed much in 802.11, since the early days we used OFDM (Orthogonal Frequency Division Multiplexing). 802.11b was really the only notable 802.11 PHY (physical layer) standard that did not use OFDM but then again it did only get 11Mbps using DSSS (Direct-Sequence Spread Spectra). OFDM is the primary modulation suite used for today’s technology. This is because of the super extensible QAM (Quadrature Amplitude Modulation) with BPSK (Binary Phase Shift Keying), and QPSK (Quadrature Phase Shift Keying) handling the simpler modulation.
Comparing Measurements of High Speed Serial Interfaces
By Paul Willis - May 19, 2014
The priorities and specifics of physical limitations differ between standards. For instance Mobile Industry Processor Interface (MIPI) physical layer (PHY) specifications prioritize low power consumption and low electromagnetic interference (EMI) whereas an enterprise storage specification such as Serial Attached SCSI (SAS) focuses on high bandwidth and reliability. The MIPI protocols that are tested at the UNH-IOL are designed for video streaming while SATA is for transmission of Block Storage data. Both send bursts of data with headers and footers to manage erroneous bits. In MIPI the breaks between bursts are relatively short because one line of pixels is usually contained in one burst. After each MIPI burst there is also a break where the transmitter stops sending data, and the receiver stops terminating the signal line, to save on power. The MIPI PHY Specification specifies the timings and voltage levels necessary for entering and exiting this low power mode. SATA on the other hand, does not take breaks between bursts. Once one frame is finished, another is sent. If no data needs to be sent, idle data is shared to maintain the connection.
Wireless Reliability
By Jackson Corson - May 14, 2014
The IEEE 802.11 specification has proven to be a very reliable way to do networking. Wireless LANs (Local Area Networks) are capable of achieving Gigabits of speed with little to no effort. This makes wireless a competitor with Ethernet and many other networking technologies. But is it as reliable and will it continue to be so?
MIPI D-PHY: The Physical Layer in your Phone
By Paul Willis - April 19, 2013
In the last 10 years, the Mobile Industry Processor Interface (MIPI) Alliance has created a set of specifications for communication between components within mobile devices, such as phones and tablets. In 2007, the UNH InterOperability Laboratory (UNH-IOL) launched its MIPI consortium, which is currently one of the only labs that tests conformance against the MIPI D-PHY specification. D-PHY is the high speed physical layer specification that is used to communicate with cameras and displays. The physical layer is the lowest layer in data transmission and defines the electrical and physical characteristics of the signal and its medium, such as wires and connectors.