 repeated here. More usefully, we will review contention protocols used in WiFi and LTE. As a reminder the LTE physical layer OFDM multiplex creates 15-kHz wide subcarriers that are grouped together as bundles of 12 subcarriers to make a 180-kHz resource block in the frequency domain that can then be resolved in the time domain in one-millisecond increments. This allows the bit stream to be mapped to specific subcarriers that are experiencing favourable propagation conditions and/or to avoid subcarriers that are suffering from fast or slow fading. This increases throughput for a single user and theoretically at least should deliver a power efficiency gain as well. However, other users will potentially end up on a less favourable combination of subcarriers. The performance gain therefore comes with an opportunity cost attached for other users. At the MAC layer contention algorithms have to arbitrate access between multiple users and manage at least four different traffic types, all with varying degrees of latency sensitivity. Voice, for example, is latency sensitive, best-effort data is not. Interactive and conversational traffic latency requirements fall between the two. Note that latency can and should be described as a first-order and second-order effect. The first-order effect is the amount of end-to-end delay. The second-order effect is how much that delay varies through a session. The theory is that the inherent variability of the channel itself is not noticeable to the user or at least not to an irritating degree. In LTE, rather poetically, a stream of happy bits and sad bits are seeded into a best-effort data exchange. These bits describe the fullness of the buffer in the sending device. If the buffer is nearly full, sad bits will be sent to indicate that more bandwidth is needed in order to prevent buffer overflow with an implied potential loss of data. Happy bits suggest that some of the bandwidth presently allocated to that user could be reallocated to another user. 3.5.1 802.11 QOS and Prioritisation L