e discussed in Chapter 5. Copper access and radio access networks presently have a problem in that while peak per user data rates have increased significantly, this has to an extent been achieved by degrading the average throughput rates of other users. We explore this in the following chapter. Interrelationship of the Physical Layer with Other Layers of the OSI Model 3.1 MAC Layer and Physical Layer Relationships Having stated unequivocally in Chapter 2 that we would leave the mysteries of the LTE physical layer in the capable hands of Harri Holma and Antti Toskala it is necessary for us to take a look at the differences between guided and unguided media at the physical layer and how these differences impact on the MAC (medium access control) layer with LTE¹ as particular and specific example. The MAC layer, OSI Layer 2 as the name implies describes the functional protocols that manage ingress and egress of traffic across each of the physical layer (Layer 1) delivery options. In terms of LTE this requires us to look at the specific challenges of delivering mobile broadband access over a particularly wide range of operational and channel conditions. The offered traffic mix may also be highly variable. The big difference between guided media, coaxial cable, copper twisted-pair and fibre and unguided media (free-space optics and RF) is that unguided media channel conditions change faster and are harder to predict, though they can be measured and managed. Signals will typically travel along multiple reflected paths that will produce differential time delays at the receiver. Phase reversals may also have occurred at each reflection point. This can also be observed in point-to-point microwave links where ground proximity within what is known as the Fresnel zone causes phase cancellation at the receiver. Early long-distance microwave links over the sea for example exhibited phase cancellation when the tide reached a particular height. Slow and fast fading and time dispersion are a dominant characteristic of mobile broa