are called the rise time and the fall time respectively. So, the leading and trailing edges are never vertical-the leading edge leans slightly to the right and the trailing edge leans slightly to the left. When the phase is not perfectly aligned, then there is a measurable phase error known as phase off- set, as shown in Figure 3-5. Phase offset Figure 3-5 Phase Offset Expressing phase offset mathematically, assume that the time reported by a clock CA at some ideal time t is CA(t). The difference between the ideal clock, which might be UTC (or an approximation of it), and the time given by the clock CA is called the clock offset and usually is represented by the lowercase Greek letter theta as follows: =CA(t) Although this could be viewed as an absolute clock offset because it is compared to a universally accepted timescale, really it is just another relative offset compared to UTC. The offset between any two network clocks, a relative offset, is defined as OAB=CA(t) - CB(t) 3.1.1 Synchronization with Packets In packet-based networks, the timing information is conveyed, from a packet master clock to a packet slave clock, by the periodic exchange of data between nodes. The time information is repres- ented in the form of timestamps and, where required, exchanged using packets. The precision time protocol (PTP) calls packets that are timestamped on transmission and reception event messages. Once the newly generated packet is timestamped at the transmitting end, the packet starts to "age," and unnecessary delay adds to this aging. The time between when the packet is timestamped on de- parture and the time recorded when it is received is time critical and must be minimized. The un- avoidable delay as a result of the transmission distance is fine, because a two-way time protocol can estimate this, but having the packets sit in a buffer queue waiting for a transmission time-slot only increases the inaccuracy. Figure 3-6 illustrates some of the basic characteristics of timing distributed by packet. First, there is the p