2.1: The Preamble field is a 7-octet field that is used to allow the (Physical Signaling Sub-layer) PLS circuitry to reach its steady-state synchronization with the received packet's timing. This mechanism allows the receiving circuitry to be aligned with the incoming signal; however, there are other requirements for synchronization that are important for such a link. For example, a circuit might need to have identical transmission speeds in both directions to reduce crosstalk or in- terference between the two data streams on adjacent copper wires. To achieve this, forms of Ethernet using copper wires can select one end of a circuit to be a master with the other end becoming a slave. The transmitter on the master end will use its local free run- ning oscillator to transmit data to the slave. The receiver at the slave end then uses the preamble to align its reception frequency with that of the incoming signal (it recovers the frequency). The slave uses that frequency, learned from the incoming signal, as the transmission frequency from the slave back to the master. Use of the terms "master" and "slave" is ONLY in association with the official terminology used in industry specifications and standards, and in no way diminishes Pearson's commitment to promoting diversity, equity, and inclusion, and challenging, countering, and/or combating bias and stereotyping in the global population of the learners we serve. So, the links themselves are frequency aligned, without any centralized notion of clocking in the node. For details, see IEEE 802.3 clause 28, "Physical Layer link signaling for Auto-Negotiation on twisted pair", clause 40.4, "Physical Medium Attachment (PMA) sub-layer" (for 1000BASE-T); and clause 40.4.2.6, "Clock Recovery function" (for 1000BASE-T). For Ethernet, the requirements for frequency accuracy are quite loose, commonly only requiring ac- curacy to within +100 ppm. Different Ethernet technologies and speeds can use different frequen- cies-as an example, the 1 Gigabit Ethernet for unshielded copper tw