ring Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols tiple traffic types on a single network fabric. This means that IP, which is a Network Layer protocol, can be transported across an ATM back- bone, allowing for the smooth and service-driven migration to an all-IP network. Eventually, ATM's overhead-heavy quality-of-service capabili- ties will be replaced by more elegant solutions but, until that time comes, it still plays a central role in the delivery of service. We will discuss all of these services in later chapters. The Physical Layer Once the CRC is calculated and the frame is fully constructed, the Data Link Layer passes the frame down to the Physical Layer, the lowest layer in the networking food chain. This is the layer responsible for the physi- cal transmission of bits, which it accomplishes in a wide variety of ways. The Physical Layer's job is to transmit the bits; this includes the proper representation of zeroes and ones, transmission speeds, and physical- connector rules. For example, if the network is electrical, then what is the proper range of transmitted voltages required to identify whether the received entity is a zero or a one? Is a one in an optical network repre- sented as the presence of light or the absence of light? Is a one repre- sented in a copper-based system as a positive or as a negative voltage, or both? Also, where is information transmitted and received? For example, if pin 2 is identified as the transmit lead in a cable, which lead is data received over? All of these physical parameters are designed to ensure that the individual bits are able to maintain their integrity and be rec- ognized by the receiving equipment. Many transmission standards are found at the Physical Layer, includ- ing T1, E1, SONET, SDH, DWDM, and the many flavors of DSL. T1 and E1 are longtime standards that provide 1.544 and 2.048 Mbps of band- 