 a common access rate of 64 kbit/s, are the norm. Within this value a customer will pay for a CIR value. This is the rate that is guaranteed for delivery by the network. Packets exceeding this rate are allowed up to the negotiated EIR but are sent marked Discard Eligible (DE). The CIR and the EIR are normally added together to make the access rate. Service Level Agreements (SLAs) often include formulas for allowable short bursts and timing of the CIR and EIR values. X.25 Services X.25 was one of the earliest data networks (ITU-T, 1996); it is a true OSI layer-3 protocol and as such was implemented long before the requirements for carrying IP data. Most incumbent network operators had extensive X.25 networks; see for example, Atkins, Cooper, and Gleen (2007). In the 1990s most of the major banks' teller machines ran over such networks, running X.25 software directly. Ironically, today X.25 is more likely to be run over TCP as "X.25 over TCP" (XOT) (Forster, Satz, & Glick, 1994), although X.25 services are still used and offered. Chapter 12 Legacy Packet-Switched Data Overlay Networks Asynchronous Transfer Mode ATM as a technology was designed to be an extremely effective way to move data across a transport medium (De Prycker, 1991); it was designed to be flexible, to use bandwidth effi- ciently through statistical multiplexing, and to be able to accommodate any future service. It was originally standardized for line rates of 155 Mbit/s and 622 Mbit/s, although lower rates were later standardized by the ATM Forum. Unlike IP (Internet Protocol), ATM's packets (or "cells") are of equal length, simplifying ATM switch and cross-connect design and making it easier to design special-purpose hardware for their implementation. Cells are 53 bytes in length, consisting of a 5-byte header followed by 48 bytes of data (see Figure 12.8). Of particular interest and importance are the Virtual Circuit Indicator (VCI) and Virtual Path Indicator (VPI) fields. Unlike IP, the destination address of a cell is not held inside its header