o a full-mesh architecture or they can move to MPLS, which is discussed in Chapter 10. Chapter 7 Wide Area Networking IP Versus ATM Table 7.1 is a simple overview comparison of IP and ATM. An upside of IP is that it is pervasive at the desktop. The downside is that it has no QoS built in. It supports data, voice, and fax. IP packet size is variable. It can be up to 64,000 bytes, but packets are segmented into 1,500-byte frames for transport, and 40 bytes of each packet is for the header information. The upside of ATM is that it is an architected QoS approach that defines key service classes (as described in Chapter 10). The downside is that it uses a small cell size (only 53 bytes, 5 bytes of which is the header information), which means it has a lot of overhead (i.e., cell tax), which could be construed as inefficient-for data transport, for voice transport, or for other traffic types-and this is an issue when bandwidth is constrained and expensive. Remember that ATM was built based on the assumption of Gbps trunks and generous bandwidth, SO the cell tax is less relevant if the prevailing condition is abundant bandwidth. ATM supports a wide variety of services, including voice, IP, Frame Relay, X.25, and leased lines. We don't really have to choose between IP and ATM. At least for the time being, we can use them together quite effectively. IP has become the universal lan- guage of computer networking, especially in the desktop environment. IP-based services-including VPNs, e-commerce, outsourced remote access, application hosting, multicasting, and VoIP, along with fax and video over IP-are used in number of areas. A benefit of IP is that there is a much larger pool of knowledge- able applications developers for IP than there is for ATM. However, all these won- derful applications that the programmers are developing for IP tend to require a lot of CoS and QoS, as well as controlled access. IP standards for QoS are not yet as effective as ATM's, and ATM therefore continues to be used to switch IP traffic because 