-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies Switching versus Routing: What's the Difference? A review of these two fundamental technologies is probably in order. The two terms are often used interchangeably, and a never-ending argument is underway about the differences between the two. The answer lies in the lower layers of the now-famous OSI Protocol Model. You will recall that OSI is a conceptual model used to study the step-by-step process of transmitting data through a network. It com- prises seven layers, the lower three of which define the domain of the typical service provider. These layers, starting with the lowest in the seven-layer stack, are the physical layer (layer one), the data link layer (layer two), and the network layer (layer three). Layer one is responsible for defining the standards and protocols that govern the physical trans- mission of bits across a medium. SONET and SDH are both physical layer standards. Switching, which lies at layer two (the data link layer) of OSI, is usu- ally responsible for establishing connectivity within a single network. It is a relatively low-intelligence function and, therefore, is accomplished quite quickly. Such technologies as ATM, frame relay, and wireless access technologies such as FDMA, TDMA and CDMA, and LAN access control protocols (CSMA/CD, token passing) are found at this layer. Routing, on the other hand, is a layer three (network layer) function. It operates at a higher, more complex level of functionality and is, there- fore, more complex. Routing concerns itself with the movement of traffic between subnetworks and therefore complements the efforts of the switching layer. ATM, frame relay, LAN protocols, and the PSTN are switching protocols; IPRIP, OSPF, and IPX are routing protocols. Switching in the Optical Domain The principal form of optical switching is really nothing more than a very sophisticated digital cross-connect system. In the early