 bitstream according to the transmission requirements of the underlying network. For example, if the data is to be transmitted across a T- or E-Carrier network, the data will be encoded using AMI and will be transmitted across the facility to the next switch at either 1.544 Mbps or 2.048 Mbps, depending on whether the network is T1 or E1. When the bitstream arrives at the next switch (not the destination), the bits flow into the Physical Layer, which determines that it can read the bits. The Physical Layer hands the bits up to the Data Link Layer, which proceeds to find the flags SO that it can frame the incoming stream of data and check it for errors. If we assume that it finds none, it strips off the Data Link frame surrounding the packet and passes the packet up to the Network Layer. The Network Layer examines the destination address in the packet, at which point it realizes that it is not the intended recipient. So, it passes it back to the Data Link Layer, which builds a new frame around it, calculating a new CRC and adding a new Data Link Layer address as it does SO. It then passes the frame back to the Physi- cal Layer for transmission. The Physical Layer spits the bits out the facil- ity to the next switch, which, for our purposes we will assume is the intended destination. The Physical Layer receives the bits and passes them to the Data Link Layer, which checks them for errors. If it finds an errored frame it requests a resend, but ultimately receives the frame cor- rectly. It then strips off the header and trailer, leaving the original packet. The packet is then passed up to the Network Layer, which, after examining the packet address, determines that it is in fact the intended recipient of the packet. As a result, it passes the packet up to the Trans- port Layer, after stripping off the Network Layer header. The Transport Layer examines the packet and notices that it has received packet three of eleven packets. Because its job is to assemble and pass entire messages up to the Session Layer, the Transpo