The transport services layer. The error handling is done here. Different protocols have different requirements for length of data per packet, e.g. Token ring, Ethernet ATM all use different lengths and these are formed and reformed in this layer. The above 4 layers use gateways. Data Link Layer layer - Look at the movies on the CD with Net essentials and see that DLL add CRC to other end of the data frames.
Bridges work here, and the book is very terse on explanations.
The LAN being the basic underlying network, that's the nitty gritty physical network, where the the actual Network cards talk to each other. This may seem difficult at first but its not too bad. This is the hardware and physical cables. It just has to send 0 and 1, hi's and lows, on's and off's. On page 176 looks like 10 of the gift Qs we got.