2.2. Transparent Bridge Architecture
In this section we look at how transparent bridges map into and provide the services for communication in the context of the OSI seven-layer reference model [IS094]. In addition, we consider how the insertion of transparent bridges between end stations affects the architectural invariants of the Data Link as seen by higher-layer protocols and applications within those stations. If abstract architecture bores you, you may safely skip this entire section with no loss of continuity.
 If only we, the authors, could do the same.
Transparent bridges, dealing exclusively with globally unique MAC addresses, provide internetworking services at the Data Link layer of the OSI model. This is depicted in Figure 2-6. For simplicity, the figure shows only a two port bridge, although the architecture is the same regardless of the number of ports present.
 Okay, to be absolutely precise, the service is provided at the MAC sublayer of the Data link layer.
Figure 2.6. Bridge architecture
Each port on the bridge comprises a MAC and Physical layer entity, which communicate with their peer entities in the end stations on the attached LANs. On any given port, all of the devices — both the end station(s) and the bridge port itself — must use the same MAC method (for example, Ethernet) and the same Physical layer data rate (for example, 10 Mb/s or 100 Mb/s). ...