768 users. This represents a network increase in efficiency of 1,500 percent. However, the picture is not as bleak (or bright, depending on your per- spective) as just described. First, the stated inefficiency would only be man- ifest on the access ring, which typically can support 3 to 4 ADMs, and hence, only have 9 to 16 Ethernet circuits on the ring, assuming our realistic esti- mate of 3 TLS users per building. Secondly, it is well understood that some kind of Layer 2 grooming is needed at the CO/SN/POP. It is not desirable or feasible to solve the entire problem within the confines of just SONET (see Figure 4-15 for an example of Layer 2 grooming). In the past, ATM was used for Layer 2 grooming; now Layer 2 switches (such as a GbE switch) can be used to groom and repack the traffic. Furthermore, although 10-Mbps TLS was of interest in 1995,¹ even then these practitioners were already deploy- ing 100-Mbps TLS. (The very idea embodied in the Fiber-Distributed Data Interface [FDDI] was to provide a MAN that had more bandwidth than the constituent subLANs.) If 100-Mbps Ethernet is mapped to an OC-3, a 2.5- Gbps ring would support 16 TLS connections, with a stranded bandwidth figure of 800 Mbps. Wrapping up these considerations, the consensus appears to approach the bandwidth efficiency issue via a Layer 2 multiplexing technique, such as RPR as a better architecture and VLAN GbE as a lower-grade architec- ture, rather than using a Layer 1 VT-based approach. Chapter 4 Figure 4-15 Management Layer 2 grooming. 10/100/1000- Mbps Ethernet Distributed Layer 2 Switch Shared Bandwidth on Self-Hearing Fiber Ring 10/100/1000- Mbps Ethernet 10/100/1000-Mbps Ethernet Next we describe another proposal. At the time of this writing, Nortel was advancing an alternative to a recently ratified standard for recovering Ethernet networks. The alternative claims to make better use of redundant switches, links, and bandwidth. The company has advanced its Split- Multilink Trunking Protocol (Split-MLT) technology as an alternative to the