When these routers are in close physical proximity,
they can be interconnected with Ethernet ports. When they are not in close
physical proximity, they must be interconnected with TDM ports such as Packet
over SONET. This connection is more expensive and often at a relatively low data
rate. A simple and economical solution from the perspective of the ISP is to utilize
Ethernet ports for all connections regardless of physical distance.
284 Chapter 10
Existing Ethernet Services
Today there are several options for services supporting Ethernet beyond the LAN.
Three common options will be described. The first option is router-based; the service
provider installs an Internet Protocol (IP) router with TDM ports in each location, as
shown in Figure 10.10a. The TDM ports (DS1s or DS3s) use the copper infrastructure
to connect to the nearest service provider Point of Presence (POP).
A service provider??™s router running many high-layer protocols must be installed and
provisioned to deliver Ethernet frames from one point to another. The router provides
the Ethernet port for the service connection and TDM ports to interface to the transport
network.
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