Fiber optics allows the ethernet cable to connect nodes tens of miles apart. It is long enough to cover the distance from the customer premise equipment (CPE) to service provider's point of presence (POP), it is even long enough to cover the distance from one city to another.
Internet service providers (ISPs) quickly turn this new technology into profit by offering ethernet based WAN services.
Your little-rock-city SOHO LAN is connect to the CPE router in the little-rock-city commercial center building with an ethernet cable. The CPE router then connects to a special ethernet switch located in the local ISP central office (CO) with an ethernet cable (fiber optics). This central office provides a point of presence (POP). Behind the ethernet switch, ISP can uses any WAN technology to connect their devices internally. In another city, say big-river-city, the ISP's central office provides another POP, where another special ethernet switch connects to your CPE router located in -- well -- big-river-city commercial center building with a fiber optic ethernet cable, this CPE router then connects to you big-river-city SOHO LAN with an ethernet cable.
The internal WAN connection can vary from ISP to ISP. However, from customer's perspective: a point-to-point connection is established between two remote CPEs connected with a (long, virtual) ethernet link.
Ethernet over Multiprotocol Label Switching (EoMPLS) is one of those WAN technologies that can provide that "long, virtual ethernet link".
Recall the routing process in HDLC WAN.When an IP packet from source PC needs to travel across WAN to reach destination PC, the IP packet has to be de-encapsulated and re-encapsulated between Ethernet frame and HDLC frame.
EoMPLS uses ethernet for both layer 1 and layer 2 functions, routing over an EoMPLS WAN uses the same Ethernet protocols as LAN. Instead of switching between Ethernet and HDLC, EoMPLs use the Ethernet protocol all the time.
|IP packet travel LAN and EoMPLS|
At the beginning, the sending node encapsulates the IP packet in an Ethernet frame with the destination MAC address of the gateway router. This Ethernet frame travels across the LAN and reaches the gateway router. At the gateway router, the Ethernet frame is de-encapsulated to extract the IP packet. This IP packet is then encapsulated in another ethernet frame, this time, the destination MAC address is the MAC address of the receiving gateway router. The ethernet frame is then forwarded to the EoMPLS WAN network. The Ethernet frame travels across EoMPLS WAN and reaches other side of the point to point link, which is the receiving LAN's gateway router. The Ethernet frame is de-encapsulated to extract the IP packet. This IP packet is then encapsulated in an Ethernet frame with the destination MAC address as the receiving node. Finally the Ethernet frame travels across the LAN and reaches the receiving node. The return journey follow the same ethernet frame header/trailer changing process.