Key Concept: The most widely used data link protocols used on serial WAN links are HDLC, PPP, Frame Relay.
In this section, we will briefly introduce you to a handful of other WAN protocols.
Ethernet WANs
Ethernet began life as a LAN technology and remained so for quite a while because distance limitations made it difficult to create longer links. However, Ethernet standards kept improving with time, both in speed and distance, especially for optical fiber media. The result is that service providers now can and do offer WAN services that employ Ethernet both on the edge for customer access links and in the core of the provider network.
Figure 12-20 Ethernet WAN Service
Different kinds of Ethernet WAN services are commercially available with many different names such as Wide Area Ethernet, Ethernet over MPLS (EoMPLS), Metropolitan Ethernet (MetroE), and Virtual Private LAN Service (VPLS). In fact the provider can use any technology inside its network to create an Ethernet WAN service for its customers. Ethernet WAN services usually offer 100 Mbps or 1 Gbps speeds to customers.
Multi-Protcol Label Switching (MPLS)
Multiprotocol Label Switching (MPLS) technology is used by service providers to offer many types of WAN services. We will mention one of those WAN services called MPLS VPN that happens to be very popular with enterprise customers. MPLS VPN has a familiar service model, with customer sites connecting to the provider’s network cloud and the cloud moving data between customer sites connected to the cloud as required. The service provider also promises to keep data from different customers separate as it passes through its network.
MPLS VPNs have many differences from other WAN services, but the most significant difference is that they are aware of IP packets from customers. They do not just promise to deliver bits like leased lines or data link frames like Frame Relay and Ethernet WAN. MPLS network is more like an IP network, routing IP packets between customers sites. Due to this IP awaresness of the MPLS network, service providers are offering many interesting services to customers.
Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) has enabled much faster Internet access speeds to both homes and businesses as compared with dial-up and ISDN technologies that DSL has almost completely replaced now.
One limitation of DSL is that it only works at certain distances from the central office (CO) to the home and as cable distance increases it suffers speed degrades. So if the site where you want to have a DSL connection happens to be far from the nearest CO, the quality of the service may become poor or the service may not be available at all. Though this is usually not a concern in urban areas, yet you may occasionally see this problem.
PPP over Ethernet (PPPoE)
PPP over Ethernet (PPPoE) is one technology overlaid on top of another. You know that PPP is a data link protocol used on serial interfaces to create point-to-point links over leased lines. PPP is also used on those links that are created from a user to an ISP with dial-up modems. Some features of PPP are very useful for ISPs. First, PPP supports a way to assign IP addresses to the other end of the PPP link. PPP also supports CHAP for authentication which allows ISPs to check their accounting records to see if the customer’s bill was paid before granting Internet access.
DSL came after dial-up and ISDN that both used PPP, so ISPs still wanted their PPP with DSL. The customer however mostly used an Ethernet link between the customer PC or the router and the DSL modem. That Ethernet link only supported Ethernet data link protocols and not PPP. ISPs demanded a way to create the equivalent of a PPP connection between the customer router and the ISP router over the various technologies used on DSL connections.
PPP over Ethernet (PPPoE) was created to allow the sending of PPP frames encapsulated inside Ethernet frames. PPPoE essentially creates a tunnel between customer router and the ISP router. PPP was originally meant for point-to-point links and there is not a single point-to-point link between the two routers here. With PPPoE and its associated protocols, the rotuer logically creates a tunnel and then creates and sends PPP frames over that tunnel as if the tunnel were a point-to-point link between the routers.
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