Ethernet LAN's connected by an IP Network
This was the grand solution for companies spread out across the USA. For years, they packaged data onto large disks and used couriers to travel between their sister sites on a daily or weekly basis. Eventually IBM mainframes were connected via T1 circuits, but the Ethernet LAN's remained small, static enclaves . . .
When it became feasible (and affordable) to connect disparate LAN's together with X.25 and then Frame Relay . . . the world of data communications exploded !!
Speed - Ethernet is not 10 Mbps !! It has a max of 10 Mbps !!
Virtually every resource states that Ethernet operates at 10 Mbps. This does apply to the clocking speed of the transmissions, but Wrong - it has a maximum operating speed of 10 Mbps.
The initial Frame Relay solution was either DS0 (56k) or DS1 (T1 at 1.536 Mbps), with Fractional T1 speeds (n x DS0) made available later. All of these speeds are substantially slower than the Ethernet max of 10 Mbps. Fortunately, the entire path, from the source station to the destination - use IP, and IP has built-in flow control at the TCP layer. The WAN connection was slow, but it did work !! The LAN's have an Ethernet "max" of 10 Mbps, but have no "min". IP is at a higher layer than Ethernet, and therefore is dictates and controls the speed.
Ethernet is viewed as a local networking protocol, and is at Layer 2 of the OSI model IP is seen as a wide-area networking protocol, and is at Layer 3 of the OSI model. LAN's can be connected together with either a single, long-distance serial link with 2 routers - or through an entire network of multiple routers.
The IP packets start out in the LAN wrapped in Ethernet frames, and as they work their way through the WAN, they are transported between router with PPP frames, which like Ethernet, are Layer 2. The LAN workstations use MAC addresses burned into their interfaces (NIC's), while the WAN routers use IP addresses assigned to their interfaces (WIC's). The router WIC IP addresses are assigned in their configs.
PPP (Point-to-Point Protocol)
*** PPP is defined in RFC 1548
The Ethernet frames use the MAC layer 2 protocol to manage their transmission. But for IP packets traveling between routers, an entirely different Layer 2 protocol is used - either PPP (3COM, Juniper, etc - non-Cisco routers) or HDLC (Cisco). PPP is an extremely, extremely complex protocol. It deals with management of the serial link, and uses LCP (Link Control Protocol) and NCP (Network Control Protocol) packets to bring the link up, down, report the link status, etc.
A summary of the PPP encapsulation is shown below. The Protocol field contains 2 bytes (4 Hex digits). The fields are transmitted from left to right. If he packet is an IP PPP frame it will be denoted as such - the protocol value will be 0021. There are many other types of PPP frames, such as protocol C021, which is an LCP (Layer Control Protocol) frame, containing link info.
+----------+-------------+---------+
| Protocol | Information | Padding |
| 16 bits | * | * |
+----------+-------------+---------+
Router WAN Port Addresses
Since the MAC protocol is not used, router WAN serial interfaces have no MAC addresses. They use IP addresses, which are assigned to the interface in the router's config. Actually, since they are direct connections between routers, they do not need addresses to simple send and receive data, but the IP protocol requires that every port has an IP address.
Encapsulation
Ethernet frames contain data passed down from the higher layers. This can be any number of protocols and formats. In the case of IP, the frame header is a wrapper for the IP packet. The IP packet has it's own data and it's own IP addresses. The Ethernet layer adds MAC addresses in front and a CRC in back of the IP packet. So, basically, you have encapsulated the IP packet with an Ethernet frame.
Imagine two ethernet LAN's connected by an IP network, with a path between them that passes through 10 routers. Initially it appears that IP is encapsulating the ethernet frames through the network and to the other side. But actually, IP frames are moving through the network, and they are being encapsulated in Ethernet frames at the endpoints, and PPP frames between the routers.
IP packets never move
directly across a link !!!
They must be framed first by a Layer 2 protocol !!!
How it Works
Here we details how an IP packet moves from one LAN to a far away remote LAN :