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2. Ethernet Basics

TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of protocols independent of the physical medium used to transmit data, but most data transmission for Internet communication begins and ends with Ethernet frames.

The Ethernet can use either a bus or star topology. A bus topology attaches all devices in sequence on a single cable. In a star topology all devices are wired directly to a central hub. 10Base-T uses a combination called a star-shaped bus topology because while the attached devices can share all data coming in on the cable, the actual wiring is in a star shape.

The access method used by the Ethernet is called Carrier Sense Multiple Access with Collision Detect (CSMA/CD). This is a contention protocol, meaning it is a set of rules to follow when there is competition for shared resources.

2.1 Ethernet Address

All Ethernet interfaces have a unique 48-bit address that is supplied by the manufacturer. It is called the Ethernet address (also known as the MAC address, for Media Access Control). Ethernet-enabled Rabbit boards store this value in Flash Memory (EEPROM) that is programmed at the factory. If you need unique Ethernet addresses for some product you are making, you can obtain them from the IEEE Registration Authority.

Run the utility program Samples/tcpip/display_MAC.c to read the MAC address of a Rabbit-based board. The utility is included with Dynamic C starting with version 7.04.

2.2 Physical Connections

A Realtek RTL8019 10Base-T interface chip provides a 10 Mbps Ethernet connection. This chip is used on many Ethernet-enabled Rabbit boards. The corresponding port can be connected directly to an Ethernet network.

By using hubs and routers, a network can include a large number of computers. A network might include all the computers in a particular building. A local network can be connected to the Internet by means of a gateway. The gateway is a computer that is connected both to the local network and to the Internet. Data that must be sent out over the Internet are sent to the local network interface of the gateway, and then the gateway sends them on to the Internet for routing to some other computer in the world. Data coming in from the Internet are directed to the gateway, which then sends them to the correct recipient on the local network.

2.2.1 Cables

Ethernet cables are similar to U.S. telephone plug cables, except they have eight connectors. For our purposes, there are two types of cables--crossover and straight-through. In most instances, the straight-through cables are used. It is necessary to use a crossover cable when two computers are connected directly without a hub (for example, if you want to connect your PC's Ethernet directly to the Rabbit Semiconductor TCP/IP Development Board.) Some hubs have one input that can accept either a straight-through or crossover cable depending on the position of a switch. In this case make sure that the switch position and cable type agree.

Figure 2.1 Ethernet Network

2.3 Frames

Bits flowing across the Ethernet are grouped into structures called frames. A frame must be between 46 and 1500 bytes in size. An Ethernet frame has four parts:

1. A Preamble of 8 bytes that helps synchronize the circuitry, thus allowing small bit rate differences between sender and receiver.

2. A Header of 14 bytes that contains a 6 byte destination address, 6 byte source address and a 2 byte type field.

3. A Data area of variable length that, along with the header, is passed to the IP layer (aka. the Network layer).

4. A Trailer of 4 bytes that contains a CRC to guard against corrupted frames.

If the destination address is all 1 bits, it defines a broadcast frame and all systems on the local network process the frame. There are also multicast frames. A subset of systems can form a "multicast" group that has an address that does not match any other system on the network. All systems in a particular subset process a packet with a destination address that matches their subset. A system can belong to any number of subsets.

A system may put its interface(s) into promiscuous mode and process all frames sent across its Ethernet. This is known as "sniffing the ether." It is used for network debugging and spying.

2.3.1 Collisions

In a star-shaped bus topology, all systems have access to the network at any time. Before sending data, a system must determine if the network is free or if it is already sending a frame. If a frame is already being sent, a system will wait. Two systems can "listen" on the network and "hear" silence and then proceed to send data at the same time. This is called a collision. Ethernet hardware has collision detection sensors to take care of this problem. This is the Collision Detect (CD) part of CSMA/CD. The colliding data is ignored, and the systems involved will wait a random amount of time before resending their data.