14 DAY TRIAL // This article will provide more in-depth information about Ethernet networks. We will mainly concentrate on data frames and alternative network types to the Ethernet design. But first, let us see the differences between several types of networks according to their area spanning.
LAN vs. WAN Ethernet network technologies may be classified in two classes:
- Local area network (LANss) technologies allow the interconnection of numerous devices that are relatively close to each other. These devices may be found in the same building or a small neighborhood. For example modern library terminals that display book information would connect over a local area network.
- Wide area network (WAN) technologies connect a considerably smaller number of devices. However, these devices can be separated by many kilometers. Take the example of two libraries at opposite ends of a city. If they want to share their book catalog information, they would probably make use of a wide area network technology. The connection medium would be in this case a dedicated line leased from the local telephone company or a specially constructed cable "conduit" intended solely to carry their data.
When comparing the two classes, LANs emerge as the faster and more reliable solution, but improvements in technology continue to blur the line of demarcation. The introduction of fiber optic cables has allowed LAN technologies to connect devices tens of kilometers apart, while at the same time greatly improving the speed and reliability of WANs.
Ethernet data frame components and frame types Remember those Ethernet frames that allow network-connected devices to communicate similarly to people using particular languages? Today we are actually going to explore the components of a basic frame.
Thus, a basic frame consists of seven elements split between three main areas.
1. The first main area is the Header, which includes most of the elements:
a) The Preamble (PRE) - this has to be seven bytes long and it consists of a pattern of alternating 1s and 0s, informing the receiving stations that a frame is incoming. This also enables the synchronization procedures.
b) The Start Of Frame Delimiter (SOF) - consists of one byte and contains an alternating pattern of 1s and 0s that necessarily ends in two1s.
c) The Destination Address (DA) - a field which contains the address of the information receiving device. The left most bit indicates whether the destination is an individual address or a group address. An individual address is denoted by a 0, while a 1 indicates a group address. The next bit into the DA indicates whether the address is globally administered, or local. If the address is globally administered the bit is a 0, while 1 represents a locally administered address. The remaining 46 bits are used for the destination address itself.
d) The Source Address (SA) - This type of address consists of six bytes, and it is used to identify the data sending device. Because of the fact that the sending device always uses an individual address the left most bit is always a 0.
e) The Length/Type Field - consists of two bytes and provides MAC information. It also indicates the number of client data types that are contained in the data field of the frame. In addition it may indicate the frame ID type if the frame is assembled using an optional format.
2. Payload area consists of the actual data that is sent. Data packets may be up to 1500 bytes long. If the length of the field is less than 46 bytes, then padding data is added to bring its length up to the required minimum of 46 bytes.
3. The Trailer includes the Frame Check Sequence (FCS). This last field is four bytes long and contains a 32 bit Cyclic Redundancy Check (CRC) which is generated over the previous DA, SA, Length / Type and Data fields.
In order to be properly identified, every Ethernet network interface card (NIC) is given a unique ID called a Media Access Control (MAC) address. This is assigned by the manufacturer of the card and each manufacturer that complies with IEEE Ethernet standards can apply to the IEEE Registration Authority for a range of numbers to use in their products, much like the registration of cell-phone numbers.
The MAC address is a 48-bit number, within which the first 24 bits identify the manufacturer and it is known as the manufacturer ID or Organizational Unique Identifier (OUI). This is assigned only by the registration authority. The remaining half of the address is assigned by the manufacturer and it is known as the extension of board ID. The MAC address is usually encrypted into the hardware so that it cannot be easily changed. Because the MAC address is assigned to the network interface card, it moves with the entire PC system that includes it. Eve if you have a notebook and have to travel to another location across the world, you can still be reached because the message is sent to the particular hardware MAC address.
Hopping that I haven't lost you yet, I'll go on with Ethernet frame standards. Here are the most important frame standards that appeared after the first experimental Ethernet stage:
* Ethernet Version 2 or Ethernet II frame, also known as DIX frame (named after DEC, Intel, and Xerox, the companies that supported 3Com in its endeavor to standardize the Ethernet technology). This is the most commonly used type of frame nowadays, because it is directly used by the Internet Protocol address (IP). This address is quite similar to the MAC address, but it is not encrypted in the PC hardware and thus, it can be changed by the user or automatically assigned by the operating system.
* IEEE 802.2 LLC frame
* IEEE 802.2 LLC/SNAP frame
* Novell's homegrown variation of IEEE 802.3. This "raw" 802.3 frame format was based on early IEEE 802.3 structures. Novell used this as a starting point to create the first implementation of its own IPX Network Protocol over Ethernet.
Alternative Network Technologies Among the most common LAN alternative to Ethernet is the IBM developed token ring technology. Ethernet relies on the random gaps between data transmissions in order to regulate access to the medium. On the other hand, token ring implements a strict and orderly access. Accordingly, token-ring networks arrange the included nodes in a logical ring. The nodes send frames in one direction around the ring, removing a frame when it has completed a full circle.
Here are the basic data transmission stages that occur in token ring networks:
1. The ring is initialized by creating a token, which is a special type of frame that gives a node permission to transmit. The Token is similar to the Header portion from a standard Ethernet frame. 2. The token circles the ring until it encounters a node that forwards a data transmission request. 3. This node which has to transmit data "captures" the token by replacing the token frame with a data-carrying frame. This latter type of frame encircles the entire network in its turn. 4. Once that data frame returns to the transmitting node, that station removes the data frame, creating a new token, which is forwarded to the next node in the ring.
Token ring nodes do not check for a carrier signals or listen for collisions; the presence of the token frame itself provides assurance that the node can transmit a data frame without being interrupted by another node. Because nodes transmit only a single data frame before passing the token along, each of the nodes in a ring will get a turn to communicate in a deterministic manner. However, because of this strict manner of communication, token ring networks can normally transmit data only up to 16 Mb/s.
In order to overcome the speed issue, fiber-distributed data interface (FDDI) were created. These are token derived technologies that operate over a pair of fiber optic rings, where each ring passes a token in opposite directions. This way, FDDI networks offered transmission speeds of up to 100 Mb/s, which initially made them quite popular for high-speed networking. In the early 1980s, however, the advent of cheaper and easier to administer 100 Mb/s Ethernet technologies triggered the fall of FDDI and token ring networks.
A few words about the asynchronous transfer mode (ATM), which is another network technology alternative to Ethernet. ATM networks focus on blurring the line between LAN and WAN. In this case, one is able to attach many different devices with high reliability and at high speeds, even across an entire country. ATM networks are suitable for carrying all sorts of data, including voice and video information, and this makes them versatile and expandable. Introduced in the early 1980, it hasn't yet gained a wide acceptance, remaining nonetheless a solid network technology for the future.
