domain is then a shared resource. Problems originating in one part of the collision domain will usually impact the entire collision domain. A repeater is responsible for forwarding all traffic to all other ports. Traffic received by a repeater is never sent out the originating port. Any signal detected by a repeater will be forwarded. If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct and regenerate the signal. Standards guarantee minimum bandwidth and operability by specifying the maximum number of stations per segment, maximum segment length, maximum number of repeaters between stations, etc. Stations separated by repeaters are within the same collision domain. Stations separated by bridges or routers are in different collision domains. Figure maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various topologies. Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its functions has limitations. Layer 2 addresses these limitations. Data link sublayers contribute significantly to technology compatibility and computer communication. The MAC sublayer is concerned with the physical components that will be used to communicate the information. The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process. Figure maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. While there are other varieties of Ethernet, the ones shown are the most widely used. Interactive Media Activity Drag and Drop: The OSI Model After completing this activity, the student will be able to identify the 7 layers of the OSI model including the two data link sublayers. Web Links The OSI Model http://www.usyd.edu.au/is/comms/ networkcourse/ USydNet_mod1_ introduction&osimodel.html#tocOSIModel
Content 6.1 Ethernet Fundamentals 6.1.4 Naming To allow for local delivery of frames on the Ethernet, there must be an addressing system, a way of uniquely identifying computers and interfaces. Ethernet uses MAC addresses that are 48 bits in length and expressed as twelve hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number, or another value administered by the specific equipment manufacturer. MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes. At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. Data from upper layer entities is encapsulated in the data link layer header and trailer. The NIC uses the MAC address to assess whether the message should be passed onto the upper layers of the OSI model. The NIC makes this assessment without using CPU processing time, enabling better communication times on an Ethernet network. On an Ethernet network, when one device sends data it can open a communication pathway to the other device by using the destination MAC address. The source device attaches a header with the MAC address of the intended destination and sends data onto the network. As this data propagates along the network media the NIC in each device on the network checks to see if the MAC address matches the physical destination address carried by the data frame. If there is no match, the NIC discards the data frame. When the data reaches the destination node, the NIC makes a copy and passes the frame up the OSI layers. On an Ethernet network, all nodes must examine the MAC header even if the communicating nodes are side by side. All devices that are connected to the Ethernet LAN have MAC addressed interfaces including workstations, printers, routers, and switches. Web Links IEEE OUI and Company_id Assignments http://standards.ieee.org/regauth/ oui/index.shtml
Content 6.1 Ethernet Fundamentals 6.1.5 Layer 2 framing Encoded bit streams (data) on physical media represent a tremendous technological accomplishment, but they, alone, are not enough to make communication happen. Framing helps obtain essential information that could not, otherwise, be obtained with coded bit streams alone. Examples of such information are: Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit. A voltage vs. time graph could be used to visualize bits. However, when dealing with larger units of data, addressing and control information, a voltage vs. time graph could become large and confusing. Another type of diagram that could be used is the frame format diagram, which is based on voltage versus time graphs. Frame format diagrams are read from left to right, just like an oscilloscope graph. The frame format diagram shows different groupings of bits (fields) that perform other functions. There are many different types of frames described by various standards. A single generic frame has sections called fields, and each field is composed of bytes. The names of the fields are as follows: When computers are connected to a physical medium, there must be a way they can grab the attention of other computers to broadcast the message, "Here comes a frame!" Various technologies have different ways of doing this process, but all frames, regardless of technology, have a beginning signaling sequence of bytes. All frames contain naming information, such as the name of the source node (MAC address) and the name of the destination node (MAC address). Most frames have some specialized fields. In some technologies, a length field specifies the exact length of a frame in bytes. Some frames have a type field, which specifies the Layer 3 protocol making the sending request. The reason for sending frames is to get upper layer data, ultimately the user application data, from the source to the destination. The data package has two parts, the user application data and the encapsulated bytes to be sent to the destination computer. Padding bytes may be added so frames have a minimum length for timing purposes. Logical link control (LLC) bytes are also included with the data field in the IEEE standard frames. The LLC sub-layer takes the network protocol data, an IP packet, and adds control information to help deliver that IP packet to the destination node. Layer 2 communicates with the upper-level layers through LLC. All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety