Key Elements Of Ethernet/802.3 Networks

Ethernet Communications
Reference the selected Ethernet Communications area in the figure.
Communications in a switched LAN network occur in three ways: unicast, broadcast, and multicast:
Unicast: Communication in which a frame is sent from one host and addressed to one specific destination. In unicast transmission, there is just one sender and one receiver. Unicast transmission is the predominant form of transmission on LANs and within the Internet. Examples of unicast transmissions include HTTP, SMTP, FTP, and Telnet.
Broadcast: Communication in which a frame is sent from one address to all other addresses. In this case, there is just one sender, but the information is sent to all connected receivers. Broadcast transmission is essential when sending the same message to all devices on the LAN. An example of a broadcast transmission is the address resolution query that the address resolution protocol (ARP) sends to all computers on a LAN.
Multicast: Communication in which a frame is sent to a specific group of devices or clients. Multicast transmission clients must be members of a logical multicast group to receive the information. An example of multicast transmission is the video and voice transmissions associated with a network-based, collaborative business meeting.
Ethernet Frame
Networking Fundamentals, described the structure of the Ethernet frame in detail. To briefly review, the Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent. Both the Ethernet header and trailer have several sections (or fields) of information that are used by the Ethernet protocol. The figure shows the structure of the current Ethernet frame standard, the revised IEEE 802.3 (Ethernet).
Destination MAC Address Field
The Destination MAC Address field (6 bytes) is the identifier for the intended recipient. This address is used by Layer 2 to assist a device in determining if a frame is addressed to it. The address in the frame is compared to the MAC address in the device. If there is a match, the device accepts the frame.
Source MAC Address Field
The Source MAC Address field (6 bytes) identifies the frame's originating NIC or interface. Switches use this address to add to their lookup tables.
Length/Type Field
The Length/Type field (2 bytes) defines the exact length of the frame's data field. This field is used later as part of the Frame Check Sequence (FCS) to ensure that the message was received properly. Only a frame length or a frame type can be entered here. If the purpose of the field is to designate a type, the Type field describes which protocol is implemented. When a node receives a frame and the Length/Type field designates a type, the node determines which higher layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal or 1536 decimal, the contents of the Data Field are decoded according to the protocol indicated; if the two-byte value is less than 0x0600 then the value represents the length of the data in the frame.
Data and Pad Fields
The Data and Pad fields (46 to 1500 bytes) contain the encapsulated data from a higher layer, which is a generic Layer 3 PDU, or more commonly, an IPv4 packet. All frames must be at least 64 bytes long (minimum length aides the detection of collisions). If a small packet is encapsulated, the Pad field is used to increase the size of the frame to the minimum size.
Frame Check Sequence Field
The FCS field (4 bytes) detects errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame. The receiving device receives the frame and generates a CRC to look for errors. If the calculations match, no error has occurred. If the calculations do not match, the frame is dropped.
MAC Address
An Ethernet MAC address is a two-part 48-bit binary value expressed as 12 hexadecimal digits. The address formats might be similar to 00-05-9A-3C-78-00, 00:05:9A:3C:78:00, or 0005.9A3C.7800.
All devices connected to an Ethernet LAN have MAC-addressed interfaces. The NIC uses the MAC address to determine if a message should be passed to the upper layers for processing. The MAC address is permanently encoded into a ROM chip on a NIC. This type of MAC address is referred to as a burned in address (BIA). Some vendors allow local modification of the MAC address. The MAC address is made up of the organizational unique identifier (OUI) and the vendor assignment number.
Organizational Unique Identifier
The OUI is the first part of a MAC address. It is 24 bits long and identifies the manufacturer of the NIC card. The IEEE regulates the assignment of OUI numbers. Within the OUI, there are 2 bits that have meaning only when used in the destination address, as follows:
Broadcast or multicast bit: Indicates to the receiving interface that the frame is destined for all or a group of end stations on the LAN segment.
Locally administered address bit: If the vendor-assigned MAC address can be modified locally, this bit should be set.
Vendor Assignment Number
The vendor-assigned part of the MAC address is 24 bits long and uniquely identifies the Ethernet hardware. It can be a BIA or modified by software indicated by the local bit.


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