key elements ethernet 802.3 networks

In this topic, you will learn about key components of the Ethernet standard that play a significant role in the design and implementation of switched networks. You will explore how Ethernet communications function and how switches play a role in the communication process.

CSMA/CD

Ethernet signals are transmitted to every host connected to the LAN using a special set of rules to determine which station can access the network. The set of rules that Ethernet uses is based on the IEEE carrier sense multiple access/collision detect (CSMA/CD) technology. You may recall from CCNA Exploration: Networking Fundamentals that CSMA/CD is only used with half-duplex communication typically found in hubs. Full-duplex switches do not use CSMA/CD.

Carrier Sense

In the CSMA/CD access method, all network devices that have messages to send must listen before transmitting.

If a device detects a signal from another device, it waits for a specified amount of time before attempting to transmit.

When there is no traffic detected, a device transmits its message. While this transmission is occurring, the device continues to listen for traffic or collisions on the LAN. After the message is sent, the device returns to its default listening mode.

Multi-access

If the distance between devices is such that the latency of the signals of one device means that signals are not detected by a second device, the second device may also start to transmit. The media now has two devices transmitting signals at the same time. The messages propagate across the media until they encounter each other. At that point, the signals mix and the messages are destroyed, a collision. Although the messages are corrupted, the jumble of remaining signals continues to propagate across the media.

Collision Detection

When a device is in listening mode, it can detect when a collision occurs on the shared media, because all devices can detect an increase in the amplitude of the signal above the normal level.

When a collision occurs, the other devices in listening mode, as well as all the transmitting devices, detect the increase in the signal amplitude. Every device that is transmitting continues to transmit to ensure that all devices on the network detect the collision.

Jam Signal and Random Backoff

When a collision is detected, the transmitting devices send out a jamming signal. The jamming signal notifies the other devices of a collision, so that they invoke a backoff algorithm. This backoff algorithm causes all devices to stop transmitting for a random amount of time,  which allows the collision signals to subside.

After the delay has expired on a device, the device goes back into the "listening before transmit" mode. A random backoff period ensures that the devices that were involved in the collision do not try to send traffic again at the same time, which would cause the whole process to repeat. However, during the backoff period, a third device may transmit before either of the two involved in the collision have a chance to re-transmit.

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

The first course in our series, CCNA Exploration: 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).

Roll over each field name to see its description.

Preamble and Start Frame Delimiter Fields

The Preamble (7 bytes) and Start Frame Delimiter (SFD) (1 byte) fields are used for synchronization between the sending and receiving devices. These first 8 bytes of the frame are used to get the attention of the receiving nodes. Essentially, the first few bytes tell the receivers to get ready to receive a new frame.

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

In CCNA Exploration: Networking Fundamentals, you learned about the 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  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.