Ethernet

IEEE 802.3

The standards set forth by IEEE 802.3 establish Medium Access Control over wired Ethernet. The relevant changes put forth by revisions of the 802.3 standards are:

Ethernet deployment Standards

The standards are named in the form xBASE-y with the elements taking the following

• x is the bitrate (confusingly, in GB or MB per second.)
• BASE stands for BASEBAND or BROADBAND. All ethernet is transmitted using Baseband.
• y is the transmission type. This indicates the cable medium. Usually this is “T” for twisted copper.

For the exam remember that the maximum transmission distances for 10GBASE-T over the following mediums:

• Ethernet standards are backwards compatible

Medium Access Control (MAC) Address

A 6 byte address that is assigned to a physical communication device. Every device is assigned a unique MAC address. These are usually represented in hexadecimal format:

11:22:33:44:55:66

The first 3 bytes are the Organizationally Unique Identifier (OUI), a code assigned to a unique manufacturer. The last three bytes are the Network Interface Controller Specific, or serial number, of the devices.

The Ethernet Frame

The Ethernet Frame encapsulates the information needed by layer 2 of the OSI Model. It will tell us the The form of an Ethernet Frame is:

[ Preamble | SFD | Destination MAC | Source MAC | Type | Payload | FCS ]

Each of the sections are described below:

Preamble

This indicates that the following message is an Ethernet Frame. It is always 7 bytes of alternating 1 and 0:

1010101010101010101010101010101010101010101010

SFD - Start Frame Delimiter

Another static set of data, this time it is one byte in a specific order:

10101011

Destination and Source MAC

Each are 6 bytes long and give the MAC address of the physical devices sending and receiving the frame.

Type

2 Bytes of data. Indicates whether the data is IPv4 or IPv6.

TODO: list kinds of payload data

Payload

46-1500 bytes. This is commonly TCP or UDP data. If the payload contains less than 46 bytes then a PAD section is added.

FCS - Frame Check Sequence

A CRC checksum that is 4 bytes long and is calculated for all of the data in frame. The CRC does not allow for retransmission, and does not catch all errors. If the frame does not agree with it’s CRC then the frame is dropped.

Types of Networks

Bus Network

In a bus network, every computer is directly connected to every other computer through a common network cable. Every ethernet frame is sent to every host. Each host determines whether or not the frame is directed to it and discards the frame if the destination MAC address does not match the device’s.

Hub Network

This is also an old format of transporting data in a network. Really, it is like putting a bus put into a small box. The advantage of this is that the bus wire can’t be cut as easily. If a wire gets cut then only the host connected to that wire is disconnected from the network.

Data can be sent both to and from a HUB, but only one direction can be sent at a time. A device cannot send data when the HUB is sending data (to all devices).

In a hub network, every ethernet frame is sent to every device on the network. 10BaseT and early 100BaseT used hub networks.

Switch Network

A network switch is an even more intelligent network device. The switch examines the ethernet frame and sends the frame only to the reciepient.

Spanning Tree Table (STT)

A switch will keep a table of MAC addresses listing all the devices on the network. This table is built from the Source MAC parts of the Ethernet Frames sent to it.

Flooding

When a destination MAC address is not in the STT it will flood the network with that frame.

Half vs. Full Duplex

A host can send and receive data, but not at the same time. Hub (and bus) networks use Half Duplex communication.

Full Duplex communication Switch networks use full duplex communication, it can send and receive data simultaneously.

Address Resolution Protocol

A protocol that a network device can use to find MAC addresses of the other devices on a network. It is used to pair IP addresses with MAC addresses, and can be run for a specific IP address or can be used to flood the network and get all the MAC addresses.

Querying your local ARP Cache: arp -a

Wireless Networks

Wireless networks use a Ready to send, Clear to send protocol. When a devices is ready to send data to the base station it requests a time slot in which to broadcast. This is the ready to send signal. The base station then queues the request. When the base station is ready to receive the data from that device it will send the device a signal allowing it to broadcast its payload.

This rigmarole is necessary because a device physically cannot send and receive data at the same time. This is because sending data overrides the antenna’s ability to listen. Furthermore, on a specific communication band, if two devices are broadcasting at the same time, their signals will interfere with each other.

Ethernet Standards

The ethernet standards are written by IEEE working group 802.3. The standards take names that take the form NBaseX, where: - N is the speed (in Mbps) - Base indicates baseband (one signal, as opposed to broadband) - X indicates cable type: - T means twisted copper wire - FL, SX, and LX mean Fiber Optic

10 Base

• Also called classic ethernet

100 Base

• Also called fast ethernet

1000 Base T

• IEEE 802.3ab
• most common ethernet nowadays

1000 Base SX / LX

• IEEE 802.3z
• optical

10G Base (10 GbE)

• 10 Gigabytes a second

Standard names take the form 10GBaseXY, where X and Y have the following meanings:

and

Synchronized Optical Networok (SONET)

• Existed before 10GBase ethernet
• Used in WANs
• Adopted by IEEE 802.3 working group as the 10GBase*W standards

Cable Lengths

• Some questions will ask for distances of sub-optimal cables. See Cables for more on this.

Carrier Sense Multiple Access and Collision Detection (CSMA/CD)

This is a protocol that allows multiple hosts to access a bus or hub network.

Multiple Access

Every host has the same access to the network.

Carrier Sense

Every host examines the network before sending a data frame, sending data only when the network is not in use.

Collision Detection

When two hosts send data at the same time both frames are lost (a collision). Both NICs on the hosts will detect that the other host is sending data and stop transmission. After a random ammount of time (called the backoff) the NIC will retransmit the frame.

Spanning Tree Protocol (STP)

If two or more switches are connected in a loop this can cause problems. A frame could be transmitted through the loop infinitely and bring down the entire network Spanning Tree Protocol solves this issue.

STP Uses special frames called Bridge Protocol Data Units (BDPU)*.

• One switch is randomly elected to be the “root switch”
• every other switch finds a path to the root switch using Bridge Protocol Data Units (BDPU)
• Ports connected to switches not on the path to the are put into the “blocking state”, and are unable to send or recieve normal frames

Of course, the topology needs to change Blocked ports can listen for a Topology Change Notification BDPU. In the case of a switch being taken out of the network then the rest of the switches will recreate the spanning tree with a newly elected root switch.