Ethernet is a medium which is used by devices like computers, switches , routers, hubs etc. If two devices wishes to send data among each other using ethernet technology, both the devices should support ethernet. For example, there are network interface cards which support ethernet, switches and routers which support ethernet network cards. Ethernet comes at the layer 2 of the OSI Model and Layer 1 of the TCP/IP Model.
Late collision can be avoided, if the maximum length of the Ethernet network segment is restricted, such that if a collision occurs, it will reach the trmitter before the trmitter completed trmitting the entire frame. In a typical 10 Mbps network, the minimum length of an Ethernet frame is 576 bits (72 bytes) and the maximum length of a single Ethernet network segment is 2.5 kms.
At any one instance, in an Ethernet network, only one device can trmit. If two devices trmit at the same instance, then the signals from both devices will collide and a "collision" will occur. When a "collision" occurs, the signals will get distorted and the frame will be lost. Collisions are very common in a Ethernet network.
Ethernet is a Local Area Network (LAN) cabling and signaling specification for baseband networks. Ethernet uses a bus or star topology for connecting different nodes in a network.
The standard data rates for Ethernet are 10 Mbps, 100 Mbps, and 1 Gbps
The different Ethernet frame formats are listed below: Ethernet II and IEEE 802.3
The value of 'length' field in an 802.3 frame must be less than 1500 and in a Ethernet II frame the value of 'type' field must be more than 15@Since the 802.3 frame 'length' field and the Ethernet II frame 'type' field are at the same offset from the header, depending on the value present, the frame can be differentiated.
CSMA/CD is a media access control mechanism used in Ethernet to recover from frame collision. The following steps are followed to recover from a collision.
Step 1: Before an Ethernet device sends a frame on the Ethernet cable, it listens to find if another device is already trmitting a frame (Carrier Sense).
Step 2: Once the device finds that other devices are not trmitting any frame, it starts trmitting the frame. If two devices detect that the Ethernet cable is free at the same time, then both will start trmitting the frames (Multiple Access). This will result in collision.
Step 3: The Ethernet devices while trmitting the frames, also listen for the collision. (Collision Detect).
Step 4: If they detect a collision, both the devices stop sending the frame (back off).
Step 5: They retry the trmission after a logarithmic time-out period. This process is repeated till the frame is trmitted successfully, for a maximum of 16 times. The frame is discarded after the 16th retry.
Ethernet uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) media access control mechanism to detect and recover from a collision.
Ethernet frames are of different types. Typically they consist of the source mac-address, destination mac-address, data from the upper layer, type of data like IP, IPX etc, preamble (for clocking) and a FCS value for identifying the validity of the frame (Checks if the frame is invalid, corrupt, etc)
The minimum size of an Ethernet frame is 64 bytes. The breakup of this size between the fields is: Destination Address (6 bytes) + Source Address (6 bytes) + Frame Type (2 bytes) + Data (46 bytes) + CRC Checksum (4 bytes). The minimum number of bytes passed as data in a frame must be 46 bytes. If the size of the data to be passed is less than this, then padding bytes are added. The maximum size of an Ethernet frame is 1518 bytes. The breakup of this size between the fields is: Destination Address (6 bytes) + Source Address (6 bytes) + Frame Type (2 bytes) + Data (1500 bytes) + CRC Checksum (4 bytes). The maximum number of bytes of data that can be passed in a single frame is 1500 bytes.
The preamble field is looked into first inside a frame. This is purely for synchronization. This is followed by the destination looking into whether the frame is intended for itself or not, by checking the destination mac-address. Once the mac-address is verified, the FCS value is calculated. If a mismatch is found , the frame is discarded. Else, the data type of the frame is looked into and the data is send to the appropriate layers of the operating system (IP Layer).
An ethernet frame is used for encapsulating data from the higher layers of the TCP/IP Model. Assume that a PC1 pings PC@The ping , which is an ICMP packet along with the IP Header is encapsulated in the ethernet frame. The frame is used for carrying the ping packet and ultimately deliver it to PC2.
The Ethernet address in which all the bits are 1 is known as a broadcast address. It is represented as FF:FF:FF:FF:FF:FF. A frame with this address is received and processed by all the nodes in the network.
Normally, a Ethernet network interface will pass a frame to the above network layers only if it is addressed to that interface. If the network interface is put in the promiscuous mode, the Ethernet network interface will send all the frames (frames addressed to any host in the network), regardless of their destination address to the above network layers. This mode is used by network analyzers to capture all the frames.
In a Ethernet network, a system broadcasts the data using a Ethernet frame. The destination system is specified in the Ethernet frame using its Ethernet address. All the systems in the network listen for an Ethernet frame with their Ethernet address in it. When a system receives an Ethernet frame with its address in it, it processes the frame and sends it to the higher layers (like IP) for further processing.
Xerox developed the first version of Ethernet, Ethernet I. The second version of Ethernet, Ethernet II, was developed by DEC, Intel and Xerox. After this the Ethernet was standardized by IEEE and the new format is known as 802.3 format. To provide backward compatibility with Ethernet II, 802.2 SNAP format was developed.
An Ethernet device will detect a collision, while it is trmitting, only if the collision reaches it before it completes trmitting the entire frame. If the collision reaches the trmitter, after it completed sending the entire frame, then the trmitter will not detect the collision, it will assume the collision occurred because of some other frame. This is called "late collision". Late collision will occur, if the length of the Ethernet network segment is greater than the standard allowed length.
The 802.2 LLC header replaces the 'protocol type' of the Ethernet II format with two SAP fields, Source SAP and Destination SAP. The value of the SAP field in the 802.2 header is equivalent to the 'protocol type' field in the Ethernet II header. The value of the SAP field will be between 1 and 255, since it is an 8 bit field. On the other hand, the 'protocol type' value for the standard protocols like IP, ARP, etc is grater than 15@Obviosuly, these values cannot be represented in the SAP fields. So to provide compatibility with Ethernet II, SNAP header was added to the 802.2 LLC header. In a SNAP frame, both the SAP values will be 0xAA and the first 5 bytes of the data will give the protocol ID. Out of the 5 bytes of data, the last 2 bytes are same as the protocol type field of the Ethernet II frame. The first 3 bytes are called as 'Organizationally Unique Identifer' (OUI) and are allocated as a vendor identifier. Typically, OUI will be zero.
The length of an Ethernet II frame is not present in the frame itself. It depends on the Ethernet network interface used. When the interface sends a frame to the network device driver, it supplies the length of the received frame.
Ethernet belongs to both the Physical Layer (Layer 1) and the Data Link layer (Layer 2) in the OSI architecture.
An ethernet frame is constructed by the network card. When data on the operating system , comes down to the network card, the frame would be constructed to send the data out on the network.
Maximum Trmission Unit (MTU) is the maximum number of bytes that can be trmitted in a single trmission unit. Every communication medium has a MTU. For Ethernet, the MTU of a frame is 1500.
Each device in an Ethernet network is uniquely identified by a 48 bit (6 bytes) address called Ethernet address. Ethernet address is also known as Media Access Control (MAC) address. Ethernet addresses are represented as six pairs of hexadecimal digits separated by a colon. Ethernet address are buried in the network adapter by the manufacturer. A Ethernet address of a device cannot be changed. Example: 00:60:08:11:B1:AB, 00:00:c0:5e:83:0e