Ethernet Q&A

1. What is Ethernet?
   
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.
2. To which OSI layer does Ethernet belong?
   
Ethernet belongs to both the Physical Layer (Layer 1) and the Data Link layer (Layer 2) in the OSI architecture.
3. What are the standard data rates for Ethernet?
   
The standard data rates for Ethernet are 10 Mbps, 100 Mbps, and 1 Gbps
4. What are the IEEE standards that cover Ethernet?
   
The following IEEE standards define Ethernet:

   +--------+----------------------------------------------------+
  IEEE                      Description                      
  Standard                                                   
  +--------+----------------------------------------------------+
  802.2   Logical Link Control (LLC) Specification.  Specifies
          the general interface between the network layer    
          (IP, IPX, etc) and the data link layer (Ethernet,  
          Token Ring, etc).                                  
  +--------+----------------------------------------------------+
  802.3   CSMA/CD Network (Ethernet) Specification.  Specifies
          the frame format, cabling and signaling standards. 
  +--------+----------------------------------------------------+

5. How two systems in an Ethernet network communicate?
   
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.
6. What is a "collision"?
   
At any one instance, in an Ethernet network, only one device can transmit. If two devices transmit 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.
7. How is "collision" handled in Ethernet networks?
   
Ethernet uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) media access control mechanism to detect and recover from a collision.
8. What is CSMA/CD?
   
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 transmitting a frame (Carrier Sense).
Step 2: Once the device finds that other devices are not transmitting any frame, it starts transmitting the frame. If two devices detect that the Ethernet cable is free at the same time, then both will start transmitting the frames (Multiple Access). This will result in collision.
Step 3: The Ethernet devices while transmitting 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 transmission after a logarithmic time-out period. This process is repeated till the frame is transmitted successfully, for a maximum of 16 times. The frame is discarded after the 16th retry.
9. What is "late collision"?
   
An Ethernet device will detect a collision, while it is transmitting, only if the collision reaches it before it completes transmitting the entire frame. If the collision reaches the transmitter, after it completed sending the entire frame, then the transmitter 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.
10. How "late collision" is avoided in Ethernet?
    
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 transmitter before the transmitter completed transmitting 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.
11. What is an Ethernet address?
    
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
12. What is a broadcast address?
    
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.
13. What are the different Ethernet frame formats?
    
The different Ethernet frame formats are listed below: Ethernet II and IEEE 802.3
14. Why there are different Ethernet frame formats?
    
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.
15. What is the format of an Ethernet II frame?
    

    +-----------+----------+----------+-----------+----------+
   DestinationSource MACFrame typeData        CRC     
   MAC AddressAddress   (IP, ARP) (46 to      Checksum
   (6 bytes)  (6 bytes) (2 bytes) 1500 bytes) (4 bytes)
   +-----------+----------+----------+-----------+----------+

16. What is the format of an 802.3 frame?
    
The various components of an 802.3 frame are shown below:

    +----------+---------+-----------+----------+
   802.3 MAC 802.2 LLCData        CRC     
   Header    Header   (43 to      Checksum
   (14 bytes)(3 bytes)1497 bytes) (4 bytes)
   +----------+---------+-----------+----------+

The first two components, MAC Header and LLC Header are further expanded below: 802.3 MAC Header:

    +-----------+----------+---------+
   DestinationSource MACLength of
   MAC AddressAddress   the frame
   (6 bytes)  (6 bytes) (2 bytes)
   +-----------+----------+---------+

802.2 LLC Header:

    +-----------+--------+--------+
   DestinationSource  Control
   SAP        SAP     Byte   
   (1 byte)   (1 byte)(1 byte)
   +-----------+--------+--------+

17. What is the format of an 802.2 SNAP frame?
    

    +----------+---------+----------+-----------+----------+
   802.3 MAC 802.2 LLC802.2 SNAPData        CRC     
   Header    Header   Header    (38 to      Checksum
   (14 bytes)(3 bytes)(5 bytes) 1492 bytes) (4 bytes)
   +----------+---------+----------+-----------+----------+

The 802.2 SNAP header is further expanded below. 802.2 SNAP Header:

    +---------------------+---------+
   OUI (OrganizationallyType    
   Unique Id)           (2 bytes)
   (3 bytes)                    
   +---------------------+---------+

18. How is the length of an Ethernet II frame calculated?
    
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.
19. What is the minimum and maximum size of an Ethernet frame?
    
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.
20. What is a SAP?
    
SAP, Service Access Point, is the logical point at which services are provided by an OSI layer. Typically, the protocols in the network layer (like IP) bind at specific SAP in the Logical Link Control Layer( LLC) for accessing the services provided by it.
21. Why Sub Network Access Protocol (SNAP) header is required?
    
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 1500. 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.
22. What are the values for SSAP, DSAP, control and org fields in a 802.2 SNAP frame?
    

    +-------+-----+
   Field  Value
   +-------+-----+
   SSAP   0xAA
   DSAP   0xAA
   Control3   
   OUI    0   
   +-------+-----+

23. How to differentiate between an 802.3 frame and an Ethernet II frame?
    
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 1500. 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.
24. What is promiscuous mode?
    
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.
25. What is MTU?
    
Maximum Transmission Unit (MTU) is the maximum number of bytes that can be transmitted in a single transmission unit. Every communication medium has a MTU. For Ethernet, the MTU of a frame is 1500.