- 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.
- 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.
- What are the standard data rates for Ethernet?
The standard data rates for Ethernet are 10 Mbps, 100 Mbps, and 1 Gbps
- 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. |
+--------+----------------------------------------------------+
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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.
- What are the different Ethernet frame formats?
The different Ethernet frame formats are listed below: Ethernet II and IEEE
802.3
- 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.
- What is the format of an Ethernet II frame?
+-----------+----------+----------+-----------+----------+
|Destination|Source MAC|Frame type|Data | CRC |
|MAC Address|Address |(IP, ARP) |(46 to | Checksum |
|(6 bytes) |(6 bytes) |(2 bytes) |1500 bytes)| (4 bytes)|
+-----------+----------+----------+-----------+----------+
- 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 LLC|Data | 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:
+-----------+----------+---------+
|Destination|Source MAC|Length of|
|MAC Address|Address |the frame|
|(6 bytes) |(6 bytes) |(2 bytes)|
+-----------+----------+---------+
802.2 LLC Header:
+-----------+--------+--------+
|Destination|Source |Control |
|SAP |SAP |Byte |
|(1 byte) |(1 byte)|(1 byte)|
+-----------+--------+--------+
- What is the format of an 802.2 SNAP frame?
+----------+---------+----------+-----------+----------+
|802.3 MAC |802.2 LLC|802.2 SNAP|Data | 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 (Organizationally|Type |
|Unique Id) |(2 bytes)|
|(3 bytes) | |
+---------------------+---------+
- 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.
- 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.
- 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.
- 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.
- What are the values for SSAP, DSAP, control and org fields in a 802.2
SNAP frame?
+-------+-----+
|Field |Value|
+-------+-----+
|SSAP |0xAA |
|DSAP |0xAA |
|Control|3 |
|OUI |0 |
+-------+-----+
- 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.
- 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.
- 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.
