TCP/IP Protocol Suite: Table Of Contents
Ethernet – Q&A
Questions
- What is Ethernet?
- To which OSI layer does Ethernet belong?
- What are the standard data rates for Ethernet?
- What are the different IEEE standards that cover
Ethernet?
- How two systems in an Ethernet network communicate?
- What is a “collision”?
- How is “collision” handled in Ethernet networks?
- What is CSMA/CD?
- What is “late collision”?
- How “late collision” is avoided in Ethernet networks?
- What is an Ethernet address?
- What is a broadcast address?
- What are the different Ethernet frame formats?
- Why there are different Ethernet frame formats?
- What is the format of an Ethernet II frame?
- What is the format of an 802.3 frame?
- What is the format 802.2 SNAP frame?
- How is the length of an Ethernet II frame calculated?
- What is the minimum and maximum size of an Ethernet
frame?
- What is a SAP?
- Why SNAP header is required?
- What are the values for SSAP, DSAP, control and org fields
in a 2.2 SNAP frame?
- How to differentiate between an 802.3 frame and an Ethernet
II frame?
- What is promiscuous mode?
- What is MTU?
Answers
- What is Ethernet?
- To which OSI layer does Ethernet belong?
- What are the standard data rates for Ethernet?
- What are the IEEE standards that cover Ethernet?
- How two systems in an Ethernet network communicate?
- What is a “collision”?
- How is “collision” handled in Ethernet networks?
- What is CSMA/CD?
- What is “late collision”?
- How “late collision” is avoided in Ethernet?
- What is an Ethernet address?
- What is a broadcast address?
- What are the different Ethernet frame formats?
- Why there are different Ethernet frame formats?
- What is the format of an Ethernet II frame?
- What is the format of an 802.3 frame?
- What is the format of an 802.2 SNAP frame?
- How is the length of an Ethernet II frame calculated?
- What is the minimum and maximum size of an Ethernet frame?
- What is a SAP?
- Why Sub Network Access Protocol (SNAP) header is required?
- What are the values for SSAP, DSAP, control and org fields in a 802.2
SNAP frame?
- How to differentiate between an 802.3 frame and an Ethernet II
frame?
- What is promiscuous mode?
- What is MTU?
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.
Ethernet belongs to both the Physical Layer (Layer 1) and the Data Link layer
(Layer 2) in the OSI architecture.
The standard data rates for Ethernet are 10 Mbps, 100 Mbps, and 1 Gbps
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. |
+——–+—————————————————-+
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.
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.
Ethernet uses the Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) media access control mechanism to detect and recover from a
collision.
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.
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.
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.
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
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.
The different Ethernet frame formats are listed below: Ethernet II and IEEE
802.3
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.
+———–+———-+———-+———–+———-+
|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)|
+———–+———-+———-+———–+———-+
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)|
+———–+——–+——–+
+———-+———+———-+———–+———-+
|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) | |
+———————+———+
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.
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.
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.
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.
+——-+—–+
|Field |Value|
+——-+—–+
|SSAP |0xAA |
|DSAP |0xAA |
|Control|3 |
|OUI |0 |
+——-+—–+
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.
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.
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.
TCP/IP Protocol Suite: Table Of Contents
