Page 129 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes the nodes connected to the hub are in the same collision domain. We may recall that hub is
basically a repeater that re-sends any signal it receives out of each one of its ports and this signal
is accessible to all nodes connected to the same hub. This explains why any message or signal
sent by any node is treated as broadcast signal and therefore all nodes on the same hub are in
same broadcast domain.

8.7.1 Ethernet Frame

This has also been explained in Section I in detail; however, a cursory look of the same is being
presented here. There are three basic elements, which makes an Ethernet. These are physical
medium, a set of medium access control rules, and the Ethernet frame. Ethernet takes packets
from upper-layer protocols, and places header and footer information around the data before it
traverses the network. This process is called data encapsulation or framing. Ethernet frames
travel at the Data Link layer of the OSI model and must be a minimum of 64 bytes and a
maximum of 1518 bytes. Figure 8.7 shows an Ethernet IEEE 802.3 frame and an Ethernet frame.

Figure 8.7 Ethernet IEEE 802.3 frame
7 1 6 6 2 46-1500 4
Byte
Bytes Bytes Bytes Bytes Bytes Bytes
Preamble SFD SA DA Length FCS
Field
(P) 10101
011
1010…10

Below is a brief description of each field in an Ethernet IEEE 802.3 frame:
Preamble (P): It is beginning of the frame and used to establish bit synchronization with
the help of an alternating pattern of ones and zeros that is used by the receiver.
SFD (Start Frame Delimiter): It lets the receiver know the beginning of the frame and
contains one byte length.
Destination Address (DA) and Source Address (SA): These are each six bytes long and are
contained in hardware on the Ethernet interface card.

Type Field: In Ethernet frames, this is the two-byte field after the source address. After
Ethernet processing, the type field specifies the upper-layer protocol to receive the data.

Length Field: It is a two-byte field following the source address. The length field indicates
the number of bytes of data that follow this field and precede the frame check sequence
field.

Data Field: It is the place where the information to be transmitted is contained in the
frame. It follows the type and length fields. After Physical-layer and Link-layer processes
are complete, this data is sent to an upper-layer protocol. With Ethernet, the upper-layer
protocol is identified in the type field. With IEEE 802.3, the upper-layer protocol must be
defined within the data portion of the frame. If the data of the frame is not large enough to
fill the frame to its minimum size of 64 bytes, padding bytes are inserted to ensure at least
a 64-byte frame.
FCS (Frame Check Sequence) or CRC (Cyclic Redundancy Check) fields: These are at the end
of the frame. The frame check sequence recalculates the number of frames to make sure
that none are missing or damaged. The CRC applies to all fields except the first, second,
and last.

122 LOVELY PROFESSIONAL UNIVERSITY