Ethernet
was developed in the 1970s by Digital Equipment Corporation (DEC),
Intel, and Xerox. Later, the IEEE defined new standards for Ethernet
called Ethernet 802.3. 802.3 is the standard that is in use today.
Ethernet
Ethernet
is one of the most widely used LAN standards. As Figure 3-1 shows,
Ethernet operates at Layers 1 and 2 of the OSI model.
Figure 3-1 Physical and Data Link Layers
The physical layer (Layer 1) defines cabling, connection specifications, and topology.
The data link layer (Layer 2) has the following functions:
- Provides physical addressing
- Provides support for connection-oriented and connectionless services
- Provides frame sequencing and flow control
One
sublayer performs data-link functions: the MAC sublayer. Figure 3-2
shows the Media Access Control (MAC) sublayer (802.3). The MAC sublayer
is responsible for how data is sent over the wire. The MAC address is a
48-bit address expressed as 12 hex digits.
Figure 3-2 MAC Sublayer
The MAC sublayer defines the following:
- Physical addressing
- Network topology
- Line discipline
- Error notification
- Orderly delivery of frames
- Optional flow control
Ethernet LAN Connection Media
The
term Ethernet encompasses several LAN implementations. Physical layer
implementations vary, and all support various cabling structures. The
following four main
categories of Ethernet exist:
- Ethernet (DIX) and IEEE 802.3:
Operate at 10 Mbps over coaxial cable, unshielded twisted-pair (UTP)
cable, or fiber. The standards are referred to as 10BASE2, 10BASE5,
10BASE-T, and 10BASE-F.
- Fast Ethernet or 100-Mbps Ethernet: Operates over UTP or fiber.
- Gigabit Ethernet: An 802.3 extension that operates over fiber and copper at 1000 Mbps, or 1 gigabit per second (Gbps).
- 10-Gigabit Ethernet: Defined in 802.3ae, runs in full-duplex mode only, over fiber.
Network Media Types
Network media refers to the physical path that signals take across a network. The most common types of media are as follows:
Twisted-pair cable: Used
for telephony and most Ethernet networks. Each pair makes up a circuit
that can transmit signals. The pairs are twisted to prevent interference
(crosstalk). The two categories of twisted-pair cables are unshielded
twisted-pair (UTP) and shielded twisted-pair (STP). UTP cable is usually
connected to equipment with an RJ-45 connector. UTP (see Figure 3-3)
has a small diameter that can be an advantage when space for cabling is
at a minimum. It is prone to electrical noise and interference because
of the lack of shielding. Examples of categories of UTP cable exist: CAT
1, CAT 2, CAT 3, CAT 4, CAT 5, CAT 5e, CAT 6, CAT 6a, CAT 7, and so on
Figure 3-3 UTP
Fiber-optic
cable: Allows the transmission of light signals. This offers better
support in bandwidth over other types of cables. The two types of
fiber-optic cables are multimode and single-mode, defined as follows:
Multimode:
With this type of fiber, several modes (or wavelengths) propagate down
the fiber, each taking a slightly different path. Multimode fiber is
used primarily in systems with transmission distances less than 2 km.
Single-mode:
This type of fiber has only one mode in which light can propagate.
Single-mode fiber is typically used for long-distance and high-bandwidth
applications.
UTP Implementation
An
RJ-45 connector is used with UTP cabling. Figure 3-4 shows an RJ-45
connector and its pin connections, following the T568B standards.
Figure 3-4 RJ-45 Connector
The
two types of Ethernet cables are straight-through and crossover.
Straightthrough cables are typically used to connect different devices
(data terminal equipment [DTE] to data communications equipment [DCE]),
such as switch-to-router connections. Figure 3-5 shows the pins for a
straight-through cable.
Figure 3-5 Straight-Through Wiring
Crossover
Ethernet cables are typically used to connect similar devices (DTE to
DTE or DCE to DCE), such as switch-to-switch connections. Exceptions to
this rule are switch-to-hub connections or router-to-PC connections,
which use a crossover cable. Figure 3-6 shows the pins for a crossover
cable.
Figure 3-6 Crossover Wiring
Role of CSMA/CD in Ethernet
All
stations on an Ethernet segment are connected to the same media.
Therefore, all devices receive all signals. When devices send signals at
the same time, a collision occurs. A scheme is needed to detect and
compensate for collisions. Ethernet uses a method called carrier sense
multiple access collision detect (CSMA/CD) to detect and limit
collisions.
In
CSMA/CD, many stations can transmit on the Ethernet media, and no
station has priority over any other. Before a station transmits, it
listens to the network (carrier sense) to make sure that no other
station is transmitting. If no other station is transmitting, the
station transmits across the media. If a collision occurs, the
transmitting stations detect the collision and run a backoff algorithm.
The backoff algorithm computes a random time that each station waits
before retransmitting.
Ethernet LAN Traffic
Three major types of network traffic exist on a LAN:
- Unicasts: The most common type of LAN traffic. A unicast frame is a frame intended for only one host.
- Broadcasts:
Intended for all hosts. Stations view broadcast frames as public
service announcements. All stations receive and process broadcast
frames.
- Multicasts: Traffic in which one transmitter tries to reach only a subset, or group, of the entire segment.
Ethernet Addresses
The
Ethernet address, or MAC address, is the Layer 2 address of the network
adapter of the network device. Typically burned into the adapter, the
MAC address is usually displayed in a hexadecimal format such as
00-0d-65-ac-50-7f. As shown in Figure 3-7, the MAC address is 48 bits
and consists of the following two components:
Organizational
Unique Identifier (OUI): 24 bits. This is IEEE assigned and identifies
the manufacturer of the card. Vendor-assigned: 24 bits. Uniquely
identifies the Ethernet hardware.
Figure 3-7 MAC Addresses
Switching Operation
Ethernet switches perform four major functions when processing packets: learning, forwarding, filtering, and flooding.
Switches perform these functions by the following methods:
- MAC address learning: Switches learn the MAC addresses of all devices on the Layer 2 network. These addresses are stored in a MAC address table.
- Forwarding and filtering: Switches
determine which port a frame must be sent out to reach its destination.
If the address is known, the frame is sent only on that port, filtering
other ports from receiving the frame. If it’s unknown, the frame is
flooded to all ports except the one it originated from.
- Flooding: Switches flood all unknown frames, broadcasts, and some multicasts to all ports on the switch except the one it originated from.
A
switch uses its MAC address table when forwarding frames to devices.
When a switch is first powered on, it has an empty MAC address table.
With an empty MAC address table, the switch must learn the MAC addresses
of attached devices. This learning process is outlined as follows using
Figure 3-8:
1. Initially, the switch MAC address table is empty
Figure 3-8 Frame Forwarding by a Switch
2.
Station A with the MAC address 0260.8c01.1111 sends a frame to station
C. When the switch receives this frame, it does the following:
a.
Because the MAC table is empty, the switch must flood the frame to all
other ports (except E0, the interface the frame was received).
b. The switch notes the source address of the originating device and associates it with port E0 in its MAC address table entry.
3.
The switch continues to learn addresses in this manner, continually
updating the table. As the MAC table becomes more complete, the
switching becomes more efficient, because frames are forwarded to
specific ports rather than being flooded out all ports.
