While many types of LANs have existed over the years, today’s networks use two general types
of LANs: Ethernet LANs and wireless LANs. Ethernet LANs happen to use cables for the links
between nodes, and because many types of cables use copper wires, Ethernet LANs are often
called wired LANs. In comparison, wireless LANs do not use wires or cables, instead using
radio waves for the links between nodes.
Typical SOHO LANs
To begin, first think about a Small Office / Home Office (SOHO) LAN today, specifically a
LAN that uses only Ethernet LAN technology. First, the LAN needs a device called an Ethernet
LAN switch, which provides many physical ports into which cables can be connected. An
Ethernet uses Ethernet cables, which is a general reference to any cable that conforms to any of
several Ethernet standards. The LAN uses Ethernet cables to connect different Ethernet devices
or nodes to one of the switch’s Ethernet ports.
Typical SOHO LANs today also support wireless LAN connections. Ethernet defines wired
LAN technology only; in other words, Ethernet LANs use cables. However, you can build one
LAN that uses both Ethernet LAN technology as well as wireless LAN technology.
Typical Enterprise LANs
Enterprise networks have similar needs compared to a SOHO network, but on a much larger
scale. For example, enterprise Ethernet LANs begin with LAN switches installed in a wiring closet
behind a locked door on each floor of a building. The electricians install the Ethernet cabling
from that wiring closet to cubicles and conference rooms where devices might need to connect
to the LAN. At the same time, most enterprises also support wireless LANs in the same space, to
allow people to roam around and still work
The Variety of Ethernet Physical Layer Standards
Ethernet supports a large variety of options for physical Ethernet links given its long history
over the last 40 or so years. Today, Ethernet includes many standards for different kinds of
optical and copper cabling, and for speeds from 10 megabits per second (Mbps) up to 100
gigabits per second (100 Gbps). The standards also differ as far as the types of cabling and the
allowed length of the cabling.
The most fundamental cabling choice has to do with the materials used inside the cable for the
physical transmission of bits: either copper wires or glass fibers.
Consistent Behavior over All Links Using the Ethernet Data Link Layer
No matter whether the data flows over a UTP cable, or any kind of fiber cable, and no matter
the speed, the data link header and trailer use the same format.
While the physical layer standards focus on sending bits over a cable, the Ethernet data link
protocols focus on sending an Ethernet frame from source to destination Ethernet node.
Transmitting Data Using Twisted Pairs
First, to create one electrical circuit, Ethernet defines how to use the two wires inside a single
twisted pair of wires, as shown Figure 2-5. The figure does not show a UTP cable between two
nodes, but instead shows two individual wires that are inside the UTP cable. An electrical circuit
requires a complete loop, so the two nodes, using circuitry on their Ethernet ports, connect the
wires in one pair to complete a loop, allowing electricity to flow.
UTP Cabling Pinouts for 10BASE-T and 100BASE-T
Straight-Through Cable Pinout
10BASE-T and 100BASE-T use two pair of wires in a UTP cable, one for each direction, as
shown in Figure 2-9. The figure shows four wires, all of which sit inside a single UTP cable that
connects a PC and a LAN switch. In this example, the PC on the left transmits using the top pair,
and the switch on the right transmits using the bottom pair.
For correct transmission over the link, the wires in the UTP cable must be connected to the correct
pin positions in the RJ-45 connectors.
(Many Ethernet UTP cables use an RJ-45 connector on both ends. The RJ-45 connector has eight
physical locations into which the eight wires in the cable can be inserted, called pin positions,
or simply pins. These pins create a place where the ends of the copper wires can touch the electronics
inside the nodes at the end of the physical link so that electricity can flow.)
As a rule,
Ethernet NIC transmitters use the pair connected to pins 1 and 2,the NIC receivers use a pair of
wires at pin positions 3 and 6.
Crossover Cable Pinout
A straight-through cable works correctly when the nodes use opposite pairs for transmitting
data. However, when two like devices connect to an Ethernet link, they both transmit on over
the same pins. In that case, you then need another type of cabling pinout called a crossover
cable. The crossover cable pinout crosses the pair at the transmit pins on each device to the
receive pins on the opposite device.
UTP Cabling Pinouts for 1000BASE-T
1000BASE-T (Gigabit Ethernet) differs from 10BASE-T and 100BASE-T as far as the cabling and
pinouts. First, 1000BASE-T requires four wire pairs. Second, it uses more advanced electronics
that allow both ends to transmit and receive simultaneously on each wire pair. However, the wiring
pinouts for 1000BASE-T work almost identically to the earlier standards, adding details for
the additional two pairs.
MAC
Ethernet addresses, also called Media Access Control (MAC) addresses, are 6-byte-long (48-bitlong)
binary numbers. For convenience, most computers list MAC addresses as 12-digit hexadecimal
numbers.
Most MAC addresses represent a single NIC or other Ethernet port, so these addresses are
often called a unicast Ethernet address.
If two PCs on the same Ethernet tried to use the same MAC address, to which PC should frames sent to that MAC address be delivered?
Ethernet solves this problem using an administrative process so that, at the time of manufacture,
all Ethernet devices are assigned a universally unique MAC address.
Ethernet addresses go by many names: LAN address, Ethernet address, hardware address,
burned-in address, physical address, universal address, or MAC address. For example, the term
burned-in address (BIA) refers to the idea that a permanent MAC address has been encoded
(burned into) the ROM chip on the NIC.
Broadcast address: Frames sent to this address should be delivered to all devices on the
Ethernet LAN. It has a value of FFFF.FFFF.FFFF.
Multicast addresses: Frames sent to a multicast Ethernet address will be copied and forwarded
to a subset of the devices on the LAN that volunteers to receive frames sent to a specific
multicast address.
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