CCNA RnS Article #9 -- Ethernet UTP cable & Ethernet Frame
CCNA RnS Article – Ethernet Communication
In this article, we are going to talk about the Ethernet Cabling specially Unshielded Twisted Pair (UTP) cable. This is a widely used connection in Ethernet LAN.
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| Fig 1.1- Ethernet Communication UTP Cable |
Above is a UTP cable. As you can see, this UTP cable
contains some copper wires (a total of 8) grouped as twisted pairs (a total of 4).
The 1000BASE-T standard requires 4 pairs while the earlier legacy standards
10BASE-T or 100BASE-T require two pairs of wires. Each pair in UTP cable is color
coded – for example, orange wire pair has one solid orange wire and one orange-and-white
striped wire.
To connect two ethernet nodes together UTP cable needs to
have connectors at each end. This connector is called RJ-45. RJ-45 Connector
has eight tiny holes into which the 8 wires can be inserted – these holes are
called the pin-positions. These pins come into contact with the Network nodes’
NIC card circuit to make an electrical circuit.
Figure 2:
RJ-34 Empty Connector
Figure 3:
RJ-35 with Cable Inserted
Figure 4:
Crimping
A UTP cable is crimped at both ends and the RJ-45 connector
is inserted into the Network Nodes’ NIC cards or Ethernet Ports. Now both the
devices are connected using the Ethernet UTP cable.
Figure 5:
RJ-45 Ports on Network Device
You got the idea, to complete a physical connection using
the UTP cable, you need UTP cables with RJ-45 connectors at each end and RJ-45
ports (Routers, Switches) or Ethernet NICs (for PC).
Now that you know some of the physical aspects of the UTP
connections, let’s deep dive into the cable arrangements in the RJ-45 connector
this is known as pinouts. Ethernet standard has defined rules for pinouts. This
pinout arrangement decides whether a cable is straight or cross-over-cable.
Straight-Through Cable
Straight-Through cable is used between the devices that send
on different ports. For example, a PC sends on pin 1 and 2; and a switch sends
on pin 3, 6. To connect unlike devices we use straight-through cable.
For successful transmission between the PC and the Switch,
the wires in the UTP cable must be put correctly into the pin position in the
RJ-45 connector. Let me show you a connector with pin numbers –
Figure 6:
RJ-45 Connect Pin Numbers
The transmitter on the PC use the orange pair (1,2 pins) and
Switch uses green pair (3, 6 pins) for transmission. In a straight-through
cable, the nodes should use the opposite pairs for transmitting data.
Figure 7:
Straight-through cable
However, when two like devices (PC to PC) that use the same
pins for transmission, the straight-through cable doesn’t work and you need a cross-over
cable. This cable crosses the pair at the transmit pins on each device to
receive pins on the opposite device.
Figure 8:
Cross-Over Cable
Figure 8 shows how UTP pairs are laid in the RJ-45 Connector
for cross-over cable. 1-3, 2-6
·
Crossover Cable: if the network devices transmit
on the same pin pair
·
Straight-through cable: if the network devices
transmit on different pairs
Who transmit on pin 1 and 2 – PC NICs, routers, Wireless
Access Points
Who transmit on Pin 3 and 6 – Hubs and Switches
So, a UTP cable connection between a switch and a PC is
using a straight-through wire. The connection between two switches using a UTP
cable is built using cross-over wires.
Data Transfer in Ethernet Networks
There can be multiple physical standards to build network connections,
but the frame format in the Ethernet network remains the same. And this format ensures
that the destination receives data from one station irrespective of the type of
physical connection (Ethernet or Fiber).
Figure 9: Ethernet Frame Format
Preamble – 7 bytes – used synchronization
Start Frame Delimiter (SFD) – 1 byte long start frame Delimiter
(SFD) – indicates that next byte holds the destination address information in
Ethernet Frame
Destination – 6 byte – indicates the MAC address of the
receiving device
Source – 6 byte – indicates the MAC address of the sending device
Type – 2 bytes – indicates the type of the L3 Packet, IPv4 or
IPv6
Data & Pad – 46 to 1500. This is L3 PDU (L3 Packet). To
meet the minimum length size while sending the data Pad is added – the minimum requirement
is 46 bytes
Frame Check Sequence (FCS) – 4 byte – using FCS receiving
device identifies whether the received frame is error-free or not.
With this, I conclude today's article. Hope you find this informative, see you soon with a new article on Ethernet Addressing.
