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.

Figure 1: 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

 The process of preparing an Ethernet UTP cable is called “Crimping” and it is done with the help of the Crimping tool.

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.  

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