Ethernet Networking and Data Encapsulation

Let start by understanding the word Ethernet in Networking, Ethernet is a technology that connects local area network (LAN) devices via a wired network and enables the devices to communicate with each other through a TCP/IP protocol. In other words, you can define Ethernet as a contention-based media access method that allows all hosts on a network

 to share the same link’s bandwidth.

Ethernet is so readily scalable, meaning that it eases the process of integrating new technologies into an existing network infrastructure, like upgrading from Fast Ethernet to Gigabit Ethernet.

Collision Domain

A collision domain is a network scenario where one device sends a frame out on a physical network segment forcing every other device on the same segment to pay attention to it.

This is not good because if two devices on a single physical segment just happen to transmit simultaneously,

 it will cause a collision and require these devices to retransmit.

Fig 1.1

 In the above image in fig 1.1, the computer connected to each hub are in the same collision domain, so if one of them transmits, all the others must take the time to listen for and read the digital signal.

The below network design in fig 1.2 has two switches that have separate collisions domains, this improves the network performance of the network due to the fact that each port on the switch is a single collision domain, due to this we gain more bandwidth for the user connected to the network. More also take note switch doesn’t break the broadcast domain by default, so we still have one broadcast domain which is not so good.

Fig 1.2

Broadcast Domain

broadcast domain refers to a group of devices on a specific network segment that hears all the broadcasts sent out on that specific network segment.

The image in fig 1.3 shows how a router breaks a collision domain, the image shows the router has 3 interfaces giving us 3 broadcast domains, and I can count 9 switch segments which means we have 9 collision domains.

Fig 1.3


Ethernet networking uses a protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD), which helps devices share the bandwidth evenly while preventing two devices from transmitting simultaneously on the same network medium.

CSMA/CD was actually created to overcome the problem of the collisions that occur when packets are transmitted from different nodes at the same time. Let understand the concept of CSMA/CD using the image in fig 1.4

Fig 1.4

When a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear and no other host is transmitting, the host will then proceed with its transmission. The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data. The nodes respond to that jam signal by waiting a bit before attempting to transmit again.

The following happens when a collision happens on Ethernet LAN:

  1. A jam signal informs all devices that a collision occurred
  2. The collision invokes a random backoff algorithm.
  3. Each device on the Ethernet segment stops transmitting for a short time until its backoff timer expires.
  4. All hosts have equal priority to transmit after the timers have expired.

Ethernet at the Data Link Layer

Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to as MAC or hardware address. The Ethernet Addressing is commonly referred to as Media Access Control (MAC) addresses which is burned into each and every ethernet network interface card(NIC). The MAC address is a 48 bit (6-byte) address written in hexadecimal.

Binary to Decimal Conversion

Binary: Binary is either 0 or1 and each digit is called a bit. When you group either 4 or 8 bits together, it is referred to as a nibble and a byte, respectively.


The nibble is added up to 15= 8+4+2+1. An example of nibble values would be 1001, meaning that the 8 bit and the 1 bit are turned on, which equals a decimal value 9. If we have a nibble binary value of 0110, then our decimal value would be 6, because the 4 and 2 bits are turned on. But the byte decimal values can add up to a number that’s significantly higher than 15.

When we add up the byte we have 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255 which is 11111111 in binary.

There are plenty of other decimal values that a binary number can equal. Let’s work through a few examples: 10010110 Which bits are on? The 128, 16, 4, and 2 bits are on, so we’ll just add them up: 128 + 16 + 4 + 2 = 150. 01101100

The Table below shows binary to a decimal value

Ethernet Cabling

We have 3 types of Ethernet cable

  1. Straight-Through Cable.
  2. Crossover cable.
  3. Rolled cable.

Let take a look at the common ethernet cable used today category 5(CAT5e) enhanced Unshielded Twisted Pair(UTP). The CAT5 can handle speeds up to gigabit with a distance of 100 meters.

Typically we’d use this cable for 100 Mbps and category 6 for a gigabit, but category 5 Enhanced is rated for gigabit speeds and category 6 is rated for 10 Gbps!

Straight-through Cable

The straight-through cable is used to connect the following devices:  

  1. Host to switch or hub 
  2. Router to switch or hub

Only four wires are used in the straight-through cable to connect Ethernet devices.

Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6 and you’ll be up and networking in no time.

Crossover Cable

The crossover cable can be used to connect the following devices:

      1. Switch to switch

     2.Hub to the hub

     3.Host to host

     4.Hub to switch

      5.Router direct to host

      6.Router to router

The same four wires used in the straight-through cable are used in this cable—we just connect different pins together.

Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

      Typical uses for straight-through and cross-over Ethernet cables.

UTP Gigabit Wiring (1000Base-T) UTP Gigabit Wiring (1000Base-T)

In the previous examples of 10Base-T and 100Base-T UTP wiring, only two wire pairs were used, but that is not good enough for Gigabit UTP transmission.

1000Base-T UTP wiring requires four wire pairs and uses more advanced electronics so that each and every pair in the cable can transmit simultaneously.

For a straight-through cable it’s still 1 to 1, 2 to 2, and so on up to pin 8. And in creating the gigabit crossover cable, you’d still cross 1 to 3 and 2 to 6, but you would add 4 to 7 and 5 to 8.

Rolled Cable

Rolled cable isn’t used to connect any Ethernet connections together, you can use a rolled Ethernet cable to connect a host EIA-TIA 232 interface to a router console serial communication (COM) port.

These are probably the easiest cables to make because you just cut the end off on one side of a straight-through cable, turn it over, and put it back on—with a new connector.

once you have the correct cable connected from your PC to the Cisco router or switch console port, you can start your emulation program such as putty or SecureCRT to create a console connection and configure the device.

Notice the Putty configuration you will see the serial line with COM4 to get the COM port number to used checked the device manager of the PC you connected to the console port of the switch or Router.

The image below shows two console ports on a switch which is USB type B and an RJ45 port. Note the USB type B port supersedes the RJ45 port if you plugged both same time, the USB port has a speed up to 115,200 Kbps.

Data Encapsulation

When a host transmits data across a network to another device, the data goes through a process called encapsulation and is wrapped with protocol information at each layer of the OSI model.

Networking three-Layer Hierarchical Model

The three-networking layer hierarchy model can help you design, implement, and maintain scalable, reliable, cost-effective hierarchical internetwork. Each layer has specific responsibilities.

The 3 layers are:

1.Core Layer

2.Distribution Layer.

3.Access Layer.

CORE LAYER: The core layer is the core of the network. The core layer is responsible for transporting large amounts of traffic both reliably and quickly. The only purpose of the network’s core layer is to switch traffic as fast as possible. Note user data is processed at the distribution layer, which forwards the requests to the core if needed. The core is likely to see large volumes of traffic, so speed and latency are driving concerns here. Take note never do the following at the core network.

1.Never do anything to slow down traffic. This includes using access lists, routing between virtual local area networks, and implementing packet filtering.

2. Don’t support workgroup access here.

3 Avoid expanding the core (e.g., adding routers when the internetwork grows). If perfor[1]mance becomes an issue in the core, give preference to upgrades over expansion.

Here’s a list of things that we want to achieve as we design the core: 

1.Design the core for high reliability. Consider data-link technologies that facilitate both speed and redundancy, like Gigabit Ethernet with redundant links or even 10 Gigabit Ethernet.

2. Design with speed in mind. The core should have very little latency.

3.Select routing protocols with lower convergence times. Fast and redundant data-link connectivity is no help if your routing tables are shot!

The Distribution Layer

The distribution layer is sometimes referred to as the workgroup layer and is the communication point between the access layer and the core. The primary functions of the distribution layer are to provide routing, filtering, and WAN access and to determine how packets can access the core if needed. The distribution layer must determine the fastest way that network service requests are handled.

The distribution layer is where we want to implement policies for the network because we are allowed a lot of flexibility in defining network operation here.

There are several things that should generally be handled at the distribution layer:


2.Implementing tools (such as access lists), packet filtering, and queuing.

3.Implementing security and network policies, including address translation and firewalls

4.Redistributing between routing protocols, including static routing

5.Routing between VLANs and other workgroup support functions

6.Defining broadcast and multicast domains

Access Layer

The access layer controls user and workgroup access to internetwork resources. The access layer is sometimes referred to as the desktop layer. The network resources most users need will be available locally because the distribution layer handles any traffic for remote services.

The image below shows the three networking Hierarchical Models.

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