Cisco has developed a three-layer hierarchical model to help you design campus networks. Cisco uses this model to simplify designing, implementing, and managing large-scale networks. With traditional network designs, it was common practice to place the networking services at the center of the network and the users at the periphery.

However, many things in networking have changed over the past decade, including advancements in applications, developments in graphical user interfaces (GUIs), the proliferation of multimedia applications, the explosion of the Internet, and fast-paced changes in your users traffic patterns. Cisco developed the three-layer model to accommodate these rapid changes.

Cisco’s hierarchical model contains three layers: core, distribution, and access. A well-designed network typically follows this topology.

Core Layer
The core layer, as its name suggests, is the backbone of the network. It provides a high speed connection between the different distribution layer devices. Because of the need for high-speed connections, the core consists of high-speed switches and will not, typically, perform any type of packet or frame manipulations, such as filtering or Quality of Service.

Because switches are used at the core, the core is referred to as a layer-2 core. The traffic that traverses the core is typically to access enterprise corporate resources: connections to the Internet, gateways, e-mail servers, and corporate applications.

Half-duplex Ethernet is defined in the original 802.3 Ethernet; Cisco says it uses only one wire pair with a digital signal running in both directions on the wire. Certainly, the IEEE specifications discuss the process of half duplex somewhat differently, but what Cisco is talking about is a general sense of what is happening here with Ethernet.

It also uses the CSMA/CD protocol to help prevent collisions and to permit retransmitting if a collision does occur. If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions. Half-duplex Ethernet typically 10BaseT is only about 30 to 40 percent efficient as Cisco sees it, because a large 10BaseT network will usually only give you 3 to 4Mbps at most.

But full-duplex Ethernet uses two pairs of wires, instead of one wire pair like half duplex. And full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device. This means that with full-duplex data transfer, you get a faster data transfer compared to half duplex. And because the transmitted data is sent on a different set of wires than the received data, no collisions will occur.
The reason you don’t need to worry about collisions is because now it’s like a freeway with multiple lanes instead of the single-lane road provided by half duplex.

Ethernet is a contention media access method that allows all hosts on a network to share the same bandwidth of a link. Ethernet is popular because it’s readily scalable, meaning that it’s comparatively easy to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet, into an existing network infrastructure. It’s also relatively simple to implement in the first place, and with it, troubleshooting is reasonably straightforward. Ethernet uses both Data Link and Physical layer specifications, and this section of the chapter will give you both the Data Link and Physical layer information you need to effectively implement, troubleshoot, and maintain an Ethernet network.

Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/ CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes. And trust me good collision management is crucial, because when a node transmits in a CSMA/CD network, all the other nodes on the network receive and examine that transmission. Only bridges and routers can effectively prevent a transmission from propagating throughout the entire network!

The types of Ethernet cables available are:
- Straight-through cable
- Crossover cable
- Rolled cable

Straight-Through Cable

The straight-through cable is used to connect
- Host to switch or hub
- Router to switch or hub

Four wires are used in straight-through cable to connect Ethernet devices. It is relatively simple to create this type; Figure as below shows the four wires used in a straight-through Ethernet cable.

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. However, remember that this would be an Ethernet-only cable and wouldn’t work with Voice, Token Ring, ISDN, etc.

Crossover Cable
The crossover cable can be used to connect :

- Switch to switch
- Hub to hub
- Host to host
- Hub to switch
- Router direct to host

The same four wires are used in this cable as in the straight-through cable; we just connect different pins together. Figure as below shows how the four wires are used in a crossover Ethernet cable.
Notice that instead of connecting 1 to 1, etc., here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

Ethernet at the Physical Layer
Ethernet was first implemented by a group called DIX (Digital, Intel, and Xerox). They created and implemented the first Ethernet LAN specification, which the IEEE used to create the IEEE 802.3 Committee. This was a 10Mbps network that ran on coax, and then eventually twistedpair and fiber physical media.

The IEEE extended the 802.3 Committee to two new committees known as 802.3u (Fast Ethernet) and 802.3ab (Gigabit Ethernet on category 5) and then finally 802.3ae (10Gbps over fiber and coax).

Figure as below shows the IEEE 802.3 and original Ethernet Physical layer specifications. When designing your LAN, it’s really important to understand the different types of Ethernet media available to you. Sure, it would be great to run Gigabit Ethernet to each desktop and 10Gbps between switches, and although this might happen one day, justifying the cost of that network today would be pretty difficult. But if you mix and match the different types of Ethernet media methods currently available, you can come up with a cost-effective network solution that works great.