Section 1.1: The Traditional Shared Campus Network
In the 1990s, the traditional campus network started as one LAN and grew until segmentation needed to take place to keep the network up and running. In this era of rapid expansion, response time was secondary to ensure the network functionality. Typical campus networks ran on 10BaseT or 10Base2, which was prone to collisions, and were, in effect, collision domains. Ethernet was used because it was scalable, effective, and comparatively inexpensive. Because a campus network can easily span many buildings, bridges were used to connect the buildings together. As more users were attached to the hubs used in the Ethernet network, performance of the network became extremely slow.
Availability and performance are the major problems with traditional campus networks. Bandwidth helps compound these problems. The three performance problems in traditional campus networks were:
Because all devices could see each other, they could also collide with each other. If a host had to broadcast, then all other devices had to listen, even though they themselves were trying to transmit. And if a device were to malfunction, it could bring the entire network down. Bridges were used to break these networks into subnetworks, but broadcast problems remained. Bridges also solved distance-limitation problems because they usually had repeater functions built into the electronics.
The bandwidth of a segment is measured by the amount of data that can be transmitted at any given time. However, the amount of data that can be transmitted at any given time is dependent on the medium, i.e. its carrier line: on its quality and length. All lines suffer from attenuation, which is the progressive degradation of the signal as it travels along the line and is due to energy loss and energy abortion. For the remote end to understand digital signaling, the signal must stay above a critical value. If it drops below this critical, the remote end will not be able to receive the data. The solution to bandwidth issues is maintaining the distance limitations and designing the network with proper segmentation of switches and routers.
Another problem is congestion, which happens on a segment when too many devices are trying to use the same bandwidth. By properly segmenting the network, you can eliminate some of these bandwidth issues.
1.1.3: Broadcasts and Multicasts
All protocols have broadcasts built in as a feature, but some protocols, such as Internet Protocol (IP), Address Resolution Protocol (ARP), Network Basic Input Output System (NetBIOS), and Routing Information Protocol (RIP), need to be configured correctly. However, there are features, such as packet filtering and queuing, that are built into the Cisco router Internetworking Operating System (IOS) that, if correctly designed and implemented, can alleviate these problems.
Multicasts are broadcasts that are destined for a specific or defined group of users. If you have large multicast groups or a bandwidth-intensive application, such as Cisco's IPTV application, multicast traffic can consume most of the network bandwidth and resources.
To solve broadcast issues, create network segmentation with bridges, routers, and switches. Another solution is Virtual LANs (VLANs). A VLAN is a group of devices on different network segments defined as a broadcast domain by the network administrator. The benefit of VLANs is that physical location is no longer a factor for determining the port into which you would plug a device into the network. You can plug a device into any switch port, and the network administrator gives that port a VLAN assignment. However, routers or layer 3 switches must be used for different VLANs to communicate. VLANs are discussed in more detail in Topic 3.