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Friday, January 11, 2013

CCNA Notes: Network Considerations and Topology/Media

Network Considerations

When designing a network, there are many factors at play. Some factors to consider:

  1. Cost - What is an acceptable budget?
  2. Speed - What is an acceptable speed?
  3. Security - How much security is needed?
  4. Topology - Where are the users? The services? What infrastructure separates them?
  5. Scalability - Is this network capable of growing larger easily?
  6. Reliability - Are the network solutions we are using reliable?
  7. Availability - Do we have redundancy for connectivity/services?
There are different types of network locations that will help drive these questions/answers:

Small Office / Home Office (SOHO) - Small site that may or may not connect to a larger network

Central Office - Where most users/services are homed

Branch Office - An office which is geographically separated from central office but uses its services

Mobile Office - Users that connect via VPN or RAS to services, not physically connected to the network

When dealing with expectations of network speed/latency, determine whether you are talking about a LAN (Local Area Network) or WAN (Wide Area Network). LANs connect geographically close users together over high-speed links. WANs connect users over large geographical areas, as in the Central/Branch office setup, typically using much slower links.

LAN Topologies and Media

10Base5 and 10Base2 - Thicknet and thinnet coax.  This is obsolete technology and you are not likely to see it in use today. Connectivity was achieved via a vampire tap that literally tapped into the line to get signal, or the connection was terminated via BNC connector. This media was very prone to electrical interference as the cables had to have ground terminators, and was made largely obsolete by the advent of ethernet Cat5 cabling. Thicknet and thinnet were very prone to network collisions, which is what happens when two or more devices try to send signals across the wire at the same time, ruining the transmission. Later implementations of thinnet used a token ring to resolve this issue.  Token ring is a topology where collisions are avoided by having one networked device at a time utilize a logical 'token', and only the device with the token is allowed to transmit. This token was passed to other devices in turn, but this resulted in much slower speeds and ultimately token ring could not compete with ethernet collision detection/avoidance techniques.

More info on Thicknet (including pictures)
More info on Thinnet (including pictures)

10BaseT Ethernet - Category 5 (and later 6, 6e) cabling defines this standard. This is the most popular medium in use for LANs today. Most ethernet networks utilize a star, or hub and spoke topology where devices connect to network devices that aggregate connections. Network collisions are handled by CSMA/CD (carrier sense multiple access / collision detection), where when two devices detect a collision, both send a jam signal to the wire and then wait a random backoff interval before trying again.

More info on Cat5 Ethernet (including pictures)

Wireless (802.11a/b/g/n) - Radio waves comprise the physical 'media' in a wireless network. There are many different wireless standards, the differences mainly involving what channels on which the standards communicate and the speeds attainable. The topology in a wireless network is a star/hub and spoke with the wireless access point being the aggregating network device. Collision avoidance is accomplished by CSMA/CA (carrier sense multiple access / collision avoidance). Because users can communicate with the access point but not see each other's transmissions, the AP uses a system of RTS/CTS (Request to Send/Clear to Send) messages to grant exclusive transmitting capability to one device at a time, similar to a token ring.

More info on wireless standards

Fiber - Fiberoptic cables define this media type. There are enough types of fiber cabling to warrant its own entry, but in general, the signal is either carried in a single line, or there will be two lines, one dedicated to transmit signals and the other to receive them.

 FDDI - A dual fiber loop which utilized a proprietary token bus standard instead of the more prevalent token ring standard. The second fiber ring provided redundancy, but as faster ethernet standards were developed this technology was outclassed both in ease of use and price.

More info on FDDI

For the purposes of the CCNA, be aware of most different topologies and media, but focus on Ethernet and star/hub and spoke topologies.

1 comment:

  1. hi..Im student from Informatics engineering, this article is very informative, thanks for sharing :)