Continuing my series as I work through the CCNA syllabus. The introduction to the series can be found here.
I will be pretty much following the CCNA Composite Exam Blueprint point for point. One post per bullet point. I’m using Version 11 (640-802).
Describe the purpose and basic operation of the protocols in the OSI and TCP models
Back in Post 3, I talked about the TCP, and OSI models, and how knowing them is pretty important. It gets even more interesting though. The reason these models is important is that without them, troubleshooting is a whole lot harder. To understand how that is (and we’ll come to it properly in a later post) you need to understand how the models relate to the real world. How the models define, and relate to the specific interfaces.
Most things that are engineered are dictated by standards, either imposed, or just codified after long use. The bolt you use on a garden gate is a specific diameter, with a specific gradient of thread (denoted by a term like M3). This is so that you can buy a an M3 bolt from anywhere, and know it will fit that M3 nut you have. Computers, and networking standards are no different. Defined by RFCs (Request for Comments) by people such as the ITEF and IEEE, the standards for everything from the physical media, to the definition of a “packet” are codified and available for scrutiny. Cisco insists that we know these standards, and how they relate to the models, since knowing the standards helps us to troubleshoot exactly which part of the network is failing.
Starting at the Physical layer, what are the important standards here? Well probably the most important is Ethernet or IEEE 802.3. This is the cabling system we all know and love, that forms the basis of most networks, and most PC connections. 802.3 started out as 10Mb connections using a ring topology, and coaxial cable. There have since been many revisions, moving to a star topology, and twisted pair. There are versions which use fibre (Single mode, and multi-mode depending on the distance required, and the bandwidth) and versions that run right the way up to 100Gb.
The second most common connection you will see on Cisco certs (although probably not so much in real life) is the Serial Connection.
In addition there are POTs (the Plain old Telephone System) SONET (a fibre connection often used in Metro-Ethernet systems), 802.11 (Wireless standards), USB, Bluetooth, and even Hubs.
The Hub is interesting, and most Layer 1 items are cabling systems, but the hub is a device. As it doesn’t have any intelligence though, it can be thought of as acting at Layer 1.
If layer 1 defines how bits get from one machine, or device to the next, Layer 2, defines the addresses. At this layer the important standards are:
PPP (Point to Point Protocol, often used over POTs with a modem to give an Internet connection from home), ATM and Frame Relay, which usually run over Serial links, and are used by many businesses (more on these later), SDLC which is used by point to point serial links, 802.2 or LLC, the part of a frame that is used to control traffic flow when needed in certain networks.
Finally switches act on MAC addresses, (a Layer 1 address) and so are considered layer 2 devices.
This is the layer of IP, and IPX, Appletalk, and ICMP. The important protocols here all get talked about later, but the important thing to remember is these are the protocols that describe network, and Internetwork wide addresses.
As routers work on path selection, they are considered Layer 3 devices.
The Transport layer, TCP and UDP are the big ones here, the ones that define which application gets which traffic flow. This is about where the CCNA pulls up, and although knowing the protocols above this level exist, and where they exist is necessary for the exam, they really aren’t focused on.
The wikipedia OSI Model Page is really quite useful for placing the other protocols.