IEEE 802.2 Logical Link Control (LLC)

The IEEE 802.2 standard specifies the Logical Link Control sublayer of the Data Link Layer in the OSI Reference Model (For more information on the OSI Reference Model, see below). LLC is one of two layers in the Data Link Layer. The second is the media access control (MAC) layer. The MAC layer, which varies for different network types, is itself defined by standards IEEE 802.3 through IEEE 802.5 (see below). The LLC sublayer provides the interface between the MAC sublayer and the Network Layer above the Data Link Layer. Therefore, the 802.2 standard also is used by IEEE 802.3 Ethernet (see next). However, this standard isn’t used by earlier Ethernet implementations.

IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

Burn this into your mind: whenever you see 802.3, logical bus (not to be confused with the physical bus topology mentioned above), CSMA or baseband, you are talking Ethernet. The IEEE 802.3 standard specifies any network that uses baseband signaling and a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) signaling method.

A note: IEEE’s 802.3u working group very recently updated 802.3 to include Ethernet 100BASET.

Understand the CSMA/CD signaling method. With CSMA/CD, a computer will “listen” to the network to be sure the way is clear for it to send its data. If it doesn’t hear another computer sending data, it will begin to send its own data. (This part is the “Carrier Sense”. The computers on the same network competing for the same network media is the “Media Access”.) In this setup, computers are aware that there may be a collision, so they are careful to listen for a packet collision (the “Collision Detection” part). If a collision occurs, both computers will wait a randomly varying period of time before retransmitting.

IEEE 802.5 Token Ring

The IEEE 802.5 standard defines the MAC layer for token ring networks. This standard is a rare instance of a product becoming so commonly known and used that it becomes a standard. Like Ethernet, Token Ring can use several types of cables, though you’ll most often see twisted-pair cabling, either shielded or unshielded. Standard transmission rates formerly were 4Mbps (Megabits per second), but today, rates as high as 16Mbps are possible. Token ring networks generally use a physical star/logical ring topology with token passing media access.

Transmission speed on a token ring network often will be determined by the slowest Network Interface Card (NIC) present. If you have 16Mbps NICs and 4Mbps NICs, the entire ring speed will be 4Mbps. Unlike Ethernet, a computer cannot talk on the network unless it has the token (compare that with CSMA/CD discussed above). This can cause problems when the token becomes “stuck.”

IEEE 802.11b Wireless

IEEE 802.11’s general standard outlines specifications for wireless networking. The Wireless topology is outlined above. 802.11 is a wireless Ethernet technology with devices using Direct Sequence Spread Spectrum (DSSS) radio technology. DSSS operates in a 2.4 GHz frequency band. When you hear “radio frequency,” think of this standard. The 802.11b standard includes higher wireless speeds—11MBps—a marked improvement over the 1MBps outlined in the 802.11 standard.

Obviously, LLC (802.2) will not break down readily into speed, access methods, topology, and cable type. The other standards referenced in CompTIA’s Network+ test objectives break down like this:

Here are some networking technologies resources available on the web:

Lantronix Ethernet Tutorial
University of New Hampshire’s Ethernet Interoperability Lab
Charles Spurgeon's Ethernet Web Site
IBM White Paper on Migrating to Switched Ethernet
Anixer’s Technical Library: UTP vs STP
Description of IEEE 802.11
Planet.com’s 802.11 Page
IEEE FAQ
IEEE Home Page
NetworkWorldFusion Article: Putting 802.11b to the test
Dan Bricklin’s Home Network: Wireless 802.11b and a router/switch
IEEE standards and the OSI Model
Excellent article on Direct Sequence Spread Spectrum