The Data Link Layer and IEEE
When we talk about Local Area Network (LAN) technology the IEEE 802 standard may be heard. This
standard defines networking connections for the interface card and the physical connections, describing
how they are done. The 802 standards were published by the Institute of Electrical and Electronics
Engineers (IEEE). The 802.3 standard is called ethernet, but the IEEE standards do not define the
exact original true ethernet standard that is common today. There is a great deal of confusion caused
by this. There are several types of common ethernet frames. Many network cards support more than one
type.
The ethernet standard data encapsulation method is defined by RFC 894. RFC 1042 defines the IP to link
layer data encapsulation for networks using the IEEE 802 standards. The 802 standards define the two
lowest levels of the seven layer network model and primarily deal with the control of access to the
network media. The network media is the physical means of carrying the data such as network cable. The
control of access to the media is called media access control (MAC). The 802 standards are listed below:
l 802.1 - Internetworking
l 802.2 - Logical Link Control *
l 802.3 - Ethernet or CSMA/CD, Carrier-Sense Multiple Access with Collision detection LAN *
l 802.4 - Token-Bus LAN *
l 802.5 - Token Ring LAN *
l 802.6 - Metropolitan Area Network (MAN)
l 802.7 - Broadband Technical Advisory Group
l 802.8 - Fiber-Optic Technical Advisory Group
l 802.9 - Integrated Voice/Data Networks
l 802.10 - Network Security
l 802.11 - Wireless Networks
l 802.12 - Demand Priority Access LAN, 100 Base VG-AnyLAN
*The Ones with stars should be remembered in order for network certification testing.
Network Access Methods
There are various methods of managing access to a network. If all network stations tried to talk at once,
the messages would become unintelligible, and no communication could occur. Therefore a method of
being sure that stations coordinate the sending of messages must be achieved. There are several methods
listed below which have various advantages and disadvantages.
l Contention
m Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) - Used by Ethernet
m Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)
l Token Passing - A token is passed from one computer to another, which provides transmission
permission.
l Demand Priority - Describes a method where intelligent hubs control data transmission. A
computer will send a demand signal to the hub indicating that it wants to transmit. The hub sill
respond with an acknowledgement that will allow the computer to transmit. The hub will allow
computers to transmit in turn. An example of a demand priority network is 100VG-AnyLAN
(IEEE 802.12). It uses a star-bus topology.
l Polling - A central controller, also called the primary device will poll computers, called secondary
devices, to find out if they have data to transmit. Of so the central controller will allow them to
transmit for a limited time, then the next device is polled.
Token passing performs better when the network has a lot of traffic, while ethernet which uses
CSMA/CD is generally faster but loses performance when the network has a lot of traffic. CSMA/CD is
basically a method that allows network stations to transmit any time they want. They, however, sense the
network line and detect if another station has transmitted at the same time they did. This is called a
collision. If a collision happened, the stations involved will retransmit at a later, randomly set time in
hopes of avoiding another collision.
