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OSI Data Link Layer Functions 95

HDLC is the default data link protocol (encapsulation) on Cisco routers serial interfaces. Frame
Relay headers are coincidentally based on the HDLC specification, but Frame Relay was
created for multiaccess networks (with more than two devices). The clear differences between
Frame Relay and HDLC provide a good backdrop to examine the functions of the data link
layer (Layer 2).

Data Link Function 1: Arbitration

Arbitration is needed only when there are instants in time during which it is not appropriate to
send data across the media. LANs were originally defined as a shared media on which each
device must wait until the appropriate time to send data. The specifications for these data link
protocols define how to arbitrate the use of the physical medium.

Ethernet uses the carrier sense multiple access collision detect (CSMA/CD) algorithm for
arbitration. The basic algorithm for using an Ethernet when there is data to be sent consists of
the following steps:

Step 1

Listen to find out whether a frame is currently being received.

Step 2

If no other frame is on the Ethernet, send.

Step 3

If another frame is on the Ethernet, wait and then listen again.

Step 4

While sending, if a collision occurs, stop, wait, and listen again.

With Token Ring, a totally different mechanism is used. A free-token frame rotates around the
ring while no device has data to send. When sending, a device claims the free token, which
really means changing bits in the 802.5 header to signify "token busy." The data is then placed
onto the ring after the Token Ring header. The basic algorithm for using a Token Ring when
there is data to be sent consists of the following steps:

Step 1

Listen for the passing token.

Step 2

If token is busy, listen for the next token.

Step 3

If the token is free, mark the token as a busy token, append the
data, and send the data onto the ring.

Step 4

When the header with the busy token returns to the sender of that
frame, after completing a full revolution around the ring, the
sender removes the data from the ring.

Step 5

The device sends a free token to allow another station to send a
frame.

The algorithm for Token Ring does have other rules and variations, but these are beyond the
depth of what is needed for the CCNA exam. Network Associates (the "Sniffer" people) have
an excellent class covering Token Ring in detail. To find out more about these classes, go to
www.nai.com.

ch03.fm Page 95 Monday, March 20, 2000 4:58 PM