Information Technology Reference
In-Depth Information
An information frame contains a send sequence number in the control field which ranges
from 0 to 127. Each information frame has a consecutive number, N(S) (note that there is a
roll-over from frame 127 to frame 0). The destination node acknowledges that it has re-
ceived the frames by sending a supervisory frame. The function of the supervisory frame is
specified by the 2-bit S field. This can either be set to receiver ready (RR), receiver not ready
(RNR) or reject (REJ). If an RNR function is set then the destination node acknowledges that
all frames up to the number stored in the receive sequence number N(R) field were received
correctly. An RNR function also acknowledges the frames up to the number N(R), but in-
forms the source node that the destination node wishes to stop communicating. The REJ
function specifies that frame N(R) has been rejected and all other frames up to N(R) are ac-
knowledged.
26.5 OSI and the IEEE 802.3 standard
Ethernet fits into the data link and the physical layer of the OSI model. These two layers only
deal with the hardware of the network. The data link layer splits into two parts: the LLC and
the MAC layer.
The IEEE 802.3 standard splits into three sublayers:
•
MAC (media access control).
•
Physical signalling (PLS).
•
Physical media attachment (PMA).
The interface between PLS and PMA is called the attachment unit interface (AUI) and the
interface between PMA and the transmission media is called the media dependent interface
(MDI). This grouping into modules allows Ethernet to be very flexible and to support a
number of bit rates, signalling methods and media types. Figure 26
.8
illustrates how the lay-
ers interconnect.
MAC
PLS
AUI
MAU
PMA
MDI
Medium
Figure 26.8
Organisation of the IEEE 802.3 standard
