Page 82 - Intelligent Communication Systems

CHAPTER 7/NEWLY DEVELOPED TELECOMMUNICATION SERVICES 65
7.6.7 Slotted Synchronizing Method
In the slotted synchronizing method, a transmission medium is partitioned into a
slot the size of a frame or a unit smaller than a frame. A message is transmitted by
using the slot assigned to the source node. This is called the TDMA method. In
TDMA, a slot is assigned to each node. TDMA gives each node an equal oppor-
tunity to use the transmission medium and stable delay time characteristics. How-
ever, it is not appropriate for a burst of traffic because a slot is assigned to each
node whether it is used frequently or not. Another method that has been proved
to be efficient, based on TDMA, is the slotted-ring method. In this method, a mes-
sage is transmitted through the ring network using any time slot in a slotted ring.
When a source node sends a message, it uses a time slot for sending and re-
leases it after finishing a transmission. In this case a source node occupies the time
slot when it has a message to send. Therefore network efficiency is substantially
high.

7.6.8 Local Area Network Standardization
Local area network standardization has been conducted mainly at the Institute of
Electrical and Electronics Engineers (IEEE) 802 committee. The International
Standards Organization (ISO) and the American National Standards Institute
(ANSI) have also studied standardization issues. The methods that have been stan-
dardized so far are CSMA/CD (as IEEE 802.3 and IS 8802/3) and the token-bus
method (as IEEE 802.4 and IS 8802/4). FDDI has been standardized by ANSI.

7.6.9 Layered Structure of a Local Area Network

A local area network protocol has a layered structure and corresponds to the OSI
reference model. The correspondence between the OSI reference model and an
LAN protocol is shown in Figure 2.4. The data link layer protocol corresponds to
the Logical Link Control (LLC) and Multiaccess Control (MAC) protocols. The
physical layer protocol of the OSI reference model corresponds to the physical
layer protocol of the LAN. The LLC protocol provides a common data transmission
function to the upper layer of the LLC. The LLC frame consists of a data service
access point (DSAP), a sender service access point (SSAP), control information,
and data. LLC has been standardized as IEEE 802.2 LLC. In MAC, there are
CSMA/CD, token-bus, token-ring, and FDDI MAC protocols. CSMA/CD, token-
bus, token-ring, and FDDI MAC protocols have been standardized as IEEE 802.3,
IEEE 802.4, IEEE 802.5, and FDDI (ANSI) protocol, respectively. In CSMA/CD,
there are 10 BASE 5,10 BASE 2,10 BASE T, and 10 BROAD 36 protocols. FDDI
has been standardized by the American National Standards Institute (ANSI) and
is used mainly for a high-speed LAN. The coverage of the network based on FDDI
is about 40-50 km.