Page 148 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes networks only offered 4 Mbit/s or 16 Mbit/s speeds. Thus it was the preferred choice of
that era for a high-speed backbone, but FDDI has since been effectively obsolesced by fast
Ethernet which offered the same 100 Mbit/s speeds, but at a much lower cost and, since
1998, by Gigabit Ethernet due to its speed, and even lower cost, and ubiquity.
FDDI, as a product of American National Standards Institute X3T9.5 (now X3T12), conforms
to the Open Systems Interconnection (OSI) model of functional layering of LANs using
other protocols. FDDI-II, a version of FDDI, adds the capability to add circuit-switched
service to the network so that it can also handle voice and video signals. Work has started
to connect FDDI networks to the developing Synchronous Optical Network (SONET).
A FDDI network contains two rings, one as a secondary backup in case the primary
ring fails. The primary ring offers up to 100 Mbit/s capacity. When a network has no
requirement for the secondary ring to do backup, it can also carry data, extending capacity
to 200 Mbit/s. The single ring can extend the maximum distance; a dual ring can extend
100 km (62 mi). FDDI has a larger maximum-frame size (4,352 bytes) than standard
100 Mbit/s Ethernet which only supports a maximum-frame size of 1,500 bytes, allowing
better throughput.

Designers normally construct FDDI rings in the form of a “dual ring of trees” (see network
topology). A small number of devices (typically infrastructure devices such as routers and
concentrators rather than host computers) connect to both rings - hence the term “dual-
attached”. Host computers then connect as single-attached devices to the routers or
concentrators. The dual ring in its most degenerate form simply collapses into a single
device. Typically, a computer-room contains the whole dual ring, although some
implementations have deployed FDDI as a Metropolitan area network.
z z Switched Multi-megabit Data Service (SMDS): Switched Multi-megabit Data Service
(SMDS) was a connectionless service used to connect LANs, MANs and WANs to exchange
data, in early 1990s. In Europe, the service was known as Connectionless Broadband Data
Service (CBDS).
SMDS was specified by Bellcore, and was based on the IEEE 802.6 metropolitan area network
(MAN) standard, as implemented by Bellcore, and used cell relay transport, Distributed
Queue Dual Bus layer-2 switching arbitrator, and standardSONET[3] or G.703 as access
interfaces.
Its a switching service that provides data transmission in the range between 1.544 Mbit/s
to 45 Mbit/s. SMDS was developed by Bellcore as an interim service until Asynchronous
Transfer Mode matured. In the mid-1990s, SMDS was replaced, largely by Frame Relay.

Did u know? SMDS was notable for its initial introduction of the 53-byte cell and cell
switching approaches, as well as the method of inserting 53-byte cells onto G.703 and
SONET.

9.1.2 Advantages of MAN

MAN can cover a wider area than a LAN. MAN networks are usually operated at airports, or
a combination of several pieces at a local school. By running a large network connectedness,
information can be disseminated more widely, rapidly and significantly. Public libraries and
government agencies typically use a MAN.
Disadvantages MAN: MAN will only apply if the personal computer or a terminal can compete.
If a personal computer is used as a terminal, move the file (file transfer software) allows users to
retrieve files (downloaded) from the hose or hose to deliver the data (upload). Download files
means open and retrieve data from a personal computer to another and deliver the data to the
computer pertaining requested by the user.

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