lan

1. Chapter 8 LAN Technologies
and Network Topology
 Direct Point-to-Point Communication
 Local Area Networks (LANs)
 LAN Topologies
 Ethernet
 Token Ring
 FDDI
 LocalTalk
 Wireless LAN

2. Direct Point-to-Point
Communication
 used in early computer communications systems
 Advantages
- Each point-to-point connection can use different network
hardware, frame format, and bandwidth (fig 8.1)
- Each point-to-point connection has exclusive access
- Easy to enforce security and privacy
 Disadvantages
- fully meshed network becomes exponentially expensive
as number of nodes increases (fig 8.2)
- number of direct connections required = (N2 - N) / 2

3. Local Area Networks (LANs)
 based on sharing the same communications medium.
 emerged in 1980’s in local communications as
alternative to expensive, dedicated point-to-point
connections
 shared networks confined to local communication
because
– more time would be spent coordinating the of sharing of
network and less time spent actually transmitting data if two
nodes are separated farther apart
– Cost of providing high bandwidth is more expensive over long
distance than over short distance
– Locality of reference: computers usually communicate to
another nearby computer.

4. LAN Topologies
 Star network
– all computers attached to a central point such as a hub (fig 8.3)
– Fault tolerant advantage
 Ring network
– computers are logically arranged in circular loop (fig 8.4)
– Physical orientation may be different ( offices on various floors
using token ring).
– One bad cable may disable the entire ring.
– Token ring may be logical ring but physical star.
 Bus network
– usually consist of a single shared long cable to which
computers attach (fig 8.5)
– One bad cable may cripple the entire bus.
– Ethernet network can be logical bus but physical star.

5. Ethernet Network
 Manchester Encoding (fig 8.6)
 Originally a bus network (ANIM06_1.MOV)
in which multiple computers share a single
transmission medium.
 While one computer transmits a frame, all
other computers must wait (fig 8.7)
 Uses CSMA (carrier sense multiple access
with collision detect) (ANIM06_2.MOV)
 Uses binary exponential backup in
recovering from collision. Delay time is
doubled after each successive collision.

6. Wireless LAN
 Uses antennas to broadcast RF signals through air
 Carrier Sense Multiple Access with Collision
Avoidance CSMA/CA (fig 8.8)
 IEEE 802.11b
– Up to 11 Mbps
– frequency hopping spread spectrum (FHSP)
– direct sequence spread spectrum (DSSS)
– radio frequencies in range of 2.4 GHz - 2.483 GHz
 IEEE 802.11a
– Up to 50 Mbps
– radio frequencies in range of 5 GHz

7. LocalTalk Network
 bus network designed by Apple.
 Uses modified CSMA/CA (a short message
is sent to reserve the medium before
transmitting a large message)
 Limited to 230 kbps
 Easy to install: daisy-chain computers (fig
10.9)

8. Token Ring Network
 token passing ring network (fig 8.9)
 IBM’s token ring operate at 16Mbps
 token is a special bit pattern or message different
from normal data frames (bit stuffing of data may
be used)
 only one token circulates the ring at any time. To
send data, computer waits for token to arrive,
transmit one frame, and then pass token to next
node (ANIM06_3.MOV)
 guarantees fair access because nodes take turns.

9. FDDI Network
 Fiber Distributed Data Interconnect (FDDI)
 token ring technology operating at 100
Mbps using fiber.
 Uses counter rotating ring for fault
tolerance (fig 8.10)
 Self healing
– Ring is still up if there is one break in the ring