Data and Computer
Communications
Chapter 15 – Local Area Network
Overview
Local Area Networks (LANs)

usually owned by the organization that is using
the network to interconnect equipment
 key elements:




topology
transmission medium
wiring layout
medium access control
LAN Topologies
 In
the context of a communication
network, the term topology refers to the
way in which the end points, or stations,
attached to the network are
interconnected.
 Historically common topologies for LANs
are bus, tree, ring, and star .
LAN Topologies
Bus and Tree
Bus:
• stations attach
through tap to bus
• full duplex allows
transmission and
reception
• transmission
propagates
throughout medium
• heard by all stations
• terminator at each
end
Tree:
• a generalization of
bus
• branching cable with
no closed loops
• tree layout begins at
headend and
branches out
• heard by all stations
Bus Topology

All stations attach, through appropriate hardware
interfacing known as a tap, directly to a linear
transmission medium, or bus.
 Full-duplex operation between the station and the tap
allows data to be transmitted onto the bus and
received from the bus.
 A transmission from any station propagates the length
of the medium in both directions and can be received
by all other stations.
 At each end of the bus is a terminator, which absorbs
any signal, removing it from the bus.
Tree topology





The tree topology is a generalization of the bus
topology.
The transmission medium is a branching cable with
no closed loops.
The tree layout begins at a point known as the
headend.
One or more cables start at the headend, and each of
these may have branches.
The branches in turn may have additional branches to
allow quite complex layouts. Again, a transmission
from any station propagates throughout the medium
and can be received by all other stations.
Frame
Transmission
on Bus LAN
Ring Topology

a closed loop of repeaters joined by point-topoint links
 receive data on one link & retransmit on another



data transmitted in frames




links unidirectional
stations attach to repeaters
circulate past all stations
destination recognizes address and copies frame
frame circulates back to source where it is removed
medium access control determines when a
station can insert frame
Frame
Transmission
Ring LAN
Star Topology
 each
station connects to common central
node

usually via two point-to-point link
• one for transmission and one for reception
central node
•
•
•
•
operate in broadcast fashion
physical star, logical bus
only one station can transmit at a time (hub)
can act as frame switch
Bus LAN
Transmission Media
twisted pair
• early LANs used voice grade cable
• scaling up for higher data rates not practical
baseband coaxial cable
• uses digital signaling
• original Ethernet
cont…
Bus LAN
Transmission Media
broadband coaxial cable
• used in cable TV systems
• analog signals at radio and TV frequencies
• expensive, hard to install and maintain
optical fiber
• expensive taps
• better alternatives available
only
baseband coaxial cable has
achieved widespread use
Ring and Star Topologies
Ring
• very high speed links over long
distances
• potential of providing best
throughput
• single link or repeater failure
disables network
Star
• uses natural layout of wiring in
building
• best for short distances
• high data rates for small
number of devices
Choice of Medium
 constrained
by LAN topology
 capacity

to support the expected network traffic
 reliability

to meet requirements for availability
 types

of data supported
tailored to the application
 environmental

scope
provide service over the range of environments
LAN PROTOCOL ARCHITECTURE

The architecture of a LAN is best described in terms of
a layering of protocols that organize the basic
functions of a LAN.

This section opens with a description of the
standardized protocol architecture for LANs, which
encompasses physical, medium access control
(MAC), and logical link control (LLC) layers.

The physical layer encompasses topology and
transmission medium,
LAN Protocol Architecture
IEEE 802 Layers
 Physical




Layer
Encoding / decoding of signals
preamble generation / removal
bit transmission / reception
transmission medium and topology
IEEE 802 Layers

Logical Link Control
Layer (LLC)


provide interface to
higher levels
perform flow and error
control

Media Access
Control




on transmit assemble
data into frame
on reception
disassemble frame,
perform address
recognition and error
detection
govern access to
transmission medium
for same LLC, may
have several MAC
options
LAN Protocols in Context
Logical Link Control
 transmission
of link level PDU protocol data
unit (PDU). s between stations
 must support multi-access, shared
medium
 relieved of some details of link access by
the MAC layer
 addressing involves specifying source and
destination LLC users

referred to as service access points (SAPs)
LLC Protocol
 modeled
after HDLC (high-level data link
control)
 asynchronous balanced mode

connection mode (type 2) LLC service
 unacknowledged

using unnumbered information PDUs (type 1)
 acknowledged

connectionless service
connectionless service
using 2 new unnumbered PDUs (type 3)
 permits
multiplexing using LSAPs
MAC Frame Format
Bridges

connects similar LANs with identical physical
and link layer protocols
 minimal processing
 can map between MAC formats
 reasons for use:




reliability
performance
security
geography
Bridge Function
Bridge Protocol Architecture

IEEE 802.1D defines architecture
 MAC level designates endpoint
 bridge does not need LLC layer
captures
frame
removes
encapsulation
encapsulates
it
forwards
it across
link
transmits to
destination
Connection of Two LANs
Bridges and
LANs with
Alternative
Routes
Fixed Routing

simplest and most common
 suitable for Internets that are stable
 a fixed route is selected for each pair of LANs
• usually least hop route

only changed when topology changes
 widely used but limited flexibility
Spanning Tree
 bridge
automatically develops routing table
 automatically updates routing table in
response to changing topology
algorithm consists of
three mechanisms:
frame forwarding
address learning
loop resolution
Address Learning






can preload forwarding database
when frame arrives at port X, it has come from
the LAN attached to port X
use source address to update forwarding
database for port X to include that address
have a timer on each entry in database
if timer expires, entry is removed
each time frame arrives, source address
checked against forwarding database


if present timer is reset and direction recorded
if not present entry is created and timer set
Spanning Tree Algorithm

address learning works for tree layout if there
are no alternate routes in the network

alternate route means there is a closed loop

for any connected graph there is a spanning tree
maintaining connectivity with no closed loops
 algorithm must be dynamic
IEEE 802.1 Spanning Tree Algorithm:
•
•
•
•
each bridge assigned unique identifier
cost assigned to each bridge port
exchange information between bridges to find spanning tree
automatically updated whenever topology changes
Interconnecting LANs - Hubs








active central element of star layout
each station connected to hub by two UTP lines
hub acts as a repeater
limited to about 100m by UTP properties
optical fiber may be used out to 500m
physically star, logically bus
transmission from a station seen by all others
if two stations transmit at the same time have a
collision
Two Level Hub Topology
Buses, Hubs and Switches
bus configuration
all stations share capacity of bus (e.g. 10Mbps)
only one station transmitting at a time
hub uses star wiring to attach stations
transmission from any station
received by hub and retransmitted
on all outgoing lines

only one station can transmit at a
time
total capacity of LAN is 10 Mbps
can improve performance using a layer 2 switch


can switch multiple frames between separate ports
multiplying capacity of LAN
Shared
Medium
Bus and
Hub
Layer 2 Switch Benefits

no change to attached devices to convert bus
LAN or hub LAN to switched LAN


have dedicated capacity equal to original LAN


e.g. Ethernet LANs use Ethernet MAC protocol
assuming switch has sufficient capacity to keep up
with all devices
scales easily

additional devices attached to switch by increasing
capacity of layer 2
Types of Layer 2 Switches

store-and-forward
switch



accepts frame on input
line, buffers briefly,
routes to destination
port
see delay between
sender and receiver
boosts overall integrity

cut-through switch




use destination
address at beginning
of frame
switch begins
repeating frame onto
output line as soon as
destination address is
recognized
highest possible
throughput
risk of propagating bad
frames
Two types of layer 2 switches are available as
commercial products:

Store-and-forward switch:
The layer 2 switch accepts a
frame on an input line,
buffers it briefly, and then
routes it to the appropriate
output line.

Cut-through switch: The
layer 2 switch takes
advantage of the fact that
the destination address
appears at the beginning of
the MAC (medium access
control) frame. The layer 2
switch begins repeating the
incoming frame onto the
appropriate output line as
soon as the layer 2 switch
recognizes the destination
address.
Layer 2 Switch vs. Bridge

differences between
switches & bridges:
Bridge
frame handling
done in software
Switch
performs frame
forwarding in
hardware
analyzes and
forwards one
frame at a time
can handle
multiple frames
at a time
uses store-andforward operation
can have cutthrough operation

layer 2 switch can be
viewed as full-duplex
hub
 incorporates logic to
function as multiport
bridge
 new installations
typically include layer
2 switches with bridge
functionality rather
than bridges
Virtual LANs (VLANs)
 subgroup
within a LAN
 created by software
 combines user stations and network
devices into a single broadcast domain
 functions at the MAC layer
 router required to link VLANs
 physically dispersed but maintains group
identity
A VLAN
Configuration
Defining VLANs
 broadcast
domain consisting of a group of
end stations not limited by physical
location and communicate as if they were
on a common LAN
 membership by:



port group
MAC address
protocol information
Communicating VLAN
Membership
Switches need to know VLAN membership
 configure
information manually
 network management signaling protocol
 frame tagging (IEEE802.1Q)
Summary
 LAN

bus, tree, ring, star
 LAN

topologies and transmission media
protocol architecture
IEEE 802, LLC, MAC
 bridges,
hubs, layer 2 switches
 virtual LANs