Transcript LANs - UQAC
Data and Computer
Communications
Chapter 15 – Local Area Network
Overview
Ninth Edition
by William Stallings
Data and Computer Communications, Ninth
Edition by William Stallings, (c) Pearson
Education - Prentice Hall, 2011
Local Area Network Overview
The whole of this operation is described in
minute detail in the official British Naval
History, and should be studied with its excellent
charts by those who are interested in its
technical aspect. So complicated is the full story
that the lay reader cannot see the wood for the
trees. I have endeavored to render intelligible
the broad effects.
—The World Crisis,
Winston Churchill
Local Area Network Overview
1- LAN topologies
2- Choice of medium
3- IEEE 802 standard
4- Medium access control (MAC) frame (next ch.)
5- Bridges
6- Spanning tree protocol (STP)
7- Hubs and switches
8-Virtual LAN (VLAN) (next ch.)
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
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 (link)
Tree:
• a generalization of
bus
• branching cable with
no closed loops
• tree layout begins at
headend and
branches out
• heard by all 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
A special frame (“token”)
is used to take control of
the ring
Star Topology
each
station connects to common central
node
usually via two point-to-point links
• one for transmission and one for reception
central node
•
•
•
•
can operate in broadcast fashion
only one station can transmit at a time (hub)
physical star, logical bus
can act as frame switch (no broadcast…)
Choice of Topology
reliability
medium
wiring layout
access control
factors:
performance
expandability
Bus LAN
Transmission Media
cont…
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 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
Media Available
The IEEE 802 Reference
model
LLC is used only
when more than
one network layer
is used.
LAN Protocols in Context
(Wireshark…)
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 (can multiplex
several network layers such as IP,
Appletalk,…)
perform flow and error
control
usually not used…
IEEE 802 Layers
Logical Link Control
Layer (LLC)
provide interface to
higher levels (can multiplex
several network layers such as IP,
Appletalk,…)
Media Access
Control (MAC)
perform flow and error
control
usually not used…
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
(Logical Link Control )
transmission
of link level PDUs 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 Services)
unacknowledged connectionless service
• data-gram style service
• delivery of data is not guaranteed
connection-mode service
• logical connection is set up between two users
• flow and error control are provided
acknowledged connectionless service
• datagrams are to be acknowledged, but no logical
connection is set up
(LLC Service Alternatives)
unacknowledged connectionless service
• requires minimum logic
• avoids duplication of mechanisms
• preferred option in most cases
connection-mode service
• used in simple devices
• provides flow control and reliability mechanisms
acknowledged connectionless service
• large communication channel needed
• time critical or emergency control signals
(LLC Protocol)
modeled
after HDLC
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
(Medium Access Control
Protocol)
controls
access to the transmission medium
key parameters:
where
• Centralized: greater control, but single point of failure…
• Distributed: more complex, but more redundant
how
• synchronous protocol (be careful here, we are not talking about the clock…)
capacity dedicated to connection, not optimal
• asynchronous protocol
response to demand
round robin, reservation, contention (conflict…)
(Asynchronous Systems)
round robin
reservation
contention
• each station given
turn to transmit
data
• divide medium
into slots
• good for stream
traffic
• all stations
contend for time
• good for bursty
traffic
• simple to
implement
• tends to collapse
under heavy load
MAC Frame Handling
MAC layer receives data from LLC layer
PDU (protocol data unit) is referred to as a MAC frame
fields:
MAC control
destination MAC address
Ex. Ethernet address EUI-48 (formerly called MAC-48)
source MAC address
LLC
Data
CRC
MAC layer detects errors and discards frames
LLC optionally retransmits unsuccessful frames
Medium Access Control
(MAC) Frame Format
In Ethernet II frame, Ethertype can be considered a LLC identifier (link)
Bridges
connects similar LANs with identical physical
and link layer protocols
minimal processing
more sophisticated bridges can map between
MAC formats (Ex. Ethernet to ring)
reasons for use:
reliability
performance
security
geography
Bridge Function
Bridge example
http://www.cisco.com/en/US/prod/collateral/wireless/ps567
9/ps5279/ps5285/product_data_sheet09186a008018495c.ht
ml
dBm definition : Wikipedia
Bridge Design Aspects
no
modification to frame content or format
no encapsulation (if only on bridge is used)
exact bitwise copy of frame
buffering to meet peak demand
contains routing and address “intelligence”
may connect more than two LANs
bridging is transparent to stations
Bridge Protocol Architecture
IEEE 802.1D defines architecture
MAC level designates endpoint
bridge does not need LLC layer
If two bridges are used: need encapsulation to
transmit data from one bridge to the other
captures
frame
removes
encapsulation
encapsulates
it
forwards
it across
link
transmits to
destination
Connection of Two LANs
(close to each other =>one bridge)
MAC header
MAC trailer
Bridges and
LANs with
Alternative
Routes
(eg. LAN A to LAN E)
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
(STP: spanning tree protocol)
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
Frame Forwarding
Bridge maintains forwarding database for each port
For a frame arriving on port X:
search forwarding database to see if MAC address is
listed for any port except X
if address not found, forward to all ports except X
if address listed for port Y, check port Y for blocking or
forwarding state
if not blocked, transmit frame through port Y
Address Learning
Option 1: can preload forwarding database
Option 2: learn addresses
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
(new version RSTP: Rapid Spanning Tree Protocol)
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 (30-60 sec)
Loop of Bridges
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 a “star” topology, logically, it is a “bus”
transmission from a station seen by all others
if two stations transmit at the same time, we
have a collision
Two Level Hub Topology
Header hub
Intermediate
hub
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
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
A Partitioned
LAN
Configuration
Explain stripping of MAC
addresses when going
through router (eg. X to V
then V to Z)
- Router V separates the LANs.
- X must use Ethernet address of V
to send a packet to Z (the packet
must contain the IP address of Z).
=>
link
router
Ethernet
switch
Virtual LANs (VLANs)
http://www.cisco.com/c/en/us/td/docs/ios/12_2/switch/configuration/guide/fswtch_c/xcfvl.html
subgroups
within a LAN
created by software
combines user stations and network
devices into a single broadcast domain
operates at the MAC layer
Router
devices (switches)
required to link VLANs
physically
identity
dispersed but maintains group
A VLAN
Configuration
(the switches must
support VLAN and IP
routing)
Defining VLANs
The broadcast domain, consisting of a group of
end stations, is not limited by physical location.
Stations on the same VLAN can communicate
as if they were on a common LAN.
Membership
can be defined using:
port group (here, “port” refers to the switch port numbers)
MAC address
protocol information
(ex. IP address, higher protocols,…)
Communicating VLAN
Membership
Switches need to know VLAN membership
We
must configure information manually
network management signaling protocol
frame
tagging (very common)
IEEE 802.1Q : consists in adding a
header to frames on interswitch trunks
(next chapter)
Summary
LAN
bus, tree, ring, star
LAN
topologies and transmission media
protocol architecture
IEEE 802, LLC, MAC
bridges,
hubs, layer 2 switches
virtual LANs