Local Area Network
Lesson 7
NETS2150/2850
Lesson Outline
Common LAN topologies
Logical Link Control sublayer
Medium Access Control sublayer
ARP protocol for IP  MAC map
LAN interconnection devices
Topologies
LAN topology refers to the ways end
systems are interconnected
Common topologies:
Tree
Bus
Special case of tree
Ring
Star
LAN Topologies
Bus and Tree
Transmission propagates throughout medium
Heard by all stations
Need to identify target station
Each station has unique address
Full duplex connection between station and tap
Allows for simultaneous transmission and reception
Need to regulate transmission
To avoid collisions
To avoid hogging
Data in small frames (fragmentation!)
Terminator absorbs frames at end of medium
Prevent from being reflected into the channel
Frame
Transmission
on Bus LAN
Ring Topology
Repeaters joined by point to point links in
closed loop
Receive data on one link and retransmit on
another
Links unidirectional
Stations attach to repeaters
Data in frames
Circulate past all stations
Destination recognizes address and copies frame
Frame circulates back to source where it is
removed
MAC protocol determines when station can
insert frame
Frame
Transmission
Ring LAN
Star Topology
Each station connected directly to
central node
Usually via two point to point links
Central node can broadcast
Only one station can transmit at a time
Or central node can act as frame switch
More stations can transmit at a time
IEEE 802 v OSI RM
802 Layers - Physical
Encoding/decoding
Preamble generation/removal
7 bytes with pattern 10101010 followed by
one byte with pattern 10101011
used to synchronise receiver, sender clock
rates
Bit transmission/reception
Transmission medium and topology
802 Layers Logical Link Control
Based on HDLC
Provides interface to higher levels
Transmission of LLC PDU between two
stations
Flow and error control
Must support multiaccess, shared LAN media
Link access handled by MAC layer
LLC Services
Unacknowledged connectionless service
No handshake and no ack (unreliable)
Connection mode service
Use handshake and ack
Acknowledged connectionless service
No handshake but uses ack
Media Access Control
Assembly of data into frame with
address and error detection fields
Disassembly of frame
Address recognition
Error detection
Govern access to transmission
medium
MAC Frame Format
MAC layer receives data from LLC layer and
adds:
MAC control
Destination MAC address (6-octet or 48-bit)
Source MAC address
CRC
MAC layer detects errors and discards frames
MAC broadcast address: FF FF FF FF FF FF16
LLC optionally retransmits unsuccessful
frames
IEEE 802.3 MAC Frame Format
Octets:
8
6
6
2
46-1500
4
Length
Addresses: 6 octets
if adapter receives frame with matching destination address,
or with broadcast address, it passes data in frame to netlayer protocol
otherwise, adapter discards frame
Length: length of data field in octets, max frame size
is 1518 octets (excluding preamble & SFD)
CRC: checked at receiver, if error is detected, the
frame is simply dropped (32-bit CRC)
MAC protocols
Assume single shared broadcast channel
Two or more simultaneous transmissions by
nodes will cause interference
only one node can send successfully at a time
MAC protocol:
distributed algorithm that determines how
nodes share channel, i.e., determine when
node can transmit
MAC Protocols: A taxonomy
Three broad classes:
Channel Partitioning or Reservation
divide channel into smaller “pieces” (time slots,
frequency, code)
allocate a piece to node for exclusive use
Random Access or Contention
channel not divided, thus can’t avoid collisions
Need to “recover” from collisions
“Taking turns” or Round Robin
tightly coordinate shared access to avoid collisions
Address Resolution Protocol (ARP)
Even if you have the IP address of your
destination, you need its MAC to get
your data across a physical network
So, we need a way to do this mapping
ARP performs dynamic mapping
between IP and MAC
Any resolved mapping is stored in a
host’s ARP cache
ARP operation
McGraw-Hill
©The McGraw-Hill Companies, Inc., 2004
Note:
An ARP request is broadcast; an ARP
reply is unicast.
An ARP reply is only generated by the
destined node.
ARP Packet Format
McGraw-Hill
©The McGraw-Hill Companies, Inc., 2004
Encapsulation of ARP Packet
Length
McGraw-Hill
©The McGraw-Hill Companies, Inc., 2004
Interconnecting LAN segments
Hubs
Bridges
Switches
Hubs
Hub acts as a repeater (physical layer device)
When single station transmits, hub repeats signal on
outgoing line to each station
Limited to about 100 m
Optical fibre may be used
Max about 500 m
Physically star, logically bus
Transmission from any station received by all other stations
Forms a single collision domain
Two stations transmit at the same time  collision!!
Interconnecting with hubs
Backbone hub interconnects LAN segments
Extends max distance between stations
But individual segments’ collision domain become
one large collision domain
when a node in CS and a node in EE transmit at same
time  collision!!
Can’t interconnect 10BaseT & 100BaseT
Bridges
Link layer device (layer-2 device)
stores and forwards Ethernet frames
examines frame header and selectively forwards
frame based on MAC dest address
transparent
stations are unaware of presence of bridges
plug-and-play, self-learning
bridges do not need to be configured
Bridges: traffic isolation
Bridge installation breaks LAN into LAN segments
bridges filter packets:
same-LAN-segment frames not usually forwarded
onto other LAN segments
segments become separate collision domains
collision
domain
bridge
LAN segment
collision
domain
LAN segment
LAN
= hub
= station
Forwarding
How to determine to which LAN segment to
forward frame?
• Looks like a routing problem...
Self learning
A bridge has a bridge table
entry in bridge table:
(Station MAC Address, Bridge Interface, Timestamp)
stale entries in table dropped (TTL can be ~ 60 min)
bridges learn which hosts can be reached through which
interfaces
when frame received, bridge “learns” location of
sender: incoming LAN segment
records sender/location pair in bridge table
Bridge example
Suppose C sends frame to D and D replies back
with frame to C.
Bridge receives frame from from C
updates bridge table, C is on interface/port 1
because D is not in table, bridge sends frame into
interfaces 2 and 3
frame received by D
Bridge Learning: example
C
1
D generates frame for C, and sends it
bridge receives frame
notes in bridge table that D is on interface 2
bridge knows C is on interface 1, so selectively
forwards frame to interface 1
Interconnection without
backbone
Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over CS
segment
Backbone configuration
Recommended !
Note: A bridge does not change the
physical (MAC) addresses in a frame.
Loop of Bridges
Spanning Tree Algorithm
Address learning works for tree layout
i.e. no closed loops (or cycles)
But not for cyclic connected graph!
Spanning Tree Algo. builds a network
including all the nodes with selected
links (i.e. edges) without closed loops
Known as a spanning tree!
Spanning Tree
for increased reliability, desirable to
have redundant, alternative paths from
source to dest but need to avoid cycles
solution: organize bridges in a spanning
tree by disabling subset of interfaces
Disabled
Some bridge features
Isolates collision domains resulting in higher
total max throughput (i.e. amount of data
transmitted within an interval)
Transparent (“plug-and-play”): no
configuration necessary
Routers vs. Bridges (1)
both store-and-forward devices
routers: network layer devices (examine network
layer headers)
bridges are link layer devices
routers maintain routing tables, implement
routing algorithms
bridges maintain bridge tables, implement
filtering, learning and spanning tree algorithms
Routers vs. Bridges (2)
Bridges pros (+) and cons (-)
+ Bridge operation is simpler requiring less data
unit processing
+ Bridge tables are self learning
- All traffic confined to spanning tree, even
when alternative bandwidth is available
- Bridges do not offer protection from
broadcast storms (i.e. forwarding of
broadcast traffic)
Routers vs. Bridges (3)
Routers + and + arbitrary topologies can be supported, cycling is
limited by TTL counters (and good routing
protocols)
+ provide protection against broadcast storms
- require IP address configuration (not plug and
play)
- require higher packet processing
bridges do well in small (few hundred hosts)
while routers used in large networks (thousands
of hosts)
Ethernet Switches
Essentially a multi-interface bridge
layer 2 (frame) forwarding, filtering using LAN
addresses
Incoming frame from particular station
switched to appropriate output line
Unused lines can switch other traffic
More than one station can transmit at a time
Multiplying capacity of LAN
Shared Hub and Switch

Types of Ethernet Switches
Store-and-forward switch
Accepts frame on input line
Buffers it briefly, then forwards it to appropriate
output line
Error checking, boosts integrity of network
Cut-through switch
Takes advantage of dest address appearing at
beginning of frame
Switch begins repeating frame onto output line as
Netgear
GS108UK
GB
Switch
soon as it recognizes dest address
Latencypossible
~ 10 µs
for 64-byte frames
Highest
throughput
Throughput
32 Mfps
Risk
of propagating
bad frames
MAC
database
(8000
entries)
Switch
unable to check
CRC
prior to retransmission
Ethernet Switch Benefits
No change to attached stations to convert
bus LAN or hub LAN to switched LAN
For Ethernet LAN, each station uses Ethernet
MAC protocol
Each station has dedicated capacity equal to
original LAN
Assuming switch has sufficient capacity to keep up
with all devices
Switch scales easily
Con: still has broadcast storm problem!
Subnetwork with layer-3 device!
Solution: break up network into
subnetworks connected by routers or
layer-3 switch (faster!)
Packet forwarding done in the hardware
MAC broadcast frame limited to stations
and switches contained within a single
subnetwork
Typical Large LAN Organization
Thousands to tens of thousands of stations
Desktop systems links 10 Mbps to 100 Mbps
Into layer 2 switch
Wireless LAN connectivity available for mobile
users
Layer 3 switches at local network's core
Form local backbone
Interconnected at 1 Gbps
Connect to layer 2 switches at 100 Mbps to 1
Gbps
Servers connect directly to layer 2 or layer 3
switches at 1 Gbps
Typical
Large
LAN
Organization
Diagram
Summary comparison
hubs
bridges
routers
switches
traffic
isolation
no
yes
yes
yes
plug & play
yes
yes
no
yes
optimal
routing
cut
through
no
no
yes
no
yes
no
no
yes
Summary
LAN topologies
IEEE 802 reference model
Types of MAC protocols
Interconnection Devices
Hubs, bridges, switches, routers
Read Stallings chapter 15
Next: Specific MAC protocols