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Chapter 5
Data Link Layer
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Computer Networking:
A Top Down Approach
Featuring the Internet,
2nd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2002.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2002
J.F Kurose and K.W. Ross, All Rights Reserved
5: DataLink Layer
5a-1
Chapter 5 outline
 5.1 Introduction and
 5.6 Hubs, bridges, and





services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 LAN addresses and
ARP
5.5 Ethernet



switches
5.7 Wireless links and
LANs
5.8 PPP
5.9 ATM
5.10 Frame Relay
5: DataLink Layer
5a-1
5a-2
Hubs
Physical Layer device
 Simplest way to interconnect LANs
 Operates on bits rather than frames
 When a bit comes into a hub interface, the
hub broadcasts the bit on all the other
interfaces.
5: DataLink Layer
5a-2
5a-3
Interconnecting with hubs
 Multi-tier hub design (hierarchy)
 Backbone hub interconnects three academic depts or “LAN
segments”
 Depts have a 10BaseT Ethernet that provides network access
 Hosts have point-to-point connections to departmental hub
5: DataLink Layer
5a-3
5a-4
Hubs: Benefits
 Provides interdepartmental communication
 Extends the max distance betw any pair of nodes
 Multi-tier design provides a degree of graceful
degradation
 If one dept malfunctions, the backbone hub can
detect the prob and disconnect the dept hub
5: DataLink Layer
5a-4
5a-5
Hubs: Limitations
 When departmental LANs are interconnected by a
hub, then the independent collision domains
become one collision domain.
 If depts use different Ethernet technologies, may
not be able to interconnect them
 Each Ethernet technology has restrictions on




max # of nodes in collision domain
max distance between two hosts
max # of tiers
Constrains # of hosts as well as geographical reach of
the multi-tier LAN.
5: DataLink Layer
5a-5
5a-6
Bridges
Link layer device
 Operate on Ethernet frames unlike hubs
 are full-fledged packet switches that forward and
filter Ethernet frames using the LAN destination
addresses
 examines frame header and selectively forwards
frame based on MAC dest address
 when frame is to be forwarded on segment, uses
CSMA/CD to access segment
5: DataLink Layer
5a-6
5a-7
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
collision
domain
bridge
LAN segment
= hub
= host
LAN segment
LAN (IP network)
5: DataLink Layer
5a-7
5a-8
Forwarding
How do determine to which LAN segment to forward
frame?
• Looks like a routing problem...
5: DataLink Layer
5a-8
5a-9
Self learning
 A bridge has a bridge table
 entry in bridge table:
(Node LAN Address, Bridge Interface, Time Stamp)
 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

5: DataLink Layer 5a-10
5a-9
Filtering/Forwarding
When bridge receives a frame:
index bridge table using MAC dest address
if entry found for destination
then{
if dest on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flood
forward to all but the interface
on which the frame arrived
5: DataLink Layer 5a-10
5a-11
Bridge example
Suppose C sends frame to D and D replies back with
frame to C.
 Bridge receives frame from from C
 notes in bridge table that C is on interface 1
 because D is not in table, bridge sends frame into interfaces
2 and 3
 frame received by D
5: DataLink Layer 5a-12
5a-11
Bridge Learning: example
C
D
 D generates frame for C, sends
 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
5: DataLink Layer 5a-13
5a-12
Bridges Spanning Tree
 for increased reliability, desirable to have
redundant, alternative paths from source to dest
 with multiple paths, cycles result - bridges may
multiply and forward frame forever
 solution: organize bridges in a spanning tree by
disabling subset of interfaces
Disabled
5: DataLink Layer 5a-14
5a-15
Some bridge features
 Isolates collision domains resulting in higher total
max throughput
 Can have limitless number of nodes and
geographical coverage
 Can be used to combine Ethernet segments using
different Ethernet technologies with Ethernet
bridges
 10Base2, 100BaseT, 10Base2 = can use a Gigabit bridge
 Transparent (“plug-and-play”): no configuration
necessary
5: DataLink Layer 5a-15
5a-16
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
5: DataLink Layer 5a-16
Backbone configuration
Recommended !
Backbone:
a network that has direct connections
to all the LAN segments
5: DataLink Layer 5a-17
Bridges vs. Routers
both store-and-forward devices
routers:
network layer devices (IP Address)
bridges are link layer devices (LAN Address)
routers maintain routing tables, implement
routing algorithms
 bridges maintain bridge tables, implement filtering,
learning and spanning tree algorithms
5: DataLink Layer 5a-18
Routers vs. Bridges
Bridges Pros
+ Bridge operation is simpler resulting in high
packet filtering and forwarding rates.
+ Bridge tables are self learning
+ “plug-and-play”
Bridges Cons
- All traffic confined to spanning tree, even
when more direct (but disconnected) path.
- Bridges do not offer protection from broadcast
storms
5: DataLink Layer 5a-19
Routers vs. Bridges
Routers Pros
+ arbitrary topologies can be supported, cycling is
limited by TTL counters (datagram)
+ provide protection against broadcast storms
Routers Cons
- 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)
- Pronunciation
5: DataLink Layer 5a-20
Ethernet Switches
Essentially a multi-interface bridge
layer 2 (frame) forwarding,
filtering using LAN addresses
Switching: A-to-A’ then
B-to-B’ simultaneously
No collisions (full duplex mode)
 often: individual hosts,
star-connected into switch
 Dedicated Access
5: DataLink Layer 5a-21
Ethernet Switches
frame forwarded from
input to output port without awaiting for
assembly of entire frame
cut-through switching:
slight
reduction in latency
ex. Caravan
 combinations of shared/dedicated,
10/100/1000 Mbps interfaces
5: DataLink Layer 5a-22
Not an atypical LAN (IP network)
Dedicated
Shared
5: DataLink Layer 5a-23
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
5: DataLink Layer 5a-24
Chapter 5 outline
 5.1 Introduction and




services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 LAN addresses and
ARP
5.5 Ethernet
 5.6 Hubs, bridges, and




switches
5.7 Wireless links and
LANs
5.8 PPP
5.9 ATM
5.10 Frame Relay
5: DataLink Layer 5a-25
Wireless Links
 The wave of the future for
networking: wireless links
 Examples of end systems:


Portable PCs, PDAs, airport
hubs, wireless telephony (such
as the cellphone pictured)
Future appliances may include
cameras, automobiles, pets,
security systems, kitchen
appliances, and plants.
 IEEE 802.11b – most popular
standard wireless LANs
 Bluetooth – new standard that
allows devices to communicate
with each other
 Three classifications:



Power, range, data rate
Bluetooth – low, short, low
802.11 – high, medium, high
5: DataLink Layer 5a-26
IEEE 802.11 Wireless LAN
 802.11b
 Currently most popular
form of wireless LAN:
wireless Ethernet, Wi-Fi
 2.4-5 GHz unlicensed
radio spectrum
 up to 11 Mbps
 physical layer and Media
Access Control (MAC)
layer for wireless local
area network


direct sequence spread
spectrum (DSSS) in
physical layer
• all hosts use same
chipping code
• Not a multi access
protocol (does not
coordinate channel
access from multiple
hosts
widely deployed, using
base stations
5: DataLink Layer 5a-27
IEEE 802.11 Wireless LAN
 Other wireless standards
802.11a – operates on 5-6GHz range and uses OFDM
(orthogonal frequency-division multiplexing, not DSSS
 Speeds can get up to 54Mbps
 802.11g – operates at 2.4GHz
 Speeds up to 54Mbps
 All use CSMA/CA for multi-access and have base stations
and ad-hoc network versions

5: DataLink Layer 5a-28
Base station approch
 Wireless host communicates with a base station
 base station = access point (AP)
 Basic Service Set (BSS) (a.k.a. “cell”) contains:
wireless hosts
 access point (AP): base station
 BSS’s combined to form distribution system (DS)

5: DataLink Layer 5a-29
Ad Hoc Network approach
 No AP (i.e., base station)
 wireless hosts communicate with each other
to get packet from wireless host A to B may need
to route through wireless hosts X,Y,Z
 Applications:
 “laptop” meeting in conference room, car
 interconnection of “personal” devices
 battlefield
 IETF MANET
(Mobile Ad hoc Networks)
working group

5: DataLink Layer 5a-30
IEEE 802.11: multiple access
 Collision if 2 or more nodes transmit at same time
 CSMA makes sense:
 get all the bandwidth if you’re the only one transmitting
 shouldn’t cause a collision if you sense another transmission
 Collision detection doesn’t work: hidden terminal
problem
5: DataLink Layer 5a-31
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 CSMA: sender
- if sense channel idle for DISF
sec.
then transmit entire frame (no
collision detection)
-if sense channel busy
then binary backoff
802.11 CSMA receiver
- if received OK
return ACK after SIFS
(ACK is needed due to hidden
terminal problem)
5: DataLink Layer 5a-32
Collision avoidance mechanisms
 Problem:
 two nodes, hidden from each other, transmit complete
frames to base station
 wasted bandwidth for long duration !
 Solution:
small reservation packets
 nodes track reservation interval with internal
“network allocation vector” (NAV)

5: DataLink Layer 5a-33
Collision Avoidance: RTS-CTS
exchange
 sender transmits short
RTS (request to send)
packet: indicates duration
of transmission
 receiver replies with short
CTS (clear to send) packet

notifying (possibly hidden)
nodes
 hidden nodes will not
transmit for specified
duration: NAV
5: DataLink Layer 5a-34
Collision Avoidance: RTS-CTS
exchange
 RTS and CTS short:
collisions less likely, of
shorter duration
 end result similar to
collision detection
 IEEE 802.11 allows:
 CSMA
 CSMA/CA: reservations
 polling from AP

5: DataLink Layer 5a-35
Cellular vs Wireless LAN
 3G Cellular mobile
 2Mbps indoor
 384kbps outdoor
 Licensed radio freq (1885
– 2025 and 2110 – 2200
MHz)
 Cons
 3G is more costly ($2000
for radio freq licenses)
 Competition from
wireless LAN tech.
 IEEE 802.11b wireless
LAN enjoys more
widespread usage




802.11 LAN capable
cards will be installed in
most all prepackaged
computers
Continue with other
devices
Bulk of traffic local
termination
Handles GSM/GPRS
5: DataLink Layer 5a-36
A word about Bluetooth
 Low-power, small radius,
wireless networking
technology

10-100 meters
 omnidirectional
 not line-of-sight infared
 Interconnects gadgets
 2.4-2.5 GHz unlicensed
radio band
 up to 721 kbps
 Interference from wireless
LANs, digital cordless
phones, microwave
ovens:

frequency hopping helps
 MAC protocol supports:
 error correction
 ARQ
 Each node has a 12-bit
address
5: DataLink Layer 5a-37