The Data-Link Layer:
Ethernet, ARP and LANs
Based on slides from the Computer Networking:
A Top Down Approach Featuring the Internet by Kurose and Ross
The Data Link Layer
Our goals:

Understand principles behind data link layer services:




2
Sharing a broadcast channel: multiple access.
Link layer addressing.
Interconnection of different LAN segments.
Instantiation and implementation of various link layer
technologies
Link Layer: Introduction
Some terminology:


hosts and routers are nodes
communication channels that
connect adjacent nodes along
communication path are links




wired links
wireless links
LANs
layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
3
“link”
Link layer: context


Datagram transferred by
different link protocols over
different links:
 e.g., Ethernet on first link,
frame relay on intermediate
links, 802.11 on last link
Each link protocol provides
different services
 e.g., may or may not provide
rdt over link
4
transportation analogy

trip from Princeton to Lausanne
 limo: Princeton to JFK
 plane: JFK to Geneva
 train: Geneva to Lausanne

tourist = datagram
transport segment =
communication link
transportation mode = link
layer protocol
travel agent = routing
algorithm



Physical Media
physical link:
Transmitted data bit propagates across link.


guided media:
signals propagate in solid media (e.g. copper, fiber).

unguided media:
signals propagate freely (e.g. radio bands).
The physical Layer defines the “representation” of bits.
It’s also provides protection by both detecting and correcting
corrupted bits (See how it works).

5
Physical Media (Examples)
Twisted Pair (TP)
 Two insulated copper wires. May be shielded or not.


6
Category 3
Traditional phone wires. Supports 10-Mbps Ethernet.
Category 5
Supports 100Mbps Ethernet (“Fast-Ethernet”).
Physical Media (Examples)
Coaxial cable

Two concentric shielded wires.



7
Baseband
single channel on cable.
broadband:
multiple channel on cable.
common uses for 10-Mbps Ethernet and TV cables.
Physical Media (Examples)
Fiber
 Glass fiber carrying light pulses.
 Single mode or multi mode.
 High point-to-point speed.
 Very low error rate (caused by low fiber’s attenuation).
 Secured.
Common used for 100-Mbps Ethernet and 1000-Mbps
Ethernet (“Gigabit Ethernet”).

8
Physical Media (Examples)
Many more:


Radio bands (WiFi, high bit error rate).

Microwave (Requires hosts in light of sight).
Satellite (very slow RTT).

9
Link Layer Services

Recall that we're actually considering the MAC layer in the IEEE
802 model.

Framing (Frame structure)
encapsulate datagram into frame, adding header, trailer

Link Access (The protocol):
 Addressing:
Introduces “MAC” addresses used in frame headers to identify hosts
(actually NICs) who are part of the network.
Different for IP addresses!

10
Channel Access:
Defines the set of rules which allows the hosts to use the (possibly
shared) medium.
Link Layer Services

Flow Control:
 pacing between adjacent sending and receiving nodes

Error Detection:
 errors caused by signal attenuation, noise.
 receiver detects presence of errors:
 signals sender for retransmission or drops frame

Error Correction:
 receiver identifies and corrects bit error(s) without resorting to
retransmission

Half-duplex and full-duplex:
 with half duplex, nodes at both ends of link can transmit, but not at same
time

RDT:
 Offers some reliability between the hosts (Why is this redundant?).
11
Link Layer Services
datagram
sending
node
frame
frame
adapter

link layer implemented in
“adaptor” (aka NIC)



12
adapter

encapsulates datagram in a
frame
adds error checking bits, rdt,
flow control, etc.
receiving side

Ethernet card, PCMCI card,
802.11 card
sending side:

rcving
node
link layer protocol



looks for errors, rdt, flow
control, etc
extracts datagram, passes to
rcving node
adapter is semi-autonomous
link & physical layers
Link Types
Three types of “links”:

point-to-point (P2P)





13

PPP for dial-up access.
Point-to-point link between
Ethernet switch and host.
Switched Networks
ATM (used for WAN).
Broadcast



Traditional Ethernet and its
predecessors.
Upstream HFC.
802.11 wireless LAN.
We’ll focus on Broadcast
media.
Multiple Access protocols
single shared broadcast channel
 two or more simultaneous transmissions by nodes: interference
 collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
 distributed algorithm that determines how nodes share channel,
i.e., determine when node can transmit
 communication about channel sharing must use channel itself!
 no out-of-band channel for coordination

14
Ossi Mokryn - Data link layer
Human Analogy

Rules for ‘party conversation’:






15
"Give everyone a chance to speak."
"Don't speak until you are spoken to."
"Don't monopolize the conversation."
"Raise your hand if you have a question."
"Don't interrupt when someone is speaking."
"Don't fall asleep when someone else is talking
MAC Protocols measures

Assume a shared medium with a channel rate of R [bpsec].




16
Efficient
When one node wants to transmit it ca send at rate R.
Fair
When N users want to transmit, each can send at average rate R/N.
Decentralized
No special node uses to coordinate transmission (no “leader”).
No synchronization of clocks or slots.
Fault tolerant.
Simple
Should be very fast and implemented in NICs firmware.
MAC Protocol Types
Three broad classes:
 Channel Partitioning



Random Access



Channel not divided, allow collisions.
“Recover” from collisions.
“Taking turns”


17
Divide channel into smaller “pieces” (Time-Slots, Frequancy-Bands
or by code).
allocate piece to a node for exclusive its use.
Nodes take turns, but nodes with more to send can take longer
turns.
Might uses a leader to coordinate the turns.
Channel Partitioning MAC protocols: TDMA
TDMA (Time Division Multiple Access)
 Access the channel in "rounds“.
 Each station gets fixed length slot (length = packets trans time) in each
round. Each slot called a Time-Slot.
 Unused slots go idle.
 Example: 6-station LAN, 1,3,4 have packtes, slots 2,5,6 idle
18
Channel Partitioning MAC protocols: FDMA
frequency bands
FDMA (Frequency Division Multiple Access)
 Channel spectrum divided into frequency bands.
 Each station assigned fixed frequency band.
 Unused transmission time in frequency bands go idle.
 example: 6-station LAN, 1,3,4 have packets, frequency bands 2,5,6 idle
19
TDM / FDM summary



FDM: Enables the division of a channel with capacity C
bits per seconds to N sub-channels, each gets a different
frequency range and capacity of C/N.
TDM: The division of channel to N sub-channels, each
gets C/N capacity, by giving the entire channel to each of
the N stations for 1/N of the time.
 The division makes each sub channel less busy, but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem).
20
Channel Partitioning MAC protocols: CDMA
Code Division Multiple Access (CDMA)
 used in several wireless broadcast channels
(cellular, satellite, etc) standards
 unique “code” assigned to each user; i.e., code set
partitioning
 all users share same frequency, but each user has
own “chipping” sequence (i.e., code) to encode data
 encoded signal = (original data) X (chipping
sequence)
 decoding: inner-product of encoded signal and
chipping sequence
 allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes
21are “orthogonal”)

Random Access MAC Protocols


From here on, we’ll focus on Random Access MAC protocols.
When node has packet to send




Two or more transmitting nodes ➜ “collision”.
Random access MAC protocol specifies:



Transmit at full channel data rate R.
No a priori coordination among nodes
How to detect collisions
How to recover from collisions.
Examples of random access MAC protocols:




22
ALOHA.
slotted ALOHA.
CSMA/CD (Ethernet).
CSMA/CA (Wireless).
Aloha Protocol



Invented at the ’70s in Hawaii.
Intended for Radio networks, but suitable for every
network where the station can listen to the channel while
broadcasting, and determine whether others also transmit.
Basic idea: every station may transmit when it wants to. If
collision is detected between frames, back off and try
again later.

23
If two frames are broadcast at the same time on the channel, a
collision occurs and the both need to retransmit.
Aloha Protocol

All hosts:


Transmit on one frequency (fT).
Receive on other frequency (fR).

There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency.

The central node used as a repeater.
Collisions are detected by the hosts:
Receiving corrupted data (host knows what should be
received).

24
Aloha Protocol
1.
2.
Accept a new frame arrives
Transmit immediately and listen:
If a collision occurred, wait a random time, and
repeat to stage 2.
Otherwise, go back to stage 1 to handle a new frame.
25
Aloha Protocol




Simple.
Robust against failure of a host.
Distributed (excluding the central node, which uses as a
repeater).
High load implicates low utilization of the channel and high
delays.
26
Aloha - efficiency
Efficiency is the long-run
fraction of successful
transmissions when there are
many nodes, each with many
frames to send

Only 18%...

Suppose there are n stations, and the probability that a
station starts transmitting in a time unit is p.
Then: The probability that exactly one node transmits
in a time unit is

27
Aloha - efficiency
Efficiency is the long-run
fraction of successful
transmissions when there are
many nodes, each with many
frames to send

Only 18%...

Maximize the utilization function by differentiation
yields maximum point at
with utilization
of
.

Trivial improvement:
Why be vulnerable for 2 time units?
Synchronize, and use slotted time. May transmit only
at integer times
28
Slotted Aloha
Assumptions
 all frames same size
 time is divided into equal
size slots, time to transmit 1
frame
 nodes start to transmit
frames only at beginning of
slots
 nodes are synchronized
 if 2 or more nodes transmit
in slot, all nodes detect
collision
29
Operation
 when node obtains fresh frame,
it transmits in next slot
 no collision, node can send
new frame in next slot
 if collision, node retransmits
frame in each subsequent slot
with prob. p until success
Slotted Aloha
Pros
 single active node can
continuously transmit at
full rate of channel
 highly decentralized: only
slots in nodes need to be
in sync
 simple
30
Cons
 collisions, wasting slots
 idle slots
 nodes may be able to
detect collision in less than
time to transmit packet
 clock synchronization
Slotted Aloha - efficiency
Efficiency is the long-run
fraction of successful slots
when there are many nodes,
each with many frames to send



Suppose N nodes with many
frames to send, each transmits
in slot with probability p
prob that node 1 has success in
a slot
= p(1-p)N-1
prob that any node has a
success = Np(1-p)N-1
31


For max efficiency with N
nodes, find p* that maximizes
Np(1-p)N-1
For many nodes, take limit of
Np*(1-p*)N-1 as N goes to
infinity, gives 1/e = .37
At best: channel
used for useful
transmissions 37%
of time!
Aloha - Summary




Very popular at the beginning of time (i.e., 70s to 80s).
Very simple to handle.
Lots and lots of basic probabilities calculations for
students!
Major problem: Nodes don’t check what’s going on in the
channel, each acting on its own. No manners!
32
CSMA (Carrier Sense Multiple Access)


CSMA: listen before transmit:
Protocol:
Listen to the channel
If channel sensed idle, transmit entire frame
If channel sensed busy, defer transmission by.
1.
2.
3.
1-Persistent CSMA:
Wait until channel is quiet and transmit immediately. If collision
occurs, wait a random time and listen again (go to 1).

Non-Persistent CSMA:
Wait a random time and listen again (go to 1).
They differ only by the treatment of 1st transmission.



CSMA human analogy: don’t interrupt others!
33
CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA




collision detection:



When transmitting, try to sense if there is a collision.
collisions detected within short time.
colliding transmissions aborted, reducing channel wastage.
easy in wired LANs: measure signal strengths, compare
transmitted, received signals.
difficult in wireless LANs: receiver shut off while transmitting.
human analogy: the polite conversationalist
34
CSMA/CD Minimum Packet Size
35
Ethernet uses CSMA/CD



No slots
adapter doesn’t transmit if it
senses that some other
adapter is transmitting, that is,
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting, that is,
collision detection
36

Before attempting a
retransmission, adapter
waits a random time, that
is, random access
Unreliable, connectionless service


Connectionless: No handshaking between sending and
receiving adapter.
Unreliable: receiving adapter doesn’t send acks or nacks to
sending adapter



37
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise, app will see the gaps
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other
transmitters are aware of
collision; 48 bits
Bit time: .1 microsec for 10 Mbps
Ethernet ;
for K=1023, wait time is about
50 msec
See/interact with Java
applet on AWL Web site:
highly recommended !
38
Exponential Backoff:
 Goal: adapt retransmission
attempts to estimated current
load




heavy load: random wait will be
longer
first collision: choose K from
{0,1}; delay is K· 512 bit
transmission times
after second collision: choose K
from {0,1,2,3}…
after ten collisions, choose K
from {0,1,2,3,4,…,1023}
802.3 CSMA/CD (Ethernet) Algorithm
39
Ethernet CSMA/CD algorithm
1. Adaptor receives datagram
4. If adapter detects another
from net layer & creates frame
transmission while transmitting,
aborts and sends jam signal
2. If adapter senses channel idle, it
starts to transmit frame. If it
5. After aborting, adapter enters
senses channel busy, waits until
exponential backoff: after
channel idle and then transmits
the mth collision, adapter
chooses a K at random from
3. If adapter transmits entire
{0,1,2,…,2m-1}. Adapter waits
frame without detecting
K·512 bit times and returns to
another transmission, the
Step 2
adapter is done with frame !
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols

What do you do with a shared media?


42
Channel Partitioning, by time, frequency or code
 Time Division, Frequency Division, Code Division
Random partitioning (dynamic),
 ALOHA, S-ALOHA, CSMA, CSMA/CD
 carrier sensing: easy in some technologies (wire), hard in
others (wireless)
 CSMA/CD used in Ethernet
 CSMA/CA used in 802.11 (Wireless).
LAN technologies
Data link layer so far:

MAC protocols. The random protocol approach.
Next: LAN technologies



43
Addressing
Ethernet
Hubs, bridges and switches
MAC Addresses and ARP

32-bit IP address:



network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address:


44
used to get datagram from one interface to another physicallyconnected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM, but can be modified.
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
45
Broadcast address =
FF-FF-FF-FF-FF-FF
LAN Address (more)




MAC address allocation administered by IEEE.
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
MAC flat address ➜ portability


can move LAN card from one LAN to another
IP hierarchical address NOT portable

46
depends on IP subnet to which node is attached
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?


237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
237.196.7.88
47
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
Each IP node (Host, Router)
on LAN has ARP table
ARP Table: IP/MAC address
mappings for some LAN
nodes
< IP address; MAC address; TTL>

TTL (Time To Live): time after
which address mapping will be
forgotten (typically 20 min)
ARP protocol: Same LAN (network)



A wants to send datagram to B,
and B’s MAC address not in A’s
ARP table.
A broadcasts ARP query packet,
containing B's IP address
 Dest MAC address = FF-FFFF-FF-FF-FF
 all machines on LAN receive
ARP query
B receives ARP packet, replies to
A with its (B's) MAC address

48
frame sent to A’s MAC address
(unicast)

A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
 soft state: information that
times out (goes away) unless
refreshed

ARP is “plug-and-play”:

nodes create their ARP tables
without intervention from net
administrator
Routing to another LAN
walkthrough: send datagram from A to B via R
assume A know’s B IP address
A

Two ARP tables in router R, one for each IP network (LAN)
R


In routing table at source Host, find router 111.111.111.110
In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc
49
B








A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for 111.111.111.110
A creates link-layer frame with R's MAC address as dest, frame
contains A-to-B IP datagram
A’s adapter sends frame
R’s adapter receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
R
50
Ossi Mokryn - Data link layer
B
Ethernet
“dominant” wired LAN technology, developed at the 70s:
 cheap $20 for 100Mbs!
 first widely used LAN technology
 Simple, cheap.
 Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch
51
Ethernet topology Through the Years
Classic Ethernet
• Shared Bus with CSMA/CD
• Bus maximal length: 500 m.
•Transmission rate: 10Mb/s.
Now star topology prevails



Connection choices: hub or
switch (more later)
Fast Ethernet: 100 Mb/s
Gigabit Ethernet: 1Gbps
hub or
switch
Through the years the only common is: The Frame
52
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Type/ length: length or type of frame
Preamble:
 7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
 used to synchronize receiver, sender clock rates: what is
the length, in clock ticks, of one bit.
53
Ethernet Frame Structure (more)

Addresses: 6 bytes





if adapter receives frame with matching destination address, or with
broadcast address (eg ARP packet), it passes data in frame to net-layer
protocol
otherwise, adapter discards frame
MAC addresses, also called : Physical addresses
Type: indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC: checked at receiver, if error is detected, the frame is
simply dropped. Before CRC there is a padding field for the
CRC to pad to 64 bytes.
54
Ossi Mokryn - Data link layer
Ethernet Technology: 10Base2




55
10: 10Mbps; 2: under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces: physical layer device only!
Ethernet technology: 100BaseT



10/100 Mbps rate; latter called “fast ethernet”
T stands for Twisted Pair
Nodes connect to a hub: “star topology”; 100 m max
distance between nodes and hub
twisted pair
hub
56
Ethernet technology: 100BaseT
Problem : must keep minimal packet size when bandwidth
increases.
With fixed cable length and propagation speed, must
increase minimal size proportionally to bandwidth
increase!
E.g. 100Mb/s, 1500m of cable: prop remains 6μs, minimal





size becomes 1200 bits.
Solutions:




Cable length limited to 100m.
Prevent collisions by “Ethernet Switches” (later).
Max distance from node to Hub is 100 meters
57
Gbit Ethernet






use standard Ethernet frame format.
allows for point-to-point links and shared broadcast channels.
in shared mode, CSMA/CD is used; short distances between
nodes to be efficiency.
uses hubs, called here “Buffered Distributors”
Full-Duplex at 1 Gbps for point-to-point links.
10 Gbpsec now!
58
Hubs
Q:Why not just one big LAN?




Limited amount of supportable traffic: on single LAN,
all stations must share bandwidth
limited length: 802.3 (Ethernet) specifies maximum
cable length
large “collision domain” (can collide with many
stations)
limited number of stations: 802.5 (token ring) have
token passing delays at each station
59
Hubs (Multiport repeaters, Bus in a box)



Physical Layer devices: essentially repeaters operating at bit
levels: repeat received bits on one interface to all other
interfaces
Can’t interconnect 10BaseT & 100BaseT (because
segments don’t share the same rate).
Hubs can be arranged in a hierarchy (or multi-tier design),
with backbone hub at its top
60
Lecture 3
Hubs (Multiport repeaters, Bus in a box)




61
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains: node may collide with any
node residing at any segment in LAN
Extends max distance between nodes, but all the segments become
one large collision domain.
Hub Advantages:
 simple, inexpensive device
 Multi-tier provides graceful degradation: portions of the LAN
continue to operate if one hub malfunctions
 extends maximum distance between node pairs (100m per Hub)
Bridges






Link Layer devices: operate on Ethernet frames, examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment, bridge uses
CSMA/CD to access segment and transmit
Store and forward element. So different types of Ethernet
types can be connected.
Transparent: no need for any change to hosts LAN adapters
Forwarding is selective: do not always flood. All connected
segments can work independently in parallel!
62
Bridge Filtering


bridges learn which hosts can be reached through which
interfaces: maintain filtering tables
 when frame received, bridge “learns” location of sender:
incoming LAN segment
 records sender location in filtering table
filtering table entry:
 (Node LAN Address, Bridge Interface, Time Stamp)
 stale entries in Filtering Table dropped (TTL can be 60 minutes)
63
Bridge Operation

bridge procedure(in_MAC, in_port,out_MAC)
lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) /* no entry found for destination */
then flood; /* forward on all but the interface on
which the frame arrived*/
if (in_port = out_port) /*destination is on LAN on which
frame was received */
then drop the frame
Otherwise (out_port is valid) /*entry found for destination */
then forward the frame on interface indicated;
64
Bridge Learning: example
Suppose C sends frame to D and D replies back with frame
to C
 C sends frame, bridge has no info about D, so
floods to both LANs


65

bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
Bridge Learning: example
C
1
 D generates reply to C, sends
bridge sees frame from D
 bridge notes that D is on interface 2
 bridge knows C on interface 1, so selectively
forwards frame out via interface 1

66
What will happen with loops?
Incorrect learning
B
2
2
A , 12
A , 12
1
1
A
67
What will happen with loops?
Frame looping
C
2
2
C,??
C,??
1
1
A
68
What will happen with loops?
Frame looping
B
2
2
B,2
B,1
1
1
A
69
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 802.1D



Note: redundant paths are good, active redundant paths are bad (they cause
loops)
70
How to Construct a Spanning Tree?



Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames:




71
Finding the root: flooding
Building a tree: Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 802.1 specification
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges





72
MAC address
Bridge id + port number
Spanning Tree Concepts:
Root Bridge
The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge



73
Spanning Tree Concepts:
Path Cost
A cost associated with each port on each bridge


default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge



74
Spanning Tree Concepts:
Root Port
The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge, the port with the
lowest ID becomes root port



75
Spanning Tree Concepts:
Root Path Cost
The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network



76
Spanning Tree Concepts: Designated
Bridge
Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge



77
Example Spanning Tree
B8
B3
B5
Protocol operation:
B7
B2
3.
B1
B6
78
1.
2.
B4
Picks a root
For each LAN,
picks a designated bridge
that is closest to the root.
All bridges on a LAN
send packets towards the
root via the designated
bridge.
Example Spanning Tree
B8
Spanning Tree:
B3
B5
B1
root
port
B2
B7
B2
B4
B5
B1
Root
B6
79
B8
Designated
Bridge
B4
B7
Spanning Tree Algorithm:
An Overview
1. Determine the root bridge among all bridges
2. Each bridge determines its root port


 The port in the direction of the root bridge
3. Determine the designated bridge on each LAN

 The bridge which accepts frames to forward towards the root bridge
 The frames are sent on the root port of the designated bridge
80
Spanning Tree Algorithm:
Selecting Root Bridge
Initially, each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs


 The bridge and port ID of the sending bridge
 The bridge and port ID of the bridge the sending bridge considers root
 The root path cost for the sending bridge
Best one wins


81
(lowest root ID/cost/priority)
Spanning Tree Algorithm:
Selecting Root Ports
Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie, the lowest uplink (transmitter) bridge ID is
used
In case of another tie, the lowest port ID is used



82
Spanning Tree Algorithm:
Select Designated Bridges
Initially, each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs


 The bridge and port ID of the sending bridge
 The bridge and port ID of the bridge the sending bridge considers root
 The root path cost for the sending bridge
3. Best one wins


83
(lowest ID/cost/priority)
Forwarding/Blocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state


84
Ethernet Switches





85
layer 2 (frame) forwarding,
filtering using LAN addresses
Switching: A-to-B and A’-to-B’
simultaneously, no collisions
large number of interfaces
often: individual hosts, starconnected into switch
 Ethernet, but no collisions!
Confused with Ethernet
bridges…
Ethernet Switches

cut-through switching: frame forwarded from input to
output port without awaiting for assembly of entire frame

slight reduction in latency

combinations of shared/dedicated, 10/100/1000 Mbps
interfaces

Offers VLANS (Virtual LANs).

Nowadays routers are actually combined with
Ethernet switches.
86
Ethernet Switches (more)
Dedicated
Shared
87
Summary comparison
hubs
routers
Bridges
traffic
isolation
no
yes
yes
plug & play
yes
no
yes
optimal
routing
cut
through
no
yes
no
yes
no
yes
88
Road-Map and Keywords

IEEE 802 Model compared to the OSI.


Physical Media


Aloha, Slotted Aloha.
LAN technology – Ethernet Protocol:


TDMA, FDMA, CDMA.
Random Access MAC protocols:


Channel Partitioning, Random Access, “Taking Turns”.
Portioning MAC protocols:


Point-to-point, Broadcast, Switched.
Different MAC protocol approaches:


Coax, Twisted Pairs, Fibers.
Link Types


LLC, MAC.
MAC Addresses, Frame Structure, ARP,
LAN interconnect:

89
Hubs Bridges and Ethernet Switches.
IEEE 802 Model Compared to the OSI

The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model:
Higher Layers
IEEE 802.1
Higher Levels Interface
IEEE 802.2
Logical Link Control (LLC)
Data-Link
Layer
Physical Layer
OSI
90
IEEE 802.3
CSMA/CD
Medium
Access
Control
IEEE 802.11
Wireless
Medium
Access
Control
IEEE 802.5
Token Ring
Medium
Access
Control
CSMA/CD
Medium
Wireless
Medium
Token Ring
Medium
IEEE 802
IEEE 802 Model Compared to the OSI





The LLC provides common interface for common LAN functionality.
There are various media which offer different methods for communication
(OSI so called Physical layer).
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias.
What kind of medias do we have?
What kind of corresponding MAC protocols do we have?
91