LANs
1
Local Area Networks: Ethernet
Prof. Jean-Yves Le Boudec
Prof. Andrzej Duda
Prof. Patrick Thiran
ICA, EPFL
CH-1015 Ecublens
Patrick.Thiran@epfl.ch
http://icawww.epfl.ch
LANs
2
Objective
o Understand shared medium access methods of Ethernet;
o Describe network aspects of an Ethernet network;
PART A: The CSMA/CD method
PART B: Network Aspects (Ethernet)
PART C: CSMA/CA and wireless LANs
The access method is a way of sharing a common transmission medium (cable,
wireless link) between several hosts. Ethernet is built upon the medium access
method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection).
The network aspects explain how a local area network is built today. We will
see that the resulting network is very far away from the original design.
LANs
3
Part A:
Motivation for LANs
o goal: connect computers in same site (building, small campus)
o experience from host centric networks:bursty traffic
o basic idea: share a cable, no complex software in the end systems
o alternatives ?
switch based LANs: connection oriented: ATM
switch based LANs: connectionless. Switched Ethernet
If you want to understand something in the world of local area networks, you
should keep in mind the design requirements. Today, they are:
•(1) interconnect many pieces of equipment without complex cabling, inside a
limited geographical area, and inside one organization
•(2a) be easy to manage, in particular, detect cable faults easily.
When Ethernet was first conceived, the requirements were a little bit
different. The second requirement was replaced by:
•(2b) use one shared cable for the entire network.
Today most people would agree that this is not necessarily a good idea,
because fault isolation is difficult on a shared cable. Originally, it was
believed to be good because it would reduce the amount of cabling, and because
traffic is bursty.
Burstiness means that, most of the time, sources are idle; once in a while,
they send a large amount of traffic. The response time is better with a shared
medium system than if you allocate a fixed share to all (see exercise).
LANs
4
Access Method
o multiaccess communication = share a communication medium
o examples
radio channel, cellular networks, satellite links
machine bus
local area cable
o multiaccess communication (= shared medium) requires an Access Method
deterministic:
Time Division Multiple Access (TDMA)
Token Passing (Token Ring, Token Bus, FDDI)
DQDB
non-deterministic
Aloha
CSMA/CD
CSMA/CA
• The purpose of the access method is to control access to the channel. If all
stations talk at the same time, then no data can be understood by receivers
(collision). Compare to a CB channel.
• Deterministic access method require that stations talk only when they are
authorized by the access protocol. With TDMA, time is divided into periodic
slots; station i can use time intervals [(i-1)δ, iδ), [T+(i-1)δ, T+ iδ),
, [2T+(i-1)δ, 2T +iδ), …, where T is the period and δ the slot duration. With
n stations, only 1/n of the channel time is usable by one station. The scheme
requires global synchronization; it does not support bursty traffic well (why
?), however it is simple to control. It is used in cellular and satellite
systems.
With a token passing schemes, there exists one global token that circulates
among stations; in order to talk, a station must have the token; while
talking, the token is kept by the station, which has to release it after a
maximum token holding time. Token passing schemes allow a very high
utilization even with sporadic traffic, as long as the bandwidth delay product
is not too large (time is wasted while passing token from one station to the
other).
DQDB uses two buses on which endpoints generate trains of cells. To use a
cell, a station must position a reservation bit in a cell on the opposite bus
and use one of the empty cells going to the destination station.
• Non deterministic (=collision based) schemes take an optimistic approach.
Collisions are avoided if possible, but they may occur, and the schemes
operate in such a way that they can be recovered from. Aloha is a primitive
scheme which evolved to CSMA/CD, the access method of Ethernet. These schemes
are simpler to implement than token passing schemes, but do not support as
high utilization (time is wasted during collisions and during collision
recovery times). Collision based schemes do not work well if the bandwidth
delay product is high. A modified version is used in a wireless LAN such as
WaveLAN: CSMA/CA (Collision Avoidance). In order to limit collisions, stations
backoff a random interval before each transmission.
LANs
5
Access Method Topology
o Logical Topology:
l
bus:
–
–
–
–
l
all bits sent by one station are propagated to all stations
data die at the end of bus (terminators)
all stations see all frames (broadcast medium)
used by Ethernet, Token Bus, LocalTalk, Wireless LANs
ring:
–
–
–
–
all bits pass from one station to the next one, then to its neighbour, etc
bits eventually return to the originating station that has to remove them
all stations see all frames
used by Token Ring and FDDI
o cabling topology = layout of cables = star in most cases
l
ring topology can be formed from the star layout of cables
CSMA/CD uses the bus logical topology, whereas token passing schemes such as
Token Ring and FDDI use ring topologies.
The cabling topology is in general different from the logical topology. A
simple network today uses a star topology: all cables go from a central point
(the hub) to all end-systems. A more complex network uses a tree of stars.
It is the Token Ring network that first introduced a star based cabling
topology; this because the designers of the Token Ring took requirement (2b)
seriously. With the first Token Rings, cables from a station go to a hub
containing electro-magnetic relays that would automatically bypass a station
not correctly functioning (or powered off).
LANs
6
ALOHA
data
central
host
ack
transmission procedure
ii == 11
while
while (i
(i <=
<= maxAttempts)
maxAttempts) do
do
send
packet
send packet
wait
wait for
for acknowledgement
acknowledgement or
or timeout
timeout
if
if ack
ack received
received then
then leave
leave
wait
wait for
for random
random time
time
increment
i
increment i
end
end do
do
• ALOHA is the basis of all non-deterministic access methods. The ALOHA
protocol was originally developed for communication between islands
(University of Hawaï) using radio channels at low bit rates.
• The ALOHA protocol requires acknowledgements and timers.
Collisions occur, and if a packet is lost, then source has to retransmit; the
retransmission strategy is not specified here; many possibilities exist. We
will see the one used for CSMA/CD.
• There is no feedback to the source in case of collision (was too complex to
implement at that time). The picture shows a radio transmission scenario;
Aloha can also be used on a cable (bus).
•The maximum utilization can be proven to be 18% (see below). This is assuming
an ideal retransmission policy that avoids unnecessary repetitions of
collisions.
LANs
7
Throughput of Aloha
o Assumptions
l
Frames of same length, take T time units to be successfully transmitted between any two nodes;
Infinite number of stations;
Fresh traffic arrival process is Poisson (λ);
Fresh arrival traffic always attempts to go through upon arrival;
Backlogged stations retransmit independently from each other and from the arrival process, waiting a
random time τ exponentially distributed, with mean 1/v, between consecutive attemps
l
l
l
l
o Number of generated frames is therefore a Poisson process of rate g(n) = (λ + nv) for n
backlogged stations
o P(a packet is transmitted | n backlogged stations) = P(no collision | n backlogged stations) =
P(no packets generated during 2 T | n backlogged stations) = exp (-2Tg(n))
o Average throughput when there are n backlogged stations is θ(n) = g(n)T exp (-2Tg(n))
o θ(n) has maximum equal to 1/2e for gT=0.5
θ
gT
• The maximum utilization is difficult to obtain and depends on a large number
of parameters. We provide an upper bound.
• We observe packet arrivals at one point on the medium. We assume that packet
arrivals (fresh + retransmissions) are Poisson, and call µ the parameter. This
assumption is not obvious. It has been shown to be valid if fresh traffic is
Poisson, and if the retransmission policy is optimal. Other retransmission
policies lead to worse utilizations, or even to unstable systems.
• We assume that packet transmission time is constant, equal to T.
• Consider a packet arriving at time t. The packet will be transmitted without
collision iff no other packet arrives during time interval [t-T, t+T]. The
probability of this to happen is exp(-2µT).
• Over a long time interval s, the total number of packet arrivals is close to
µs, the fraction of packets transmitted without collision is close to exp(2µT), therefore the maximum utilization is :
µs exp(-2µT)T / s = µT exp(-2µT)
• µ is unknown and depends on the retransmission policy. However we can
compute the maximum value of the utilization over all possible values of µ.
The function is maximum for 2µT =1, and the value of the maximum is 1/2e = ca.
0.18.
LANs
8
Detailed Analysis : Slotted Aloha
o The analysis is simpler for slotted ALOHA.
o Assumptions
l
l
l
l
l
Transmissions are synchronized to start at the beginning of a time slot and last for exactly one
time slot. Let T denote its length;
Number of stations is m;
Fresh arrival with probability qa per unbacklogged station, 0 otherwise. Assuming total fresh
arrival traffic for the m stations is Poisson(λ), one has thus qa = 1 - exp(- λT/m);
Fresh arrival traffic always attempts to go through upon arrival;
Backlogged stations retransmit independently from each other wit h probability qr at each slot;
o Let X(t) denote the number of backlogged stations at the end of the tth time slot
o X(t) is a Markov chain (exercise).
In this and the following slides we do a more detailed analysis.
The analysis is considerably simpler for slotted Aloha, which we assume in the
rest of this chapter.
LANs
9
Detailed Analysis : Slotted Aloha
o Define
l
l
l
l
A(n) = number of fresh arrivals during the tth time slot knowing that X(t-1) = n. Then
m − n  i
m − n −i
P( A( n) = i) = 
q a (1− qa )
E[ A(n)] = ( m − n)qa
i


B(n) = number of attempts from the backlogged stations to retransmit during the tth time slot
knowing that X(t-1) = n.Then
 n
n −i
P (B (n) = i ) =  q ir (1 − q r )
E[ B(n)] = nqr
i
 
G(n) = A(n) + B(n) = total number of frames generated during the tth slot when X(t-1) = n.
g(n) = E[G(n)] = average frame generating rate during the tth slot when X(t-1) = n.
g (n) = E[G (n)] = (m − n)qa + nqr
l
Θ(n) = number of frames successfully transmitted during the tth slot knowing that X(t-1) = n
P(T (n) = 1) = P ( A(n) = 1)P (B (n) = 0) + P( A(n) = 0)P (B (n) = 1)
( ) (1− q ) + nq (1− q ) (1 − q )
≈ (m − n) q (1 − q ) (1 − q ) + nq (1 − q ) (1− q )
= (m − n ) q a 1 − q a
m− n −1
m− n
a
l
a
m− n
n
r
r
m− n
n
r
n −1
r
a
r
a
n
r
≈ g (n) exp (− g (n))
θ(n) = E[Θ(n)] = P(Θ(n)=1) average throughput during the tth slot when X(t-1) = n.
In this and the following slides we do a more detailed analysis.
The analysis is considerably simpler for slotted Aloha, which we assume in the
rest of this chapter.
LANs
10
Numerical Examples
oThroughput is thus approximately θ = g exp(-g)
oThe maximum utilization is bounded by 1/e ≈ 0.36
θ
θ
g
g
m = 10 stations
m = 50 stations
o The figure illustrates that the relation throughput ≈ g exp(-g) holds well for large m
The figure shows results of the Markov chain analysis.
We have considered a number of possible values for the parameters m, qa and
qr. For a given value of m, we vary qa and q r as explained above. Every value
of (m, qa, qr) gives one point on one curve. A point is defined by
x = G = offered load
y = achieved throughput
The dots represent the exact values for our model. The curve is the ideal
relation y = x exp(-x).
LANs
11
ALOHA Instability
o Define D(n) = E[A(n)] - θ(n) = (m-n)qa – g(n)exp(-g(n)) as the “drift” in the
system
l D(n) > 0 ⇒ More arrivals than departures, on the average
l D(n) < 0 ⇒ Fewer arrivals than departures, on the average
l D(n) = 0 ⇒ Equilibrium
oFor m = ∞ all states of the Markov chain are transient
θ(n)
g(n)exp(-g(n))
( m − n ) qa
mq a
g (n) = ( m − n)qa + nqr
mqr
LANs
12
CSMA
o Improvement 1: Listen before you talk: ”Carrier Sense Multiple Access“
ii == 11
while
while (i
(i ≤≤ maxAttempts)
maxAttempts) do
do
listen
listen until
until channel
channel idle
idle
transmit
transmit immediately
immediately
wait
wait for
for acknowledgement
acknowledgement or
or timeout
timeout
if
ack
received
then
leave
if ack received then leave
wait
wait random
random time
time /*
/* collision*/
collision*/
increment
increment ii
end
end do
do
CSMA improves on Aloha by requiring that stations listen before transmitting
(compare to CB radio).
Some collisions can be avoided, but not completely. This is because of
propagation delays. Two or more stations may sense that the medium (= the
channel) is free and start transmitting at time instants that are close enough
for a collision to occur. Assume propagation time between A and B is 2 ms and
that all stations are silent until time 0. At time 0, station A starts
transmitting for 10 ms, at time 1 ms, station B has not received any signal
from A yet, so it can start transmitting. At time 2ms, station B senses the
collision but it is too late according to the protocol.
The CSMA protocol requires that stations be able to monitor whether the
channel is idle or busy (no requirements to detect collisions). It is a simple
improvement to Aloha, at the expense of implementing the monitoring hardware.
The effect of the CSMA protocol can be expressed in the following way. Call T
the maximum propagation time from station A to any other stations; if no
collision occurs during a time interval of duration T after A started
transmitting, then A has seized the channel (no other station can send).
CSMA works well only if the transmission time is much larger than propagation,
namely bandwidth-delay product << frame size. It has the same stability
problems as Aloha.
In order to avoid repeated collisions, it is required to wait for a random
delay before re-transmitting. If all stations choose the random delays
independently, and if the value of the delay has good chances of being larger
than T, then there is a high probability that only one of the re-transmitting
stations seizes the channel.
LANs
13
CSMA/CD
o improvement 2: detect collisions as soon as they occur :
“Carrier Sense Multiple Access / Collision Detection”
o improvement 3: acknowledgments replaced by CD
ii == 11
while
while (i
(i <=
<= maxAttempts)
maxAttempts) do
do
listen
listen until
until channel
channel is
is idle
idle
transmit
transmit and
and listen
listen
wait
wait until
until (end
(end of
of transmission)
transmission) or
or (collision
(collision detected)
detected)
if
collision
detected
then
if collision detected then
stop
stop transmitting
transmitting /*
/* after
after 32
32 bits
bits (“jam”)*/
(“jam”)*/
else
else
wait
wait for
for interframe
interframe delay
delay
leave
leave
wait
wait random
random time
time
increment
increment ii
end
end do
do
o This is Ethernet (- 802.3, the standard conformant version of Ethernet)
CSMA/CD is the protocol used by Ethernet. In addition to CSMA, it requires
that a sending station monitors the channel and detects a collision.
The benefit is that a collision is detected within a propagation round trip
time.
Collisions may still occur.
LANs
14
CSMA/CD Time Diagram 1
o A senses idle channel, starts
transmitting
0
o shortly before T, B senses T
idle channel, starts
transmitting
A
B
LANs
15
CSMA / CD Time Diagram 2
o A senses collision,
continues to transmit 32
bits (“jam”)
o B senses collision,
continues to transmit 32
bits (”jam“)
A
B
0
T
Jam bits are simply there to make sure the collision is long enough to be
detected by the hardware.
LANs
16
CSMA / CD Time Diagram 3
o A waits random time t1
o B waits random time t2
o B senses channel idle and
transmits
o A senses channel busy
and defers to B
o A now waits until
channel is idle
A
B
0
T
t2
t1
• CSMA/CD improves on CSMA by requiring that stations detect collisions and
stop transmitting (after 32 bits, called jam bits, in order to ensure that all
circuits properly recognize the presence of collisions).
• CSMA/CD has better performance than Aloha or CSMA, but suffers from the same
stability problems
• After a collision is detected, stations will re-attempt to transmit after a
random time.
• Acknowledgements are not necessary because absence of collision means that
the frame could be transmitted (see ”Minimum Frame Size“).
• The interframe delay (“gap”) is 9.6 µs. It is used to avoid blind times,
during which adapters are filtering typical noise at transmission ends.
• The random time before retransmission is chosen in such a way that if
repeated collisions occur, then the time increases exponentially. The effect
is that in case of congestion (too many collisions) the access to the channel
is slowed down.
LANs
17
Exponential Backoff
o random time before re-transmission is given by:
kk == min
min (10,
(10, AttemptNb)
AttemptNb)
k
rr == random
random (0,
(0, 22k -1)
-1) ** slotTime
slotTime
“AttemptNb” is the number of the re-transmission attempt that will be
attempted after the random time (k=1 for the first retransmission);
“random” returns an integer, uniformly distributed between the two
bounds given in argument;
o examples:
first retransmission attempt:
k = 1; r = 0 or r = slotTime
second retransmission attempt (if preceding one failed):
k = 2; r = 0, 1, 2 or 3 * slotTime
o when AttemptNb = 15, then abort transmission
LANs
18
Minimum Frame Size
A
B
A
B
A
B
A
B
t = 0: A begins transmission
t = 1- ε: B begins transmission
t = 1 : B detects collision, stops
transmitting
t = 2- ε: A detects collision
LANs
19
Minimum Frame Size
o a minimum frame size equal to number of bits transmitted during one round trip is
required to detect all collisions
o slotSize = number of bits transmitted by a source during the maximum round trip
time for any Ethernet network
l
l
l
slotSize= bandwidth - delay product + jam size + safety margin
= 512 bits at 10 and 100 Mb/s, = 512 bytes at 1 Gb/s
slotTime = slotSize / 10Mb/s = 51.2 µs
o rule: in Ethernet, all frames must be as large as slotSize
l
the minimum data field: 46 bytes
o properties:
P1: all collisions are detected by sources while transmitting
P2: collided frames are shorter than slotSize
Proof:
P1 see previous slide
P2 because collided frame are aborted by source at the latest after slotTime, including jam bits
LANs
20
CSMA / CD performance
o Maximum utilization of Ethernet is difficult to determine analytically.
l
Approximation:
θ ≈
1/(1+ Cα)
where α = β / L = 2Db / bT = 2D/T
l
L = frame size, β = bandwidth-delay product, D = propagation delay, T = transmission time
l
of a frame of size L, b = bit rate of the channel.
C is a constant : C = 3.1 is a pessimistic value; C = 2.5 is an approximate value based on
simulations
o for a large network, β is close to 60 Bytes; for traffic with small frames (L = 64 bytes),
the utilization is less than 30 %. For large frames (1500 Bytes), it is around 90%.
o Key for high utilization is: bandwidth delay product << frame size
The formula with C= 3.1 is proven in the next slide. It is a pessimistic
estimate.
LANs
21
Proof
o Assumptions
l
l
l
l
l
l
Frames of same length, take T time units to be transmitted between any two nodes;
Maximal propagation delay D = αT/2 (worst case assumption: arrivals are always at alternate ends of the
network, namely, separated by the maximum propagation delay);
Infinite number of stations;
Fresh traffic arrival process is Poisson (λ);
Fresh arrival traffic always attempts to go through upon arrival;
Backlogged stations retransmit independently from each other and from the arrival process, waiting a
random time τ exponentially distributed, with mean 1/v, between consecutive attempts
o Number of frames is therefore a Poisson process of rate g(n) = (λ + nv) for n backlogged stations
o P(a generated packet is transmitted | n backlogged stations) = P(no collision | n backlogged
stations) = P(no packets generated during D | n backlogged stations) = exp (-Dg(n))
o T cyc = E[Cycle time starting from a successful or aborted transmission | n backlogged stations] =
D + E[Time until next frame generated] +
E[Time until transmission completes|success] P(successful transmission | n backlogged stations) +
E[Time until transmission aborted|collision] P(collision | n backlogged stations)
= T + 1/g(n) + T exp (-Dg(n)) + 2D (1 - exp (-Dg(n)))
o Average throughput when there are n backlogged stations is θ(n) = Tusefu l/Tcyc= T exp (-Dg(n))/Tcyc
o θ(n) has maximum for gD = 0.43, from which we get the formula
LANs
22
Part B: Ethernet / IEEE 802.3
o Ethernet =
CSMA/CD with exponential backoff
as shown in part A
originally over a coaxial cable
10 Mb/s to 1 Gb/s
local area only (<= 0.2 to 2 kms )
o Ethernet history
1980 : Ethernet V1.0 (Digital, Intel,
Xerox)
1982 : Ethernet V2.0
1985 : IEEE 802.3 standard
small differences in both specifications;
adapters today support both
1995 : IEEE 802.3 100Mb/s standard
802.3 frame
preamble
Ethernet V.2 frame
7 B
preamble
SFD
1 B =
10101011
DA
6 B
DA
SA
6 B
SA
Length
2 B
Type
NSAP
data
SFD
[46,1500]B
data
pad
FCS
4 B
FCS
DA = destination address
SA = source address
The preamble is used for the receivers to synchronize (01010101… terminated by
0). With Ethernet, transmission starts asynchronously (stations start
independently), and between transmissions, the channel is idle.
SFD (start frame delimiter) is used to validate the beginning of a frame.
Destination length is used to indicate the total length before padding.
Padding is required if the minimum frame size of 512 bits = 64 bytes is not
reached. With the Ethernet proprietary (=non standard) format, this field is
not present. It is up to the layer using Ethernet to know that frames have to
be at least 512 bits, and perform the padding. Maximum size of data part is
1500 Bytes (limitation imposed by buffer size considerations in adapters).
The type field indicates the type of upper layer that uses the protocol (for
example: IP or Appletalk). With 802.3, either the field length indicates the
type user instead of the frame length, either this field is absent. It is
replaced by an intermediate layer, called LLC that provides mainly this
multiplexing function. LLC is not needed with the non-standard Ethernet. Type
values are larger than the maximum size so both formats can exist on the same
network (even on the same station).
The FCS (frame check sequence) is a 32-bit cyclic redundancy check. It can
detect all single, double, triple errors, all error bursts of length <= 32, most double
bursts of length up to 17. The probability that a random collection of bit errors is
undetected is 2e-10.
Ethernet works for a local area only. This is because the CSMA/CD protocol has
poor utilization as the bandwidth-delay product becomes large compared to the
frame sizes.
LANs
23
Ethernet v2 / IEEE 802.3
o Data link layer divided in 2 sublayers:
o Medium Access Control
(MAC):
l
l
Manages access to the physical
layer
Independent of physical layer
o Logical Link Control (LLC):
l
l
l
IEEE/802.3 frame
Network
Network Layer
Layer
}
LLC
LLC Sub-Layer
Sub-Layer
MAC
MAC Sub-Layer
Sub-Layer
Data
Data link
link Layer
Layer
preamble
Physical
Physical Layer
Layer
Identifies protocol user
destination in the header
Error , flow control possible
Independent of MAC sub-layer
SFD
DA
SA
Length
LLC Type
data
data
NSAP(LLC)
data
pad
FCS
Network Layer
LLC Sub-Layer MAC Sub-Layer
LANs
24
Addressing
o MAC address: 48 bits (6 bytes) = adapter name
o MAC addresses are provided by manufacturer.
l
Examples. CISCO: 00:00:0C:::,
3COM: 02:60:8C:::
o sender puts destination MAC address in the frame
o all stations read all frames; keep only if destination address matches
o all 1 address (FF:FF:FF:FF:FF:FF ) = broadcast
MAC address A
08:00:20:71:0d:d4
B
C
D
00:00:c0:3f:6c:a4
01:00:5e:02:a6:cf (group address)
• Ethernet addresses are known as MAC addresses. Every Ethernet interface has
its own MAC address, which is in fact the serial number of the adapter, put by
the manufacturer.
MAC addresses are 48 bit-long. The 1st address bit is the individual/group
bit, used to differentiate normal addresses from group addresses. The second
bit indicates whether the address is globally administered (the normal case,
burnt-in) or locally administered. Group addresses are always locally
administered.
• When A sends a data frame to B, A creates a MAC frame with source addr = A,
dest addr = B. The frame is sent on the network and recognized by the
destination.
• Some systems like DEC networks require that MAC addresses be configured by
software; those are so-called locally administered MAC addresses. This is
avoided whenever possible in order to simplify network management.
• Data on Ethernet is transmitted least significant bit of first octet first
(a bug dictated by Intel processors). Canonical representation thus inverts
the order of bits inside a byte(the first bit of the address is the least
significant bit of the first byte); examples of addresses:
01:00:5e:02:a6:cf
08:00:20:71:0d:d4
00:00:c0:3f:6c:a4
00:00:0c:02:78:36
FF:FF:FF:FF:FF:FF
(a group address)
(a SUN machine)
(a PC )
(a CISCO router)
the broadcast address
LANs
25
Ethernet Cabling
o Ethernet cabling is originally shared cable
o Today: mainly point to point UTP - twisted pair
o How is that possible ?
l
l
Thick Coax
repeaters
bridges
Thin Coax
UTP
Contrary to the original design requirement, Ethernet cabling is today mainly
point to point.
Why do network managers prefer point to point cabling?
- because fault isolation is simpler
- because configuration management is simpler
How is point to point cabling possible with a shared medium protocol?
- using repeaters (shown on the next slide)
- or using bridges (called Ethernet Switches)
LANs
26
Repeaters
o Extend network beyond cable length
limit
o Function of a simple, 2 port repeater:
- repeat bits received on one port to
other port
- if collision sensed on one port,
repeat random bits on other port
o One network with repeaters = one
collision domain
o Even with repeaters, network is
limited
l
l
l
Repeater
propagation time
51.2µs slotTime includes repeaters
at most 4 repeaters in one path
o Repeaters perform only physical
layer functions (bit repeaters)
From ethernet.faq:
There are limitations on the number of repeaters and cable segments
allowed between any two stations on the network. There are two different ways of
looking at the same rules:
1. The Ethernet way: A remote repeater pair (with an intermediate point-topoint link) is counted as a single repeater (IEEE calls it two repeaters). You
cannot put any stations on the point to point link (by definition!), and there can be
two repeaters in the path between any pair of stations.
This seems simpler to me
than the IEEE terminology, and is equivalent.
2. The IEEE way: There may be no more than five (5) repeated segments, nor
more than four (4) repeaters between any two Ethernet stations; and of the five cable
segments, only three (3) may be populated. This is referred to as the "5-4-3"
rule (5 segments, 4 repeaters, 3 populated segments).
From 3Com, for 10 Mb/s Ethernet:
The 100BASE-T standard defines two classes of repeaters, called Class I and Class II
repeaters. A collision domain can include at most one Class I or two Class II repeaters.
Key topology rules are as follows:
• Using two Class II repeaters, the maximum diameter of the collision domain is 205
meters (typically 100m + 5m + 100m). With just a single Class II repeater in the
collision domain, the diameter can be extended to 309 meters using fiber (typically
100m UTP + 209m fiber downlink). With a single Class I repeater in the collision
domain, the diameter can be extended to 261 meters using fiber (typically 100m UTP +
161m fiber downlink).
• Connecting from MAC to MAC (switch to switch, or end-station to switch) using halfduplex 100BASE-FX, a 412-meter fiber run is allowed.
• For very long distance runs, a nonstandard, full-duplex version of 100BASE-FX can be
used to connect two devices over a 2-kilometer distance. The IEEE is currently working
on a standard for full duplex, but at this time all full-duplex solutions are
proprietary.
LANs
27
From Repeaters to Hubs
o Multiport repeater (n ports)
logically equivalent to:
- n simple repeaters
- connected to one internal
Ethernet segment
o Multi-port repeaters make it possible to
use point-to-point segments (Ethernet in
the box)
o Value of point to point cabling ?
- ease of management
- fault isolation
Multiport
Repeater
Ethernet Hub
S1
S2
Multiport
ReUTP segment peater
S3
to other hub
Repeaters are the first building block that made it possible to have point-topoint star based cabling.
LANs
28
From Bus to Star and Tree
o Ethernet today = active
concentrators allow star wiring
fiber
o UTP on point-to-point
configurations only
Intermediate
o remote network management
Hub
NMA
o How many frames can be
transmitted in parallel in this UTP
network ? ______
Head
hub
NMA
coax
console
Intermediate
Hub
NMA
coax
Intermediate
Hub
NMA
NM Application
transceiver
cable
The figure shows the “tree of stars topology” which is now typical for a large
shared medium Ethernet.
However, we see on the next slides that large shared medium Ethernet are not
common anymore, due to the introduction of switching or bridging.
LANs
29
Bridges
port 1
port 3
Bridge
A
C
Forwarding Table
MAC
MAC Port
Port
addr
addr nbr
nbr
port 2
Repeater
B
D
AA
BB
CC
DD
11
22
33
22
o Bridges are intermediate systems, or switches, that forward MAC frames to
destinations based on MAC addresses
o Bridges perform connectionless data forwarding
o Bridges separate collision domains
l
l
a bridged LAN maybe much larger than a repeated LAN
there may be several frames transmitted in parallel in a bridged LAN
A bridge is an intermediate system for the MAC layer. It receives MAC frames
and forwards them further.
LANs
30
Repeaters and Bridges in OSI Model
Application
5 to 7 Presentation
Session
4
Transport
3
Network
2
1
LLC
Application
Presentation
Session
5 to 7
Transport
L2 PDU
(MAC Frame)
MAC
Physical
End System
L2 PDU
(MAC Frame)
Network
4
LLC
3
MAC
MAC
2
Physical
Physical
Physical
1
Repeater
Bridge
End System
o Bridges are layer 2 intermediate systems
o Repeaters are in layer 1 intermediate systems
o There also exist layer 3 intermediate systems (IP routers) -> see next chapter
LANs
31
Bridge learning
o Bridges build their forwarding tables by themselves (plug-and-play devices :
management very easy). They are called transparent bridges.
o Initially: empty table
port 1
A
Bridge
port 3
C
port 2
Repeater
B
D
Forwarding Table
MAC
MAC Port
Port
Addr
Addr Nbr
Nbr
LANs
32
Bridge learning (2)
o Frame arrives at one of the ports, destination not in the forwarding table : bridge
forwards copies of the frame to all its other ports
o Bridge stores
l
l
l
the source number of the incoming frame
the port from which the frame arrived
the current time
port 1
A
Bridge
port 3
C
port 2
Repeater
AA
B
D
Forwarding Table
MAC
MAC Port
Port Time
Time
addr
addr Nb
Nb
11
10:37
10:37
LANs
33
Bridge learning (3)
o Frame arrives at one of the ports, destination address is in the forwarding table :
bridge forwards copies of the frame only to the port
o Bridge stores
l
l
l
the source number of the incoming frame
the port from which the frame arrived
the current time
port 1
A
Bridge
port 3
C
port 2
Repeater
AA
BB
B
D
Forwarding Table
MAC
MAC Port
Port Time
Time
addr
addr Nb
Nb
11
22
10:37
10:37
10:43
10:43
LANs
34
Bridge learning (4)
o No frame arrived from a particular source after some period of time (the aging
time): this address is purged from the table.
port 1
A
Bridge
port 2
Repeater
D
port 3
C
Forwarding Table
MAC
MAC Port
Port Time
Time
addr
addr Nb
Nb
AA
11
11:45
11:45
LANs
35
Spanning tree
o Works well as long as there is no loops.
o If there is a loop: learning will cause a broadcast storm.
o Bridges need to run a spanning tree algorithm first. The spanning tree will keep all
the nodes reahable by disabling some ports of some bridges to prevent loops.
o Why is it useful to have loops in the connection of different LA Ns ?
port 1
Br A
Connected
port
C
port 3
Blocked
port
A
port 2
Repeater
port 2
B
port 3
Br B
D
port 1
E
LANs
36
Switched Ethernet
o
o
o
o
Switched Ethernet = Bridge in the box
Total bandwidth is not shared: parallel frame transmission
An Ethernet Switch = Multiport Bridge is a connectionless data switch
Ethernet used as a point-to-point mechanism!
Frame Switching Hub
B1
1
A
2
B
Frame Switching Hub
Bridge
3
C
4
B2 Bridge
1 2
5
D
U
3
V
4
W
5
X
LANs
37
Today’s Concentrators
concentrators (=hub) combine frame switching and port switching
o frame switching = bridging
o port switching = assign repeater ports to collision domains
How many Ethernet segments (=collision domains) on the picture ?
Frame Switching Hub
Frame Switching Hub
Bridge
B1
B2
Bridge
3a
1
A
2
B
3
C
4
1
5
D
U
2
3
V
4
W
5
repeater
X
LAN concentrators perform both bridging and repeating. They can be configured
by a network management application.
LANs
38
Today’s Concentrators
concentrators (=hub) combine frame switching and port switching
o frame switching = bridging
o port switching = assign repeater ports to collision domains
example: ports 3 and 4 at H2 are on one Ethernet segment
Frame Switching Hub
Bridge
B1
Frame Switching Hub
H1
B2
H2
Bridge
3a
1
A
2
B
3
C
4
1
5
D
U
2
3
V
4
W
5
repeater
X
LANs
39
Today’s Concentrators
concentrators (=hub) combine frame switching and port switching
o frame switching = bridging
o port switching = assign repeater ports to collision domains
example: port 5 is switched to same Ethernet segment as 3 and 4
Frame Switching Hub
H1
Bridge
B1
Frame Switching Hub
B2
H2
Bridge
3a
1
A
2
B
3
C
4
1
5
D
U
2
3
V
4
W
5
repeater
X
LANs
40
Virtual LANs
o several bridged LANs consolidated on one physical layer
o uses ATM or proprietary methods
A
B
C
X1
X2
Virtual
LAN
Concentrator
Virtual
LAN
Concentrator
D
L
M
N
P
Virtual
LAN
Concentrator
U
X3
V
The picture shows two virtual LANs: (ACLNV) and (BDMPU). For each of the virtual
LANs, there exists one or more collision domains per concentrator, plus one per
inter-concentrator link. The concentrators perform bridging between the
different collision domains of the same virtual LAN.
Between X1 and X2, the two virtual LANs use the same physical link. If ATM is
used, there is one VCC per virtual LAN.
The advantage is that physical location becomes independent of LANs. For
example, all servers and routers can be concentrated in the same rooms (ex: U
and V)
There is no communication between the different virtual LANs at layer 2.
LANs
41
Full duplex Ethernet
o A shared medium Ethernet cable is half duplex
o Full duplex Ethernet = a point to point cable, used in both directions
l
no access method, no CSMA/CD
o 100 Mb/s and Gigabit Ethernet switches use full duplex links to avoid distance
limitations and to guarantee bandwidth for stations
LANs
42
Congestion Control
o A network of buffers require some form of congestion
control
P0
l
otherwise congestion collapse may occur
o Known forms of congestion control are
l
l
l
reservations (ex: ATM)
end-to-end (ex: TCP)
hop by hop (ex: machine bus)
P1
P2
P3
P=0
P=1
P=2
STOP
P=3
P=4
STOP
o Ethernet concentrators use hop-by-hop flow control
l
l
STOP signal can be simulated by collisions on half
duplex links
on full duplex links: PAUSE ( n ) frames, where n is the
duration of required stopping time
GO
P=5
P=6
P=7
LANs
43
Architecture versus Products
o architecture = set of protocols and functions
defined by standards or proprietary specifications (SNA, Decnet, AppleTalk)
examples:
MAC layer, Ethernet Physical Layer
Bridge, Repeater
o Products = implementations of various architecture components
examples:
a concentrator that performs repeating, bridging
an adapter that performs MAC + PHY
frame switching performed
store and forward
cut through
Bridging is a well defined architecture concept. Switching is a commercial
name with different meanings depending on the context. In a LAN context, a
switching Ethernet concentrator is simply a bridge.
LANs
44
Part C: CSMA/CA / IEEE 802.11
o Wireless LANs IEEE 802.11 architecture
l
l
l
l
l
Basic block = BSS (Basic Service Set)
All stations within a BSS share the same
medium
No central control, no connections to the
outside world: ad-hoc network
With a central control, connection to the
outside world: base station = AP (Access
Point)
Multiple APs are interconnected by a
distribution system (DS) to form an
ESS (Extended Service Set)
BSS
BSS
BSS
AP
AP
DS
IEEE 802.11 is the standardized protocol for wireless LANs. Nodes can be fixed
or mobile: it supports mobility, handover
Bit Rate: 1 to 2 Mbps. Much less than in any Ethernet !
LANs
45
Why not just wireless Ethernet ?
o Problems with CSMA/CD:
l
l
l
l
Difficult for the transmitting station to
detect collision in a radio environment,
and therefore to abort due to collision
Even if transmitter detects collision, it
can be hidden by physical obstruction
from other stations, which can not detect
its transmission. A collision may then
occur at the receiver.
Fading can cause the same problem as a
hidden terminal. Time and space varying.
Transmissions by users in other LANs
can interfere with CSMA/CD operations.
LANs
46
o Source:
CSMA/CA (DCF)
ii == 11
while
while (i
(i <=
<= maxAttempts)
maxAttempts) do
do
set
set frame
frame duration
duration length
length in
in duration
duration field
field
listen
until
(channel
is
idle
listen until (channel is idle ++ DIFS)
DIFS)
transmit
transmit
wait
wait for
for acknowledgement
acknowledgement or
or timeout
timeout
if
ack
received
then
leave
if ack received then leave
wait
wait random
random time
time /*
/* collision*/
collision*/
increment
increment ii
end
end do
do
o Destination:
if
if data
data received
received then
then
wait
wait SIFS
SIFS /*
/* SIFS
SIFS << DIFS
DIFS */
*/
send
send ack
ack
end
end if
if
o Others: add virtual CSMA to physical CSMA:
if
if data
data heard
heard then
then
set
set NAV
NAV == frame
frame duration
duration ++ SIFS
SIFS ++ Ack
Ack duration
duration
defer
defer access
access until
until now
now ++ NAV
NAV
end
end if
if
The first network of Apple (Appletalk) was CSMA/CA (collision avoidance) at
230.4 kb/s.
The mode of CSMA/CA described here is valid for the Distributed Coordination
Function (DCF), which is the contention (best-effort) service always
implemented in IEEE 802.11. The contention-free service, called Point
Coordination Function (PCF), is optional.
SIFS = Short Interframe Space
DIFS = Distributed Interframe Space
CS (carrier sensing) is done at the physical and MAC layers:
Physical CS is performed by stations at the air interface, by analyzing the
presence of all detected packets and relative strength of signals from other
sources.
Virtual CS is used at by a source station to inform all stations (except maybe
those hidden to the source) inside a BSS the time during which the channel
will be busy. The frame duration of the source is set in a field contained in
the header. The stations adjust their NAV (Network Allocation Vector), which
indicate the amount of time that will elapse before the current transmission
is complete.
LANs
47
CSMA / CA (1)
A (source) B (destination) others
DIFS
Data
SIFS
NAV
o Because SIFS < DIFS,
acks have priority over
data packets
o NAV does not solve
completely the hidden
terminal (and fading)
problems
o Retransmissions use
exponential backoff
ACK
The NAV is helpful for stations hidden from the receiver, but not from the
transmitter.
LANs
48
CSMA / CA (2)
l
l
A (source) B (destination)
DIFS
RTS
RTS (Request-to-send)
CTS (Clear-to-send)
o CTS reserves the channel to the
source for the duration
indicated in duration field.
o Collisions can only occur
during the short RTS and CTS
o All stations in the range of A
adjust their NAV thanks to
duration field indicated in RTS.
o All stations in the range of B
adjust their NAV thanks to
duration field indicated in CTS.
o Stations can choose to use
CTS/RTS or not.
others
SIFS
CTS
DIFS
Data
SIFS
ACK
NAV (RTS)
NAV (CTS)
o To solve the hidden terminal
problem, use a handshake
procedure:
LANs
49
Facts to Remember
o
o
o
o
o
o
o
o
o
Computers communicate in a local area network using Ethernet and MAC addresses
A MAC address is the serial number of the Ethernet adapter
Original Ethernet is a shared medium: one collision domain per LAN
Using bridging we can have several collision domains per LAN
An Ethernet switch uses bridging
Repeaters are bit-forwarding devices inside one Ethernet segment
Bridges are connectionless intermediate systems that separate Ethernet segments
IEEE 802.11 (wireless LANs) : no collision detection and hidden terminal problem
Concepts you should know
l
l
l
l
Aloha
CSMA/CD
CSMA/CA
shared medium access protocol
Further recommended reading:
Ethernet
[Walrand Varaiya]chapter 3.1-3.2
[Halsall] chapters 6.3.2 and 7.3.2
[BG] chapter 4
Big-LAN FAQ
Ethernet FAQ
Token Bus, Token Ring, FDDI, 100VG, Wireless LANs, other LANs
[Walrand Varaiya]chapter 3.3-3.8
[Halsall] chapters 6
and 7
[BG] chapters 4.5.3 and 4.5.5
Token Ring Network Architecture reference
(IBM doc number SC30-3374-02)