Objectives
Understand shared medium access methods of Ethernet;
Describe network aspects for an Ethernet network;
Local Area Networks: Ethernet
Dots. Toomas Tommingas
Chair of Real-Time Systems (RAS)
PART A: The CSMA/CD Access Method
PART B: Network Aspects
©1998 T.Tommingas, TTU
©1998 T.Tommingas, TTU
LAN 1
LAN 2
Motivation for LANs
goal: connect computers in same site (building, small
campus)
experience from host centric networks: bursty traffic
basic idea: share a cable, no complex software in end
system
Part A: The CSMA/CD Access Method
altenatives?
❍ switch based LANs: connection oriented: ATM
❍ switch based LANs: connectionless. Switched Ethernet
©1998 T.Tommingas, TTU
Access Method
❍ multiaccess communication = share a communication
medium
LAN 4
Access Method Topology
Two topologies are used
❍bus:
❑
❑
❑
❑
❑ radio channel, cellular networks, satellite links
❑ machine bus
❑ local area cable
❍ shared medium requires an Access Method
all bits sent by one station are propagated to all stations
data die at end of bus
all stations see all frames
used by Ethernet, Token Bus
❍ring:
❑ deterministic:
❑ all bits are passed from one station to next station, then to
next’s neighbour, etc
❑ bits eventually return to originating station which has to
remove them
❑ all stations see all frames
❑ used by Token Ring and FDDI
¤ Time Division Multiple Access (TDMA)
¤ Token Passing (Token Ring, Token Bus, FDDI)
❑ non-deterministic
¤ Aloha
¤ CSMA/CD
©1998 T.Tommingas, TTU
©1998 T.Tommingas, TTU
LAN 3
LAN 5
©1998 T.Tommingas, TTU
LAN 6
ALOHA
Access Method Topology (cont.)
Central
host
Access Method topology = physical
topology = topology used by bits
❍logical topology = topology used by the token in case
of Token Bus (logical ring over a physical bus)
❍cabling topology = layout of cables = star in most
cases (see later)
©1998 T.Tommingas, TTU
data
ack
transmission
i procedure
= 1
while (i ≤ maxAttempts) do
send packet
listen for ack during one
RTD
if ack received then leave
else resend
increment I
end do
©1998 T.Tommingas, TTU
LAN 7
LAN 8
CSMA
G
CSMA
improvement 3: detect collisions as soon as they occur
: “Carrier Sense Multiple Access / Collision Detection”
Improvement 1: Listen before you talk:
“Carrier Sense Multiple Access”
J
# 7 ≤ 1 # /1 !"#$
. % 3
.% 4 0 /& ' '
% D F 5 . /0 & '0 5 0 /
F D# * /& % % /. & * .
6'& !K6/0 6 6/& !
* #% %(,
& /. & 6 * 5' .0 /& '
5 6 6/+ & 9
% /&2% 6/< 2 0 /& ' & D FK 5 /0'L,MN O P / -Q ! F D
' 4 #0 /& . 6'>
*
#& ? " 0 /& ≤ !#"#$
%
%*'+
%-
./%
'&)('
& '.&'
&, % 3 1*'8 2 4 % .'9
4 0 / &,
1 5 6'07
'1 //A,& B': 6/01 2'6/( ; * 8 * < & 6 .>= ,#?@
4 70 '& 0 . 6/ C ED F * 6 % % 6'&1F D
*
? " & 0 //& H
I
Improvement 2: Wait random time before
retransmission
©1998 T.Tommingas, TTU
©1998 T.Tommingas, TTU
LAN 9
CSMA/CD Time Diagram 1
R
S
A senses idle
channel, starts
transmitting
A
©1998 T.Tommingas, TTU
A
T
A senses collision,
continues to
transmit 32 bits
(“jam”)
0
T
U
LAN 11
LAN 10
CSMA/CD Time Diagram 2
B
shortly before T, B
senses idle channel,
starts transmitting
improvement 4: acknowledgments replaced by CD
This is Ethernet (≈ 802.3, the standard conformant
version of Ethernet)
B
0
T
B senses collision,
continues to
transmit 32 bits
(“jam”)
©1998 T.Tommingas, TTU
LAN 12
CSMA/CD Time Diagram 3
A
Z
Y
X
W
V
A waits random time t1
B waits random time t2
B senses channel idle
and transmits
A senses channel busy
and defers to B
A now waits until
channel is idle
CSMA/CD
[
B
0
\
]
_
`
t2
Stations detect collisions and stop
transmitting.
Re-attempt to transmit after a random time.
Acknowledgements ^ absence of collision.
The interframe delay (“gap”) is 9.6 µs.
If repeated collisions occur, then the time
before retransmission increases
exponentially.
t1
©1998 T.Tommingas, TTU
©1998 T.Tommingas, TTU
LAN 13
Minimum Frame Size
Exponential Backoff
a
random time before re-transmission is given by:
c
t = 0:
d
t = 1- ε: B begins transmission
e
t=1:
f
t = 2- ε: A detects collision
k = min (10, AttemptNb)
r = random (0, 2k - 1) * slotTime
b
LAN 14
examples:
❍ first retransmission attempt:
B
A
B
A
B
A
B
B detects collision,
stops transmitting
k = 1; r = 0 or r = slotTime
❍second retransmission attempt (if preceding one failed):
A
A begins transmission
k = 2; r = 0, 1, 2 or 3 * slotTime
©1998 T.Tommingas, TTU
©1998 T.Tommingas, TTU
LAN 15
SlotTime and Minimum Frame Size
g
CSMA/CD Performance
a minimum frame size equal to number of bits transmitted during
one round trip is required to detect all collisions
slotTime = number of bits transmitted by a source during the
maximum round trip time for any ethernet network
❑ slotTime = round trip time + jam time + safety margin
❑ = 512 bits (corresponding to 51.2 µs)
h
k
Bound on throughput:
θ ≥ 1 / ( 1 + 3.1 α )
where α = β / L = 2 * propagation delay / transmission time
with L = frame size, β = bandwidth-delay product
Approximation:
includes propagation time in repeaters + margin:
4 repeaters + 5 segments + 2 stations = 2*21.2 µs + 2*1 µs = 44.4 µs
j
i
l
o rule: in Ethernet, all frames must be as large as slotTime
o properties:
P1: all collisions are detected by sources
P2: collided frames are shorter than slotTime
LAN 16
m
θ ≈ 1 / ( 1 + 2.5 α )
o Key for high utilization is: bandwidth delay product <<
frame size
Proof: P1 see previous slide
P2 because collided frame are aborted by source at the latest after slotTime, including
jam bits
©1998 T.Tommingas, TTU
LAN 17
©1998 T.Tommingas, TTU
LAN 18
Bandwidth Delay Product
n
Bandwidth Delay Product
L [m]
Interpretation of bandwidth-delay product
time
A
v [m/s]
o
β= 2DR
LAN 19
2.3E+08 8.7E-07
2.3E+08 4.3E-06
2.3E+08 4.3E-05
2.3E+08 0.15652
100000
100000
100000
100000
0.17
0.87
8.70
31304.35
0.02
0.11
1.09
3913.04
0.00
0.00
0.00
3.82
100000000
100000000
100000000
100000000
86.96
173.91
869.57
8695.65
10.87
21.74
108.70
1086.96
0.01
0.02
0.11
1.06
1E+09
869.57
108.70
1E+09
1739.13
217.39
1E+09
8695.65
1086.96
1E+09
86956.52 10869.57
1E+09 ########## #########
0.11
0.21
1.06
10.61
38213.32
We have:
x1 = R
x2 = 1/µ in average
E(x3 | collision occured) = 2R; Prb (collision occured) = 1 - exp(-Rµ)
E(x3 | successful transmission ) = T; Prb (successful transmission ) = exp(Rµ)
The last formula is because collisions can occur only if an arrival
occurs during the propagation time R, because of collision avoidance. The
average cycle time is thus, for this worst case scenario:
τ
= R + 1/µ + 2R(1- exp(-Rµ)) + T exp(-Rµ)
and the corresponding utilization:
θmax = average useful time per cycle / average cycle duration
= T exp(-Rµ) / τ
computing the maximum of θmax with respect to x = R µ gives the
formula (maximum obtained for x = 0.43). Note that α = 2R / T.
r
q
100
200
1000
10000
2.3E+08
2.3E+08
2.3E+08
2.3E+08
2.3E+08 4.3E-07
2.3E+08 8.7E-07
2.3E+08 4.3E-06
2.3E+08 4.3E-05
2.3E+08 0.15652
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%.
©1998 T.Tommingas, TTU
LAN 21
LLC
MAC Frame
MAC
MAC
MAC
PHY
PHY
bits
PHY
Application
Presentation
Session
Transport
LLC
Logical Link Control
MAC
Media Accass Control
PLS
Physical Layer Signaling
Network
-DTE
DTE
(AUI not
exposed)
AUI
Data Link
AUI = Attachment Unit Interface
MAU = Medium Attachment Unit
MDI = Medium Dependent Interface
MAU
PMA
MDI
Medium
PMA = Physical Medium Attachment
©1998 T.Tommingas, TTU
LAN 22
❍Extend network beyond cable
length limit
❍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
Repeater
❍One repeated network = one
collision domain
❍Even with repeaters, network is
limited:
❑ propagation time
❑ 51.2us slotTime includes repeaters
❑ at most 4 repeaters in one path
❑ Repeaters perform physical layer
functions only (bit repeaters)
❍data packet = MAC service data unit (SDU)
❍MAC frame = MAC protocol data unit (PDU)
©1998 T.Tommingas, TTU
LAN
ETHERNET
LAYERS
Repeaters
•data packet
•status information
data packet
LAN 20
OSI
REFERENSE
MODEL LAYERS
MAC Service
LLC
0.00
0.01
0.11
382.13
©1998 T.Tommingas, TTU
Physical
the approximation shown is based on simulations
LLC
4.3E-07
8.7E-07
4.3E-06
4.3E-05
17.39
2.17
86.96
10.87
869.57
108.70
3130434.78 391304.35
IEEE Architecture Model
CSMA/CD Performance
p
[KB]
200
1000
10000
36000000
100
200
1000
10000
36000000
©1998 T.Tommingas, TTU
ß=2*D*R [bit] [Byte]
10000000
10000000
10000000
10000000
last bit sent by A arrives
large β means: delayed feedback
R [bit/s]
2.3E+08 8.7E-07
2.3E+08 4.3E-06
2.3E+08 4.3E-05
2.3E+08 0.15652
B
B says: “stop”
D[s]
200
1000
10000
36000000
LAN 23
©1998 T.Tommingas, TTU
LAN 24
From Repeaters to Hubs
❍Multiport repeater:
From Bus to Star and Tree
❑ (n ports) logically
equivalent to n simple
repeaters connected to one
internal Ethernet segment
t
❑ Multi-port repeaters it is
possible to use point-topoint segments (Ethernet in
the box)
❑ Value of point to point
cabling ?
u
Ethernet Hub
S1
¤ ease of management
¤ fault isolation
UTP segment
S3
v
Multiport
Repeater
S2
©1998 T.Tommingas, TTU
Ethernet today = active
fiber
coax
console
concentrators allow star
Intermediate
Intermediate
wiring
Hub
Hub
NMA
NMA
UTP on point-to-point UTP
coax
configurations only
Intermediate
remote network
Hub
NM Application
NMA
management
transceiver
How many frames can be
cable
transmitted in parallel in
this network?________
©1998 T.Tommingas, TTU
LAN 25
Repeaters and Bridges in OSI
7
6
5
4
3
2
1
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
LLC
(MAC
Frame)
(MAC Frame)
MAC
Network
LLC
MAC
MAC
Physical
Physical
Physical
Physical
End system
Repeater
Bridge
End system
❑ Bridges operate at layer 2
❑ Repeaters operate in layer 1
❑ Layer 3 intermediate systems - routers
LAN 27
LAN 26
Switched Ethernet
7
6
5
4
3
❑ Switched Ethernet = Bridge in the Box
❑ Total Bandwidth is not shared: parallel frame transmission
❑ An Ethernet Switch is a connectionless data switch
❑ Ethernet used as a point-to-point mechanism!
2
Frame Switching Hub
1
Frame Switching Hub
Bridge
B1
1
A
©1998 T.Tommingas, TTU
Head
hub
Head hub
NMA
s
Multiport
Repeater
2
B
3
Bridge
B2
4
1
5
C
D
U
2
3
V
4
5
W
©1998 T.Tommingas, TTU
X
LAN 28
Today’s Concentrators
❑ Concentrators (=hub) combine bridging (frame switching) and port
switching (assign repeater ports to the same collision domain)
❑ NB! Broadcast!
Frame Switching Hub
1
A
B
2
3
4
C
Bridge
3a
B2
1
5
D
MAC
Frame
U
2
V
3
4
W
1 octet
Repeater
5
LLC
PDU
LLC PDU
CRC
DSAP
1
SSAP
1 or 2
variable
LLC control
Information
X
LLC
Address
Fields
©1998 T.Tommingas, TTU
MAC
Destination
Source
Preabmle control
MAC address MAC address
Frame Switching Hub
Bridge
B1
MAC Frame Format
7+1
LAN 29
I/G
DSAP value
©1998 T.Tommingas, TTU
I/G
SSAP value
LAN 30
LLC - LSAP
MAC Addresses
7+1
MAC
Frame
LSAP - LLC Access point, SAP Source and Destination
VALUE
00
10
AA
E0
F0
F5
Assignment
TCP/IP (DoD)
NetWare
SNAP (Subnetwork Access Protocol)
NetWare
NetBios
LAN Network Manager Group
MAC
Destination
Source
Preabmle control
MAC address MAC address
Manufacturer
ID
NIC
ID
I/G U/L
Address in 46 bits
Address in 15 bits
©1998 T.Tommingas, TTU
LAN 31
LAN 32
Token Ring (IEEE 802.5)
Frame
pa
SD
AC
FC
Token
pa
SD
AC
ED
PPP
AC
JK1JK1
ED
ACrr
FS
T
M
I
DA
RRR
E
ACrr
SA
Data
FCS
FDDI
ED
FS
PPP Priority bits; T Token bit
M monitor bit; RRR Reservation bits
Frame
pa
SD
FC
DA
Token
pa
SD
FC
FS
FC
C L
J,K non-dada (CPFSK);
I intermiediate frame; E error bit
ED
A address recognition bit;
C Copy bit; r r - reserved
FS
©1998 T.Tommingas, TTU
EACr
I
E
EACr
Data
FCS
ED
FS
C synchr./asynchr.; L 16/48 bit addressing
FF (LLC/MAC/..); ZZZZ control frame type
J,K non-dada (CPFSK);
I intermiediate frame; E error bit
E error recognized; A address recognized;
C frame copied; r - reserved
LAN 34
FDDI Capacity Allocation
y
❑ seize token by aborting the token transmission instead of
flipping the T-bit;
❑ early token release (already in 16 Mbps version)
z
Frame Status (FS) field
❑ each station checks any passing frames and sets E bit
accordingly
❑ MAC protocol does not attempt to retransmit the frame with E
bit set. It is the responsibility of LLC or some higher-layer
protocol.
©1998 T.Tommingas, TTU
ZZZZ
SA
©1998 T.Tommingas, TTU
LAN 33
Key differences compared to 802.5:
x
FF
JK1JK1
FDDI MAC protocol
w
CRC
I/G = 0 Individual address
I/G = 1 Group address
U/L = 0 Globally administered
U/L = 1 Locally administered
3 octets
Address
Field
I/G
©1998 T.Tommingas, TTU
LLC PDU
LAN 35
{
The priority scheme of 802.5 not applicable due to
“early token release”
FDDI capacity-allocation scheme seeks to
accommodate a mixture of stream and bursty
traffic
use of synchronous and asynchronous frames
©1998 T.Tommingas, TTU
LAN 36
100 BASE-X
FDDI Capacity Allocation
IEEE 802.3 (10O-Mbps)
|
TTRT - Target Token Rotation Time:
10OBASE -X
DMax + FMax + TokenTime + Σ Sai ≤ TTRT
10OBASE -TX
Sai
DMax
FMax
TokenTime
2 Cat 5 UTP
©1998 T.Tommingas, TTU
LAN 37
100 BASE-TX
2 pair, STP
4B5B,
MLT-3
Signalling
technique
2 Optical Fiber
4 Cat 3 or Cat 5 UTP
2 STP
©1998 T.Tommingas, TTU
LAN 38
Bus A
100 BASE-FX 100 BASE-T4
2 pair,
2 optical fibers 4 pair, Cat 3,
Cat 5 UTP
4 or 5 UTP
4B5B,
4B5B,
8B6T,
MLT-3
NRZI
NRZ
100 Mbps
100 Mbps
100 Mbps
100 Mbps
Maximum
segment length
100 m
100 m
100 m
100 m
Network span
200 m
200 m
200 m
200 m
Data rate
10OBASE -T4
DQDB - Distributed Queue Dual Bus
IEEE 802.3 Physical Layer
Transmission
medium
10OBASE -FX
= synchronous allocation for station i
= propagation delay for one complete cirquit of the ring
= time required to transmit a max length frame (4500 o)
= time required to transmit a token
Transmit data when
no downstream (A)
requests are pending;
transmit free slot for
each pending request
Receive
segments
addressed to x
Node 0
=
head (A)
Keep a count of
downstream (A)
requests
Transmit data when
free slot passes and no
downstream (A)
requests are pending
Node x
Set request
bit to reserve
future slot
on bus A
Receive
segments
addressed
to(N-2)
Transmit
when free
slot passes
Node (N-2)
Keep a count of
downstream (A)
requests
Receive
segments
addressed
to (N-1)
Node (N-1)
=
head (B)
Set request
bit to reserve
future slot
on bus A
Bus B
©1998 T.Tommingas, TTU
LAN 39
DQDB (IEEE 802.6)
Frame, 125µs
Header
m Byte
Slot 0
53 Byte
Slot N
53 Byte
Stuffing
0-52 B
Slot 53 Byte
Header
m Byte
Busy
1 bit
Segment, 52 Byte
Sl-Type
1 bit
PSR
1 bit
Reserv
2 bits
Request Header
3 bits
4 Byte
Payload 48 Byte
Previous Slot Reserved
©1998 T.Tommingas, TTU
LAN 41
©1998 T.Tommingas, TTU
LAN 40