Multiplexing/MAC
Malathi Veeraraghavan
Univ. of Virginia
• Outline
• Types of links
• Multi-access (shared single) link
• Point-to-point link
• Types of multiplexing techniques
• Circuit-based multiplexing
• Packet-based multiplexing
1
Purpose of multiplexing/MAC
• To share link bandwidth
2
MAC
• MAC: Medium Access Control or Media Access
Control
– Set of functions to support the sharing of a single link
by multiple endpoints
• MAC vs. Multiplexing (MUX)
– The term "MAC" is used to describe sharing techniques
on multi-access links
– The term "multiplexing" is used to describe sharing
techniques on point-to-point links
3
Types of links
• Multi-access links
–
–
–
–
Typically used to connect multiple hosts to a switch
Cheaper than point-to-point links
Host
Host ......
Mostly used in wireless networks
Sometimes in wired networks through hub
• Point-to-point links
Host
Switch
Host
Switch
– Typically used between switches
– Increasingly typical between hosts and switches in
wired networks (port costs are decreasing)
Host
Host
4
Analogy of a
MULTI-ACCESS
LINK
- several driveways
attached to one road
Courtesy: http://mars.gmu.edu/dspace/bitstream/1920/2497/1/pca_608_23_16n.jpg
5
Recall our
multiple-link network
Point-to-point link
Point-to-point link
shared using MUX
techniques
Switch
Host
Host
Switch
Host
Switch
Host
e.g., roadways network (an intersection is comparable to a switch)
6
Multi-access wireless link
between hosts and switch
Multi-access wireless link
Host
Switch
Host
Switch
Host
Host
Switch
802.11 wireless
access point
Switch
Equivalent to:
Host
Host ...... Host
7
Image courtesy: http://compnetworking.about.com
Multi-access wireless link
(cell phones)
Base station/cell site
Switch
Switch
Switch
8
Images courtesy of cnet.com and wikipedia
Multi-access wired link
between hosts and switch
Multi-access wired link
Host
Switch
Host
Switch
Ethernet hub
Switch
Host
Host
Recall a hub is a multipoint repeater
from tasks/layers class notes: shared single-link network
9
Image courtesy: wikipedia
Usage of links
• Point-to-point links
– Host-to-switch, or more generally endpoint-toswitch (e.g., telephone, video camera)
– Switch-to-switch
• Multi-access links
– Endpoint-to-switch
10
Back to outline (status check)
• Outline
• Types of links
• Multi-access link
• Point-to-point link
 Types of multiplexing techniques
• Circuit-based multiplexing
• Packet-based multiplexing
11
Classification of
Multiplexing/MAC techniques
Multiplexing/MAC techniques
Circuit-based multiplexing
Position based:
• space
• time
• frequency
Each multiplexed data
stream occupies a
different position
Packet-based multiplexing
Packet header based:
• header carries
destination address
Each multiplexed data
stream consists of
packets with headers
carrying corresponding
destination addresses
12
Circuit-based multiplexing
• Frequency Division Multiplexing (FDM)
– Called Wavelength Division Multiplexing
(WDM) in the optical range
• Time Division Multiplexing (TDM)
• Space Division Multiplexing (SDM)
– Each fiber in a fiber bundle
13
Frequency Division Multiplexing
Signal 1
Signal 2
Signal 1
Signal 3
Signal 2
Signal 3
(a) The original
signals
(b) The signals modulated
on to different carrier frequencies
(c) The multiplexed
signal
Tanenbaum
14
Frequency Division Multiplexing (FDM)
• Each communication session is assigned its own frequency for the
session
• A few control channels (frequencies) are set aside to allow users to
send requests for frequencies for their communication sessions (these
requests are called "call setup signaling protocol messages")
• Frequencies are released upon completion of the session (with "call
release signaling protocol messages")
• Modulation technique determines the required carrier spacing (e.g., 30
kHz for analog cellular) and correspondingly the number of
simultaneous sessions
• Examples
– Each broadcast radio and TV station is assigned a different carrier
frequency – long-held “sessions”
– Analog cellular systems: two frequencies are assigned – one for reception,
another for transmission to each cellular caller
15
Time Division Multiplexing (TDM)
• Each communication session is assigned time slots for its
session on one or more frequencies
• A few control channels (time slots) are set aside to allow
users to use for signaling messages, i.e., to send requests
for timeslots for their communication sessions
• Timeslots are released upon completion of the session
• Examples
– Classroom being shared by multiple classes one after another in
time
– Digital cellular systems: US system has three users sharing one
carrier frequency for a cellular call
16
Frequency and time:
many practical systems are hybrid
Carrier
Time
Hybrid FDM/TDM
Frequency
TDM
Frequency
Frequency
FDM
Time
Time
Basic principle of communication: Two regions in the time-frequency
plane with equal areas can carry the same amount of information
17
Use of circuit-based multiplexing on
different types of links
• Multi-access wireless link
– Cellular phones
• Point-to-point links between switches
18
Use of FDM and TDM techniques on a
multi-access link
• FDM sharing on a multi-access link is referred to
as FDMA (Frequency Division Multiple Access)
• TDM sharing on a multi-access link is referred to
as TDMA (Time Division Multiple Access)
19
Forward and reverse channels
Reverse
channels
Switch
Forward channels
Switch
Switch
Base station/cell site
20
Images courtesy of cnet.com, wikipedia, google
Example of FDMA scheme: Advanced
Mobile Phone System (AMPS)
• FDMA/FDD
– Spectrum allocation by FCC: A and B allocations to
different providers
Reverse
A
Original
B
825
Extended A A
824 825
Forward
A
B
845
B
870
AB
A A
845 849 869 870
890
B
AB
890 894
20MHz
 666channels
30kHz
25MHz
 832channels
30kHz
20 MHz is the width of the allocated band: 845-825 and 890-870
30 kHz is the per-channel bandwidth required for an AMPS phone call
21
Duplex techniques
• Separates signals transmitted by base
stations from signals transmitted by
terminals
– Frequency Division Duplex (FDD): use
separate sets of frequencies for forward and
reverse channels (upstream and downstream)
– Time Division Duplex (TDD): same
frequencies used in the two directions, but
different time slots
22
Dimension of space
• Reuse the frequencies used at a cell C at
another cell D that is far enough from C
23
Hexagonal cell frequency plan
•
R
D
•
•
•
•
D: Distance between a base station and
the nearest base station that uses the
same channels
R: Radius of a cell
Reuse distance = D/R
Channel plan: method of assigning
channels to cells to guarantee a
minimum reuse distance between cells
that use the same channel
S/I: Signal to Interference Ratio
D D
S S
 
which is the minimum reuse distance for which
 
R  R  req
I  I  req
24
Reuse factor
(frequencies are reused every N cells)
• Divide the set of available channels into N groups
• N: reuse factor; select N such that cells assigned the same
frequencies will have a D:R ratio greater than (D:R)req
• For hexagons, reuse factor N is given by
1  D 2
N  
3  R  req
• Practical values of N
– range from 3 to 21
– most commonly used: 7 (D/R = 4.6)
25
Different reuse patterns (factors)
26
Service provider A
•
•
•
•
•
Has a total of 832/2 = 416 channels
Set aside 21 for control channels for each provider
Therefore 416-21 = 395 traffic channels per provider
Per cell, we can have 56 and 3/7 channels if N=7
Four cells are given 56 channels and three cells are given
57 channels
27
Practice: IS136 NA-TDMA
• NA-TDMA is a hybrid FDMA/TDMA scheme
• Therefore each frequency will have time slots that
are shared by multiple calls
• Typical: three calls share one frequency
• NA-TDMA is three times as efficient
• Same frequency allocation as for AMPS
• Carriers are 30 kHz apart - Bandwidth
28
The TDMA aspect:
frames and time slots
base station to mobile
6
1
2
3
4
5
6
1
2
3
4
45 MHz
or
80 MHz
mobile to base station
6
1
2
3
4
5
6
1
2
3
4
5
40ms
•
Every frame is 40ms long and consists of 6 time slots
29
What data (transmission) rate can be
used at each carrier frequency?
•
•
•
•
Each time slot carries 324 bits
There are 6 timeslots/frame
1 frame is 40ms
Therefore, data rate per carrier (frequency) is:
324bits / timeslot  6timeslots / frame
 48.6kb / s
40ms / frame
30
Data rate of a carrier (frequency)
• Voice calls in NA-TDMA
–
–
–
–
–
Speech codec rate: 7.95kbps
Channel coding rate: 13kbps
Fits within 16.2kbps
One voice call needs two timeslots per frame
So three calls can be carried at each carrier frequency
31
Use of circuit-based multiplexing on
different types of links
• Multi-access wireless link
– Cellular phones
Point-to-point links between switches
32
Techniques used in
practice today
• Time-division multiplexing
– Plesiochronous Digital Hierarchy (PDH)
• DS0, DS1, DS3 (one phone call: DS0)
– Synchronous Optical Network (SONET) - US
– Synchronous Digital Hierarchy (SDH) - others
• Wavelength-division multiplexing (on fiber)
33
Time Division Multiplexing:
The T1 carrier (1.544 Mbps)
Input links
Output link
1
2
.
.
24
1
2
24
...........
8 bits
every 125s
8 * 24 + 1 = 193 bits
every 125s
RATE: ?
RATE: ?
Round-robin cycle through and
transmit 8 bits each from input 1 through 24
onto output link and then start again from input 1
34
North American Digital
Multiplexing Hierarchy
1
DS1 signal, 1.544Mbps
Mux
24
1
DS2 signal, 6.312Mbps
24 DS0
4 DS1
Mux
4
1
7 DS2
DS3 signal, 44.736Mpbs
Mux
7
•
•
•
•
•
DS0,
DS1,
DS2,
DS3,
DS4,
64 kbps channel
1.544 Mbps channel (T1)
6.312 Mbps channel
44.736 Mbps channel (T3)
274.176 Mbps channel
1
6 DS3
Mux
6
DS4 signal
274.176Mbps
35
Synchronous Transport Signal
(STS-1) Frame
90 columns
90 x 9 = 810 bytes
810 x 8 bits/125 sec = 51.84Mbps
STS-1 rate = 51.84Mbps
Byte Byte
1
Order of
2 transmission
9 rows
SPE is 87 columns
Frame period = 125 sec
Overhead (3 columns):
error detection bits, etc.
Line overhead +
Section overhead
Payload called Synchronous
Payload Envelope (SPE)
Path overhead:
one column inside SPE
36
SONET/SDH rates
(number is the multiplier)
Optical
Carrier
How many bits are carried within a single SONET frame of an OC48 signal?
How many bits of the above number can be used to carry user data (payload)?
Verify that the SPE data rate in the table above is correct for the OC48 signal.
37
Tanenbaum
Back to outline (status check)
• Outline
• Types of links
• Multi-access link
• Point-to-point link
• Types of multiplexing techniques
• Circuit-based multiplexing
 Packet-based multiplexing
38
Packet-based multiplexing
• For point-to-point links
– Scheduling techniques
• For multi-access links
– Random access MAC schemes
39
Packet-based multiplexing on a
point-to-point link
Scheduling algorithms: FCFS, priority
Header
Data payload
A
Packetbased
multiplexer
B
Output link
C
Input links
Packet: a string of bits
Payload: user-data bits being sent
Header: overhead added by protocol
40
Packet buffering
• Buffers hold packets waiting to be transmitted on output
link
– FCFS: Single buffer
– Priority: As many buffers as there are priority classes
• When the transmitter is ready to choose the next packet for
transmission
– FCFS: Select next packet in line if buffer is non-empty
– Priority: Check buffers in order of priority and transmit packet
from the first non-empty buffer
41
Rules for order of
packet transmission
• If a packet P arrives when the transmitter is free, and all
buffers are empty, then the packet will be transmitted
immediately. Add packet transmission time to packet
arrival time to determine packet departure time.
• If a packet P arrives into a buffer while another packet Q is
being transmitted, packet P is served only after
– Packet Q is fully transmitted (non-preemption), and
– Higher-or-equal priority packets that are already in the buffers are
transmitted.
– Based on scheduling discipline, and the number of packets ahead
of packet P in the buffers, determine departure time for P.
42
Examples of packet multiplexing
• Point-to-point links between switches
• Point-to-point link between endpoint and
switch
43
Packet-based multiplexing on a
point-to-point link between switches
Switch
Host 2
Host 1
input
link
Host 3
input
link
output
link
Packet-based
multiplexer
Switch
Host 4
44
Packet-based multiplexing on a
point-to-point link between switches
Switch
Host 2
Host 1
App. 2
App. 2
Switch
Host 3
Host 4
App. 1
App. 1
Two communication
sessions sharing this switch-to-switch link
45
Packet-based multiplexing on a point-topoint link between a host and a switch
Host 1
App. 2
Packet-based
multiplexer
Host 2
App. 3
App. 2
Switch
Host 3
Switch
Host 4
App. 3
46
Packet-based multiplexing on a point-topoint link between a host and a switch
Host 1
App. 2
Host 2
App. 3
App. 2
Switch
Host 3
Host 4
App. 3
Two communications sessions sharing this host-to-switch link
47
Back to outline (status check)
• Outline
• Types of links
• Multi-access link
• Point-to-point link
• Types of multiplexing techniques
• Circuit-based multiplexing
• Packet-based multiplexing
o For point-to-point links
- Scheduling techniques
 For multi-access links
- Random access schemes
48
Random access MAC protocols
• No reservations are made; instead a host
just sends data packets
• What can happen?
– Collision (recall multi-access)
• Need to avoid collisions or detect collisions and retransmit
– What’s the cost of being too careful to avoid collisions?
• Utilization will be sacrificed
49
Analogy of a
MULTI-ACCESS
LINK: a car pulls up
to a road; driver
looks right/left before
entering - called
"carrier sensing"
Courtesy: http://mars.gmu.edu/dspace/bitstream/1920/2497/1/pca_608_23_16n.jpg
50
Random-access MAC
(packet based sharing on multi-access links)
•
•
•
•
ALOHA: just send & wait for ACK
Slotted ALOHA: send in slots
CSMA: sense carrier, but wait for ACK
CSMA/CD: detect collisions instead of
waiting for ACK
• CSMA/CA
51
ALOHA
• Simplest scheme
• True free-for-all. When a node needs to send, it
does so. It listens for an amount of time equal to
the maximum round trip delay plus a fixed
increment. If it hears an acknowledgment, fine;
otherwise it resends after waiting a random
amount of time. After several attempts, it gives up.
• Low delay if light load
• Max. utilization: 18%
52
Slotted ALOHA
• All frames are of same size L
• If link rate is C, time is divided into slots of size L/C (a slot
equals the time to transmit one frame)
• Nodes can transmit frames only at the beg. of slots
• Nodes are sync’ed so each node knows when slots start
• If more than one node sends, all nodes detect the collision
even before the slot ends
• Vulnerable period is one-way prop. delay, not two-way as
in ALOHA. So maximum throughput is double: 37%.
From Kurose and Ross
53
CSMA
• Carrier Sense Multiple Access
– sense carrier
– if idle, send
• wait for ack
– If there isn’t one, assume there was a collision, retransmit
– if busy, wait
• Vulnerable period: one tprop
54
Types of CSMA schemes
•
1-persistent:
– if busy, constantly sense channel
– if idle, send immediately
– if collision is detected, wait a random amount of time before retransmitting
•
Non-persistent:
–
–
–
–
–
•
sense channel when station has a packet to send
if busy, wait a random amount of time before sensing again;
if idle, send immediately
collisions reduced because sensing is not rescheduled immediately
drawback: more delay
p-persistent: combines 1-persistent goal of reduced idle channel time with the
non-persistent goal of reduced collisions.
– sense constantly if busy and the station needs to send a packet
– if the channel is idle, transmit packet with probability p
– with probability 1-p station waits an additional tprop before sensing again
55
CSMA/CD
• CSMA/Collision Detection (CSMA/CD):
– In CSMA, if collision occurs, need to wait until
damaged frames have fully propagated. For long frames
compared to propagation delay, this could lead to
significant waste of capacity. So add collision
detection.
– Listen for collision and immediately suspend sending
data if collision is detected.
– Rule: Frames should be long enough to allow collision
detection prior to the end of transmission
56
CSMA/CA: 802.11
• Why CA (Collision Avoidance) and not
CD?
– difficult to receive (sense collisions) when
transmitting due to weak received signals
(fading)
– hidden station problem:
• Two mutually far away stations A and C want to
send to B.
• At A and C, channel appears idle
• But collision occurs at B
57
Kurose and Ross’ slides
Random-access MAC protocols
used in practice today
• Multi-access link
• Wired: Ethernet
– CSMA/CD scheme
• Wireless: IEEE 802.11
• Wireless and mobile networks class
58
Example of CSMA/CD: Ethernet
Ethernet protocol:
1.
Each station listens before it transmits.
2.
If the channel is busy, it waits until the channel goes idle, and then it transmits.
3.
If the channel is idle it transmits immediately. Continue sensing.
4.
If collision is detected, transmit a brief jamming signal, then cease transmission, wait
for a random time, and retransmit.
•
collision detection is not by waiting for an ACK
59
Collisions in Ethernet
•
•
The collision resolution process of Ethernet requires that a collision is detected
while a station is still transmitting.
Assume: Maximum propagation delay on the bus is tprop.
t0
A
A Begins Transmission
B
A
B Begins Transmission
B
t0+tprop-
60
Collisions in Ethernet
t0+tprop
A
B Detects Collision
B
A
A Detects Collision
Just Before End
of Transmission
B
t0+2tprop
•
Restrictions: Frame should be at least as long as 2tpropr, where r is the
transmission rate of the link, and tprop is the max. one-way propagation delay
61
Exponential Backoff Algorithm
• If a station is involved in a collision, it waits a random
amount of time before attempting a retransmission.
• The random time is determined by the following
algorithm:
• Set “slot time” to 2tprop.
• After first collision wait 0 or 1 slot time.
• After i-th collision, wait a random number between 0 and 2i-1
time slots.
• Do not increase random number range if i=10.
• Give up after 16 collisions.
62
Ethernet performance
• max: maximum efficiency under
heavy load conditions
• S: length of frame; r: data rate
 max 
S
r
S t
 2t prop e
r prop
e  2.718
• tprop: one-way propagation delay
• e: average number of contention slots
before success in getting the medium
• 2tprop is contention slot
• Frame length assumed to be fixed in
deriving this formula; not true for
Ethernet (approximation)
63
Ethernet frame format
Dst. Src.
Type
Addr. Addr.
6
6
2
Example MAC address:
04-3C-5A-11-26-78
Type
0800
2
Data
CRC
46-1500
4
IP
datagram
46-1500
number of bytes
Dst. Addr: Destination address (6-byte MAC address)
Src. Addr: Source Address (6-byte MAC address)
Type: What type of payload is being carried in frame
- e.g., IP datagram: 0800 (hexadecimal)
CRC: Cyclic Redundancy Code (Error detection)
64
Ethernet protocol
support for DLL functions
• Destination and source MAC address fields
– for multiplexing
• CRC
– for error detection
• No sequence numbers/ACK numbers
– for error correction
• Pause feature
– for ON/OFF flow control (special control frame)
65
Summary
Multiplexing/MAC
schemes
Types of links
Multi-access wireless link
Circuit-based
multiplexing
Cellular
(FDMA/TDMA)
Multi-access wired link
Packet-based
multiplexing
IEEE 802.11
(WiFi)
Ethernet hub
Point-to-point switch-toswitch link
PDH, SONET, WDM
Ethernet switch
Point-to-point endpoint-toswitch link
Plain Old Telephone
Service (POTS)
(space division
multiplexing)
Ethernet
Phone links from residences carry only one phone call and
hence it is space-division multiplexing;
DSL: new technology for multiplexing data with voice
66
Supplemental reading
• Tanenbaum's 4th edition
– Section 4.3.3 (The Ethernet MAC sublayer protocol)
– Section 4.3.4 (The binary exponential backoff algorithm)
• Leon-Garcia/Widjaja's 2nd edition
–
–
–
–
–
Section 4.1 (Multiplexing)
Section 4.2.1 (SONET multiplexing)
Section 5.7.1 (Statistical multiplexing)
Section 6.4.1 (FDMA)
Section 6.4.2 (TDMA)
67
Acknowledgment
• Few of the slides in this talk are taken from
A. Tanenbaum’s textbook web site and a
few from A. Leon-Garcia and I. Widjaja’s
textbook web site.
68