Video Codec for Multimedia Communications

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COSC 3213 Computer Networks I Instructor Dr.
Amir Asif Department of Computer Science York
University Handout 7 Section M Multiple Access
Control (MAC) Topics 1. Multiple Access
Communications Channelization vs MAC 2. Data
Link Layer MAC and LLC sublayers 3. MAC Random
Access Protocols vs Scheduled Approaches 4. LAN
Standards Token Bus, Token Ring, FDDI and
Wireless LANs 5. LAN Bridges Garcia Sections
6.1 6.4 and 6.6
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Classification of Networks (1)

• There are two basic types of networks
• Switched Networks
• Interconnect users by transmission lines,
  multiplexers, and switches
• Addressing of hosts is hierarchical to scale to
  large sizes
• Multiple paths available between the source and
  destination
• Addressing of hosts is hierarchical to scale to
  large sizes
• Transmission of packets require routing table

Subnet
Router
Host
LAN
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Classification of Networks (2)

• Broadcast Networks
• All machines connected to each other using a
  shared transmission medium.
• Addressing of hosts is flat (based on NIC) due
  to a small number of host machines.
• Only one path is available between source and
  destination, Routing is not needed
• Requires medium access protocol (MAC) to
  coordinate transmissions between different
  machines.
• Pros low cost infrastructure complex routing
  algorithms are not required
• Cons A MAC protocol is needed to minimize
  collisions and ensure fair sharing of the medium
  Transmission efficiency is low due to collisions.

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MAC Approaches
MAC Multiple users share the communication
channel so a scheme (medium sharing technique)
must be devised to prevent collision of packets.
The strategy is referred to as the MAC protocol.
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Example of MAC (1)

• Satellite Communications
• Two frequency bands one for uplink (5M 42M)
  one for downlink (550M 750M)
• Each station is allocated a channel (2M) in the
  uplink frequency band
• Satellite is a repeater that changes the
  carrier frequency repeats message
• Each station has a channel (6M) in the downlink
  frequency band
• Activity 1 Under which category of MAC schemes
  does satellite communications fall?

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Example of MAC (2)

• Multidrop Telephone Line
• Set of M stations share an inbound and an
  outbound line
• Stations transmit information to host using
  inbound line, one at a time
• Host transmits information to station using
  outbound line
• Host computer issues polling message to
  stations granting permission to transmit
• Activity 2 Under which category of MAC schemes
  does multidrop telephone line fall?

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Example of MAC (3)

• Ring Networks
• Hosts are connected in a ring
• One station with the token transmits packet in
  the form of bits
• Each connected station received data, bit by
  bit
• Destination host copies data but leaves data on
  the ring, Others ignore
• Transmitting host extracts data from the ring
• Activity 3 Under which category of MAC schemes
  does Ring Networks fall?

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Example of MAC (4)

• Multitapped Bus
• Uses coaxial cable where a signal can propagate
  in both directions
• A station listens and if no one else is
  transmitting, starts to transmit
• If a collision occurs, it waits for a random
  duration before transmitting again
• All stations receive the transmitted message
• Destination station accepts the message others
  reject it
• Activity 4 Under which category of MAC schemes
  does multitapped bus fall?

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Example of MAC (5)

• Wireless LAN
• Set of devices (workstations, laptops,
  cordless, etc.) share a wireless medium
• Message transmitted have different bit rates
  (hence different BW requirements)
• Different strategies used
• A central authority accepts all messages and
  redirect them to its destination
• Messages can be communicated directly to each
  other
• A combination of the two

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Delay Bandwidth Product

1. Station A wants to talk to station B
2. Station A listens to the medium begins
   transmitting as no signal is present
3. Signal from Station A will take tprop seconds to
   reach station B and vice versa
4. Station B listens and begins transmitting before
   tprop seconds as no signal is detected at its
   end.
5. Result collision of packets
6. Station A will not know of collision till 2tprop
   seconds
7. Station B knows almost immediately
8. Resolution Who stops transmitting? Protocol is
   the one who started transmitting last.

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Delay Bandwidth Product (2)

• Time wasted in coordinating 2tprop seconds.
• If transmission rate of the medium R bps of
  bits wasted 2tprop R bits
• If size of packet L bits, efficiency in the use
  of channel is
• Efficiency L / (L 2tprop R)
• 1 / (1 2a) where a tprop R / L
• where tprop R is the delay-bandwidth and a is
  the ratio of delay-bandwidth to average packet
  length
• For a 0.01, efficiency is 98 For a 0.5,
  efficiency is 50

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Delay Bandwidth Product (3)
Distance 10 Mbps 100 Mbps 1 Gbps Network
1 m 3.33 x 10-2 3.33 x 10-1 3.33 x 100 Desk area
100 m 3.33 x 101 3.33 x 102 3.33 x 103 LAN
10 km 3.33 x 102 3.33 x 103 3.33 x 104 MAN
1000 km 3.33 x 104 3.33 x 105 3.33 x 106 WAN
100000 km 3.33 x 106 3.33 x 107 3.33 x 108 Global area
Table 6.1 Delay Bandwidth product for a number
of Networks
Activity 5 In Ethernet, the size of packets is
limited to a maximum size of 1500 bytes (12,000
bits). Calculate the value of a (ratio of
delay-bandwidth to average packet length) for a
local area network (LAN) at 10 Mbps, 100 Mbps,
and 1Gbps using the values of the delay BW
product from the above table?
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Definitions
Frame Transfer Delay (X) Duration between the
time when the first bit of frame leaves the MAC
layer of the source to the time when the last
bit reaches the MAC layer of the
destination. Throughput (Sout) Effective rate
of transmission (based on successful deliver of
frames) in frames/s across a
network Suppose that the transmission rate of
a network is R bps Length of a frame is L
bits Maximum throughtput R / L
frames/s Actual throughput lt R / L frames/s,
Why? Load (G) Load on the channel in frames per
X seconds. Topology Way a network is
structured, i.e., ring versus bus versus star
topology. Technology Set of protocols used for
a network to function.
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Random Access ALOHA (1)
MAC protocols allow sharing of a common
transmission medium by several hosts. Recall MAC
protocols can be divided into two different
categories
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Random Access ALOHA (2)

• Random Access MAC protocols include
• ALOHA
• Earliest random access method. Developed at
  University of Hawaii in 1970s to interconnect
  university campuses on different islands through
  a microwave link
• Transmitter Transmits the frame as soon as the
  MAC layer receives it
• Channel If a collision occurs, frames received
  by the receiver will contain errors
• Receiver In case of errors, no acknowledgement
  is transmitted to the receiver. (Alternatively, a
  request for retransmission may be made in case of
  errors)
• Transmitter If the transmitter receives no ACK
  within timeout (2 propagation delay), it backs
  off for a random period of time. On the expiry
  of backoff tine, the transmitter retransmits the
  frame.
• Aloha is successful for light traffic.
• Note that collision is different from errors
  since it affects two host stations. For the
  scheme to work, it is vital that the host
  stations wait for a random period of time before
  retransmitting. If both stations wait an equal
  time before retransmitting, there will always be
  collisions.

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Random Access ALOHA algorithm (4)
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Random Access ALOHA throughput (5)
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Random Access ALOHA analysis (6)

• Assumptions
• All frames are of equal length (L).
• All hosts are similar such that the frame
  transfer time X L/R is the same for each host.
• Traffic flows in one direction. (Two seprate
  channels are available in each direction).
• Assume that a frame is transmitted at t0 seconds,
  then
• Vulnerable time in which collision can occur is
  (t0 X t t0 X).
• After transmission, host times out and wait for
  the ACK frame for (t0 X t t0 X
  2tprop).
• In case no ACK is received, the host times out
  for B seconds.
• Retransmission is attempted at t t0 X
  2tprop B seconds.

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Random Access ALOHA analysis (7)

• Assume that the number of frames transmitted in
  any time interval follows a Poissons
  distribution
• where l is the average number of frames
  transmitted per second.
• Given that the total load on the channel is G
  frames per X seconds, l G/X.
• Based on the Poissons distribution,

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Random Access ALOHA analysis (8)

• To prevent collisions, there should be no
  transmissions within the vulnerable period
• The throughput S is defined as the product of the
  total arrival rate and probability of a
  successful transmission.
• Activity 6 Determine the maximum value of
  throughput S and the value of G at which it is
  possible.

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Random Access ALOHA Performance (9)

• Mode 1 (low traffic) As we increase the load G
  from 0, the throughput increases steadily. In
  fact for low values of G, S G. This matches our
  intuition that for low traffic, there will be no
  collisions and the throughput would equal the
  load.
• The throughput S achieves its maximum value of
  0.184 at G 0.5. This corresponds to an arrival
  rate of exactly one frame per vulnerable period.
• Mode 2 (high traffic) As G gt 0.5, the throughput
  drops. Again, this is consistent with our
  intuition that a high load would backlogg the
  channel with a large number of collisions.

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Random Access ALOHA (10)
Activity 7 Suppose that the ALOHA protocol is
used to share a 56 kbps satellite channel.
Suppose that the frames are 1000 bits long. What
is the maximum throughput of the system in
frames/s if the propagation time is
ignored. Solution Maximum throughput for
ALOHA 0.184 frames / X seconds. Frame transfer
delay (X) 1000/56000 1/56 seconds Maximum
throughput in frames/s 0.184 56, or
approximately 10 frames/sec.
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Random Access Slotted ALOHA (1)

• The maximum throughput of ALOHA is 0.184 frames
  per frame transfer time.
• The first attempt to increase the throughput is
  called the Slotted ALOHA, which includes the
  following modifications to ALOHA.
• Divide the time into slots.
• Any station is only allowed to transmit at the
  beginning of a slot.
• Rest of the procedure is same as for ALOHA.
• Drawback Additional complexity in the protocol
  as stations must be synchronized with the
  beginning of slots.
• Advantage Number of collisions are reduced as
  frames would collide only at the beginning of a
  time slot.

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Random Access ALOHA throughput (5)
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Random Access CSMA (1)

• Carrier Sensitive Multiple Access (CSMA)
• An improvement over ALOHA by providing carrier
  sense to the station.
• Before transmitting, the station sense if any
  carrier (or signal) is present on the shared
  medium.
• If a carrier is present, the station waits and
  transmits again when the medium indicates no
  carrier.
• Depending upon how the duration of the wait
  period is decided, CSMA can be classified in
  different categories
• 1-Persistent CSMA Transmit as soon as the medium
  is again idle (free of any carrier).
• Non-persistent CSMA If the medium is busy,
  station runs a backoff algorithm to reschedule a
  future sensing period. It senses again after a
  certain wait and transmits only if the medium is
  free.
• P-persistent CSMA Station senses for carrier. If
  the carrier is absent, it transmits with a p
  probability. If busy, it waits using the
  non-persistent CSMA approach.

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Random Access CSMA-CD

• Random Access MAC protocols include
• Carrier Sensitive Multiple Access with Collision
  Detection (CSMA-CD)
• An improvement over CSMA by giving the station
  capability of detecting collisions
• Procedure is same as CSMA except that if a
  collision is detected, the station stops
  immediately without transmitting the complete
  packet.

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Local Area Networks (LAN) An Overview

• Size serve smaller areas (reduced geographic
  scope)
• generally connected by high-speed
  communications channels
• greater capacity over shorter distances
• Transmission Technology
• switched point-to-point (used by WAN)
• shared-medium packet broadcasting (used by
  LAN)
• Transmission Rates
• Traditional 1-20Mbps
• High speed 100Mbps
• Topology Various topologies are possible varying
  from token bus to token ring

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Local Area Network (1)

• IEEE 802, a committee of Institute of Electrical
  and Electronics Engineers, developed the
  standards.
• Standards include CSMA-CD (Ethernet) referred to
  as 802.3, Token-ring referred to as 802.5, and
  Wireless LAN referred to as 802.11
• Structure
• Number of computers and network devices (jointly
  called hosts) are connected using a shared
  medium.
• Cabling system used is twisted pair, coaxial,
  optical fiber, or wireless.
• Topologies used are bus, ring, or star

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Local Area Network (2)

• Structure (contd.)
• Each host contains a network interface card
  (NIC) Laptops have the smaller PCMCIA card, an
  alternative to NIC.

Ethernet Processor
ROM

• NIC performs the following functions
• Converts parallel data (computer) to serial data
  (medium)
• Buffers data since CPU and network speeds are
  different
• Each NIC has a unique physical address (hardware
  address) burned on it. NIC is responsible for
  addressing within a LAN.

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Data Link Layer in LAN

• Data link layer is divided into two sublayers
• Logical link control (LLC) enhances the services
  offered by its lower sublayer
• Multiple access control (MAC) coordinates access
  of the shared physical medium
• Accepts a block of data from LLC or directly from
  network layer
• Constructs a PDU (frame) including source and
  destination hardware addresses as well as frame
  check sequence using CRC
• Provides for connectionless transfer of frames
  using a MAC protocol

High LevelData LinkControl
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LLC

1. LLC operates over the MAC standards
2. LLC enhances the services offered by the MAC
   layer
3. LLC hides the details of the MAC layer from the
   Network layer
4. LLC also provides a mechanism for exchanging
   frames between LANs using different MAC protocols

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LLC

1. LLC operates over the MAC standards
2. LLC enhances the services offered by the MAC
   layer
3. LLC hides the details of the MAC layer from the
   Network layer
4. LLC also provides a mechanism for exchanging
   frames between LANs using different MAC protocols

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LLC (2)

• LLC provides three kinds of services to its upper
  (network) layer
• Unacknowledged Connectionless Service uses
  unnumbered frames to communicate transfer
  unsequenced information.
• Reliable Connection-oriented Service connection
  setup and release is required with error control,
  sequencing, and flow control features.
• Acknowledged Connectionless Service Individual
  frames are acknowledged.
• LLC PDU contains SAP address of the port that is
  generating the PDU

1 byte
1 or 2 bytes
1 byte
Source SAP Address
Destination SAP Address
Information
Control
Individual or group address
Command or response frame
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LLC (3)

1. LLC datagram is encapsulated into a MAC frame