medium by ROcq7NRA

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									       Chapter 4

The Medium Access Control

      The Medium Access Control
• Network Classification
  1. Use point-to-point connections - most WANs,
     except satellite.
  2. Use broadcast channels - most LANs.
• This chapter deals with broadcast networks and
  their protocol.
• The objective is to allocate the channel to:
  –   maximize channel utilization, and
  –   minimize channel access delay.
          Channel Allocation
• Static Channel Allocation in LANs and MANs
• Dynamic Channel Allocation in LANs and

       Static Channel Allocation
• In static channel allocation, a subchannel is
  statically assigned to each station (computer or
• For example, in FDM, a frequency band is
  assigned to each station.
• This is inherently inefficient (w.r.t. channel
  utilization) for bursty traffic.
• Note, however, the channel access delay is
        Static Channel Allocation
• We use the queueing system to analyze the above
• Single station case:
  Let C = channel capacity (in bps)
      T = mean time delay to send one frame (in sec.)
      λ = arrival rate (in frames/sec.)
      1/μ = mean frame size (in bits/frame)
  From queueing theory, we obtain: T = 1 / (μ C - λ)
  For example, if 1/ μ = 10,000 bits/frame, C = 100
  Mbps, and λ = 5000 frames/sec, then T = 1 /
  ((1/104)(108) - 5000) = 1 / 5000 = 200 μsec. per frame.
        Static Channel Allocation
• N station case:
  Divide the channel up into N subchannels, each with
  capacity C/N.
  Let: λ /N = arrival rate at each station (divide the load).
  Then, TFDM = 1 / (μ(C/N) - λ/N) = N / (μC - λ) = N T.
  So, the N station case is N times worse than the 1
  station case.
  For example, as above, N = 10 TFDM = 2 msec. per

    Dynamic Channel Allocation
• Assumptions:
  1. Station Model – N independent stations generate
  2. Single Channel – A single channel is available for
     all communication.
  3. Collision – Frames that overlap in time destroy
     each other; this is called a COLLISION. All
     stations can detect collisions. The only errors are
     those caused by collisions.

    Dynamic Channel Allocation
• Assumptions:
  4. Continuous Time means that transmission of
     frames can begin at any time.
     Slotted Time means that time is divided into
     discrete intervals, and frame transmission always
     begins at the start of a slot.
  5. Carrier Sense means that stations can tell if the
     channel is in use by listening to the channel.
     No Carrier Sense means that stations cannot tell
     if the channel is in use by listening to the channel.

                Pure ALOHA
• In the Pure Aloha Protocol (by Abramson in
  1970s), a station transmits the data whenever
  there is data to be sent. Then, the station listens
  to the channel to see if a collision occurred. If
  the frame was destroyed, the station waits for a
  random length of time and tries again.
• Systems in which multiple users share a
  common channel in a way that can lead to
  conflicts are widely known as contention
  systems.                                          9
                     Pure ALOHA

In pure ALOHA, frames are transmitted at completely arbitrary times.
         Pure ALOHA - Analysis
• Let FRAME TIME be the time required to transmit a
  standard, fixed-length frame; that is, (1/μC).
• Assume there is an infinite population of stations that
  transmit frames according to a Poisson process with a
  mean of N frames transmitted per frame time.
• Note, if N>1, the channel will not be able to handle the
  load. So, we expect 0<N<1. The offered load, G, is the
  total number of frames sent on the channel per frame
  time; that is, G = N + (number of retransmissions per
  frame time).

         Pure ALOHA - Analysis
• Let P0 = probability that no collisions occur during one
  frame time; that is, a transmission is successful. Then:
  the throughput, S = GP0
• Since the offered load is a Poisson distribution, with
  mean G, the probability that k frames are generated
  during a frame time is: Pr[k] = (Gk * e-G) / k!
• So, Pr[0] = e-G. The probability of no other traffic
  during the vulnerable period is e-G e-G = e-2G. Thus, P0
  = e-2G, and it follows that S = Ge-2G
• dS/dG = e-2G (-2G + 1) = 0  G = 0.5
• Note, the maximum occurs when G = 0.5. S = 0.184.
         FYI: Poisson Distribution
• The Poisson Distribution is used to model the number of
  events occurring within a given time interval.
   – It is often used to model events such as the number of
     telephone calls at a business or the number of accidents at an
     intersection in a specific time period.
   – It is also useful in ecological studies, e.g., to model the
     number of prairie dogs found in a square mile of prairie.
• The formula for the Poisson probability mass function
   p(x, λ) = e-λ λx / x! for x = 0, 1, 2, 3, ….
   λ is a is the shape parameter which indicates the
   average number of events in the given time interval.
         FYI: Poisson Distribution
• The following is the plot of the Poisson probability density
  function for four values of λ.

       Pure ALOHA

Vulnerable period for the shaded frame.
               Slotted ALOHA
• In slotted Aloha (by Roberts in 1972) A computer is
  not permitted to send whenever a carriage return is
  typed but wait for a time slot.
• Time is divided into fixed slots of one frame time each.
  A station waits until the start of the next slot before
  transmitting a frame. Thus, P0 = e-G (the vulnerable
  period is only one time slot). S = G e-G Note that as G
  increases, the number of collisions increases
• Note, the maximum occurs when G = 1. S = 0.368.
• Slotted Aloha can be used to allocate a shared cable
              Pure ALOHA

Throughput versus offered traffic for ALOHA systems.
Carrier Sense Multiple Access (CSMA)
 • Protocols in which stations listen for a carrier (i.e.
    transmission) and act accordingly are called carrier
    sense protocols.
 1. 1-persistent CSMA
    Channel Busy  Continue sensing until free and then grab.
    Channel Idle  Transmit with probability 1.
    Collision  Wait for a random length of time and try again.
 2. nonpersistent CSMA:
    Channel Busy  Does not continually sense the channel. Wait
    for a random length of time and try again.
    Channel Idle  Transmit.
    Collision  Wait for a random length of time and try again.
Carrier Sense Multiple Access (CSMA)
 3. p-persistent CSMA:
   Channel Busy  Continue sensing until free (same as idle).
   Channel Idle  Transmit with probability p, and defer
   transmitting until the next slot with probability q = 1-p.
   Collision  Wait for a random length of time and try again.
 • The nonpersistent CSMA has better channel utilization but
   longer delays than 1-persistent CSMA.
 • CSMA are an improvement over ALOHA because they ensure
   that no station begins to transmit when it senses the channel
 • Another improvement is for stations to abort their
   transmissions as soon as they detect a collision. Quickly
   terminating damaged frames saves time and bandwidth. This
   protocol is called CSMA/CD (CSMA with Collision
   Detection).                                                 19
Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various
                   random access protocols.                   20
CSMA with Collision Detection

 CSMA/CD can be in one of three states: contention,
               transmission, or idle.
   CSMA with Collision Detection
• Stations detect collisions using analog hardware
  and abort transmissions immediately.
• Let τ be the propagation delay (the time for a
  signal to propagate between the two farthest
  stations be τ). The contention interval is
  modeled as a slotted Aloha system with slot
  width 2τ.
• Then, 2τ is the time required for a station to
  detect collision with certainty.
  For example, on a 1-km long coaxial cable, τ = 5
  μsec. 2τ = 10 μsec to detect a collision.     22
         Collision-Free Protocols
• Assumptions:
   – There are N stations, uniquely numbered 0, 1, 2, …, N – 1.
   – Each contention period consists of N slots.
   – Data frames consist of d time units.
1. Basic Bit-Map Protocol
   – Low load  bit map is simply repeated over and over.
     • Low # stations: wait ~ 1.5N slots
     • High # stations: wait ~ 0.5N slots
   – The overhead to transmit a data frame is N bits. The
     channel efficiency at low load is d/(N + d)
   – The channel efficiency at high load dN /(N + dN) = d/(1 +
2. Binary Countdown – stations overwrite the low
   numbered stations, and low numbered stations give up.
Collision-Free Protocols

    The basic bit-map protocol.
      Collision-Free Protocols

The binary countdown protocol. A dash indicates silence.
     Limited-Contention Protocols
• Contention  Limited  Collision free
   (CSMA)             contention (Binary Countdown)
  small delay for                  Good channel efficiency
 low load stations                 at high load
• Analysis: Contention protocols (symmetric case)
    Let k = # of stations
    P = prob. that one station successfully requires the
      channel during a given slot.
    ρ = prob. that a station transmits a frame during a
      given slot.
     Limited-Contention Protocols
• Analysis: Contention protocols (Symmetric case)
  P = kρ(1 – ρ)k-1
       prob. That the other stations do not transmit
  dP/dρ = –kρ(k – 1)(1 – ρ)k-2 + k(1 - ρ)k-1
         = k(1 – ρ)k-2 [(ρ(k – 1) + (1 – ρ)]
         = k(1 – ρ)k-2 (-ρk + ρ + 1 – ρ) = 0
    ρk = 1  ρ = 1/k
   P = k (1/k)(1 – 1/k)k-l = [(k – 1)/k]k-1    (4.4)

  Limited-Contention Protocols

Acquisition probability for a symmetric contention channel.
    Limited-Contention Protocols
• Idea: Limit the number of stations contending
  for a slot.
• Question: How to assign stations to a slot?
• Static assignment:
  One station/slot (group) Binary countdown
  Two stations/slot        P = 1 – ρ2 ≈ 1
                            (ρ  0 collision is small)
  All stations/slot        Slotted ALOHA

     Adaptive Tree Walk Protocol
• Use the algorithm devised by U.S. Army test for
  syphilis in 1943)
• Example: There are 8 stations. Suppose that stations 0,
  2, 4, and 5 want to transmit
   Slot 0 – All try Collision
   Slot 1 B Subtree try  Collision
   Slot 2 D Subtree try No collision – 0
   Slot 3 E Subtree try No collision – 2
   Slot 4 C Subtree try Collision
   Slot 5 F Subtree try Collision
   Slot 6 4
   Slot 7 5
   Slot 8  G Subtree try  No Collision              30
Adaptive Tree Walk Protocol

      The tree for eight stations.
Wavelength Division Multiple Access Protocols
 • Each station is assigned two channels: a narrow
   band control channel and a wide band data
 • Each channel is divided into groups of time
   slots. All channels are synchronized by a single
   global clock.
 • The protocol support three traffic classes:
    1. Constant data rate connection-oriented traffic such as
       uncompressed video.
    2. Variable data rate connection-oriented traffic such as file
    3. Datagram traffic such as UDP packet-oriented traffic. 32
Wavelength Division Multiple Access Protocols

          Wavelength division multiple access.
             Wireless LAN Protocols
• Because signal strength is not uniform throughout the space in
  which wireless LANs operate, carrier detection and collision
  may fail in the following ways:
   - Hidden nodes:
       • Hidden stations: Carrier sensing may fail to detect another station.
         For example, A and D.
       • Fading: The strength of radio signals diminished rapidly with the
         distance from the transmitter. For example, A and C.
   - Exposed nodes:
       • Exposed stations: B is sending to A. C can detect it. C might want to
         send to E but conclude it cannot transmit because C hears B.
       • Collision masking: The local signal might drown out the remote
• The result scheme is carrier sensing multiple access with
  collision avoidance (CSMA/CA).
Wireless LAN configuration

         Wireless LAN Protocols
• Hidden station problem: A is transmitting to B. C
  cannot hear A. If C starts transmitting, it will interfere
  at B.
• Exposed station problem: B is transmitting to A. C
  concludes that it may not send to D but the interference
  exists only between B and C.

    A wireless LAN. (a) A transmitting. (b) B transmitting.
            MACA and MACAW
• The sender transmits a RTS (Request To Send) frame.
• The receiver replies with a CTS (Clear To Send)
• Neighbors
   – see CTS, then keep quiet.
   – see RTS but not CTS, then keep quiet until the CTS is back
     to the sender.
• The receiver sends an ACK when receiving an frame.
   – Neighbors keep silent until see ACK.
• Collisions
   – There is no collision detection.
   – The senders know collision when they don’t receive CTS.
   – They each wait for the exponential backoff time.
• MACAW (MACA for Wireless) is a revision of
  MACA.                                                      37
Wireless LAN Protocols

The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.
•   Ethernet Cabling
•   Manchester Encoding
•   The Ethernet MAC Sublayer Protocol
•   The Binary Exponential Backoff Algorithm
•   Ethernet Performance
•   Switched Ethernet
•   Fast Ethernet
•   Gigabit Ethernet
•   IEEE 802.2: Logical Link Control
•   Retrospective on Ethernet
              Ethernet Cabling
• 10Base2 means that is operates at 10 Mbps, uses
  baseband signaling, and support segments up to 200
• 10Base-T became dominant due to its use of existing
  wiring and the ease of maintenance .

        The most common kinds of Ethernet cabling.   40
    Ethernet Cabling

     Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.   41
                 Ethernet Cabling

• Different ways of wiring a building are shown as

 Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
              Ethernet Cabling
• Devices used for connecting Ethernet cable:
  – A repeater is a physical layer device that receives,
    amplifies and retransmits the signal.
  – A hub will take a network packet and transmit it to
    all other ports on the hub.
  – A switch will take a network packet and transmit it
    only to a specific port on the switch that the packet
    is addressed to. So, a switch greatly reduces
    network transmissions thus providing better
    network throughput.
               Ethernet Cabling
• All Ethernet systems use Manchester encoding due to
  its simplicity (high at 0.85 V and low at -0.85 V).

        (a) Binary encoding, (b) Manchester encoding,
               (c) Differential Manchester encoding.    44
   Ethernet MAC Sublayer Protocol
• Preamble – used for sender and receiver to synchronize their
• Addresses
   –   unique, 48-bit unicast address assigned to each adapter
   –   example: 8:0:e4:b1:2
   –   broadcast: all 1s, the set of all recipient nodes
   –   Multicast: first bit is 1,a group of recipient nodes

         Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.        45
     Ethernet Transmit Algorithm
• If line is idle:
   – send immediately
   – upper bound message size of 1500 bytes
   – must wait some time (9.6µs) between back-to-back
• If line is busy:
   – wait until idle and transmit immediately
   – called 1-persistent (a frame to send transmits with
      probability 1)

The Binary Exponential Backoff Algorithm
 • If collision
    – Time is divided into discrete slots whose length is equal to
      the worst-case round-trip propagation time on the ether (2τ).
        • minimum frame is 64 bytes (header + 46 bytes of data) =
          512 bits
        • Channel capacity 10 Mbps, 512/10 M = 51.2µ
    – delay and try again (binary exponential backoff)
       • 1st time: waits 0 or 1 slotted time (51.2µs)
       • 2nd time: waits 0, 1, 2, or 3 slotted time
       • nth time: k slotted time, for randomly selected k=0..2n - 1
       • Frozen at 1023 slots and give up after several tries
         (usually 16)
Ethernet MAC Sublayer Protocol

        Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
                 Switched Ethernet
• To deal with increased load, the switched Ethernet is devised
  with a switch to route the frame to the destination.

              A simple example of switched Ethernet.
                  Fast Ethernet
• The 802.3 committee decided to keep 802.3 for the
  fast Ethernet (802.3u).
   – Backward compatible
   – A new protocol might have problems.
   – Get job done before the technology changed.
• 100Base-T4 – 4 twisted pairs achieve 100 Mbps.

             The original fast Ethernet cabling.      51
                 Gigabit Ethernet
• 802.3z is the standard for gigabit Ethernet.
• All configurations of gigabit Ethernet are point-to-
  point rather than multidrop with two possible
   – Two computers are directly connected to each other.
   – Multiple computers are connected through a switch
     and a hub.
• Two operation mode:
   – Full-duplex – connected by a switch. CSMA/CD is not used.
   – Half-duplex – connected by a hub.
• 802.3z increase the radius by carrier extension
  (padding to extend the frame to 512 bytes).
• Gigabit Ethernet supports flow control.
• 802.3ae is 10-gigabit Ethernet standard.                 52
           Gigabit Ethernet

(a) A two-station Ethernet. (b) A multistation Ethernet.
Gigabit Ethernet

 Gigabit Ethernet cabling.
    IEEE 802.2: Logical Link Control

• All Ethernet protocols offer a best-efforts datagram
• LLC (Logical Link Control) forms the upper half of
  the data link layer. MAC forms the lower sublayer.
   – error-controlled, flow-controlled
   – Adds an LCC header, containing sequence and
     acknowledgement numbers.
• LLC provides three service options:
   – Unreliable datagram service
   – Acknowledged datagram service
   – Reliable connection-oriented service
IEEE 802.2: Logical Link Control

   (a) Position of LLC. (b) Protocol formats.
          Retrospective on Ethernet
• Ethernet has been around for over 20 years.
• The main reason for its success is simple and flexible.
   – Simple translates into reliable, cheap, and easy to
• IP is a connectionless protocol, so it fits perfectly with
  Ethernet. IP fits much less well with ATM.
• Ethernet evolves with time. FDDI, Fibre Channel, and
  ATM were faster when introduced, but they are
  incompatible with Ethernet, complex, and hard to
  manage. Only ATM is used within the telephone
             Wireless LANs
•   The 802.11 Protocol Stack
•   The 802.11 Physical Layer
•   The 802.11 MAC Sublayer Protocol
•   The 802.11 Frame Structure
•   Services

        802.11 Physical Layers
• IEEE 802.11 physical layers:
  – Infrared – 1 Mbps and 2 Mbps
  – FHSS (Frequency Hopping Spread Spectrum) uses
    79 channels, each 1 MHz wide, starting in the 2.4
    GHz band.
     • A psudorandom number generator is used to produce
       the sequence of frequencies hopped to.
     • The amount of time spent at each frequency, dwell time,
       is adjustable.
  – DSSS (Direct Sequence Spread Spectrum) delivers
    1 or 2 Mbps in the 2.4 GHz band.
        802.11 Physical Layers
• IEEE 802.11 physical layers:
  – 802.11a uses OFDM (Orthogonal Frequency
    Division Multiplexing) to deliver up to 54 Mbps in
    the 5 GHz band.
  – 802.11b uses HR-DSSS (High Rate Direct
    Sequence Spread Spectrum) to achieve 11 Mbps in
    the 2.4 GHz band.
  – 802.11g uses OFDM to achieve 54 Mbps in the 2.4
    GHz band.

The 802.11 Protocol Stack

    Part of the 802.11 protocol stack.
          Wireless LAN Standard
Standard Modulation   Spectrum   Max physical Working
                                 Rate         distance

802.11    WDM, FHSS 2.4 GHz      2 Mbps       ≈100 m
802.11a   OFDM      5 GHz        54 Mbps      ≈ 50 m

802.11b   HR-DSSS     2.4 GHz    11 Mbps      ≈ 200 m

802.11g   OFDM        2.4 GHz    54 Mbps      ≈ 200 m

     Wireless LANS Devices

a)wireless router   b) wireless network
                       card           63
          802.11 MAC Sublayer
• Problems: Hidden and exposed stations
• 802.11 supports two modes of operation:
   – DCF (Distributed Coordination Function) does not use
     any kind of central control.
   – PCF (Point Coordination Function) uses the base station
     to control all activity in its cell. Optional.
• CSMA/CA (CSMA with Collision Avoidance) is
  based on MACAW.
   – Use NAV (Network Allocation Vector) to indicate
     the channel is busy.
The 802.11 MAC Sublayer Protocol

       (a) The hidden station problem.
       (b) The exposed station problem.   65
The 802.11 MAC Sublayer Protocol

   The use of virtual channel sensing using CSMA/CA.
The 802.11 MAC Sublayer Protocol
• The deal with the problem of noisy channels,
  802.11 allows frames to be fragmented.

     A sequence of fragments is called a fragment burst.   67
The 802.11 MAC Sublayer Protocol
• In the PCF mode, the base station polls the other
  stations, asking them if they have any frames to send.
   – Completely controlled by the base station. No collisions
   – A beacon frame which contains system parameters is
     periodically (10 to 100 times per second) broadcasted to
     invite new stations to sign up for polling service.
• PCF and DCF can coexist within one cell by
  carefully defining the interframe time interval.

The 802.11 MAC Sublayer Protocol
• The four intervals are depicted:
   – SIFS (Short InterFrame Spacing) is used to allow the
     parties in a single dialog the chance to go first including
     letting the receiver send a CTS and an ACK and the sender
     to transmit the next fragment.
   – PIFS (PCF InterFrame Spacing) is used to allow the
     base station to send a beacon frame or poll frame.
   – DIFS (DCF InterFrame Spacing) is used to allow any
     station to grab the channel and to send a new frame.
   – EIFS (Extended InterFrame Spacing) is used only by a
     station that has just received a bad or unknown frame to
     report the bad frame.
The 802.11 MAC Sublayer Protocol

        Interframe spacing in 802.11.
            802.11 MAC Sublayer
• Three different classes of frames: data, control,
• The fields of data frame:
   – Frame control – 2 bytes and 11 subfields
   – Duration – 2 bytes indicate how long the frame and its
     acknowledgement will occupy the channel
   – Addresses – 24 bytes identify source and destination
     addresses and source and destination addresses of base
   – Sequence – 12 bits identify the frame and 4 identify the
   – Data – the payload up to 2312 bytes
   – Checksum – 4 bytes
            802.11 MAC Sublayer
• The Frame control field:
   – Version – allows two versions of the protocol operating at the
     same time
   – Type – data, control, management
   – Subtype – RTS, CTS
   – To/Frame DS indicate the frame is going to or coming from
     the intercell distribution
   – MF – more fragments will follow
   – Retry – marks a retransmission frame
   – Power management – puts the receiver into sleep stateor take
     it out of sleep state
   – The W bit specifies that the frame body has been encrypted
     using the WEP (Wired Equivalent Privacy) algorithm.
   – The O bit tells a sequence of frames with this bits must be
     processed strictly in order.
The 802.11 Frame Structure

       The 802.11 data frame.
              802.11 Services
• Distribution Services
  – Association – mobile stations connect themselves to
    base stations
  – Disassociation – the station or base station breaks
    the association
  – Reassociation – a station may change its preferred
    base station
  – Distribution – determines how to route frames sent
    to the base station
  – Integration – handles the translation from the
    802.11 format to the format of the destination
              802.11 Services
• Intracell Services
  – Authentication – a station must authenticate itself
    before permitted to send data.
  – Deauthentication – a authenticated station wanting
    to leave the network is deauthenticated.
  – Privacy – manages the encryption and decryption.
    The algorithm specified is RC4 by Ronald Rivest of
  – Data Delivery – not reliable.

           Broadband Wireless
•   Comparison of 802.11 and 802.16
•   The 802.16 Protocol Stack
•   The 802.16 Physical Layer
•   The 802.16 MAC Sublayer Protocol
•   The 802.16 Frame Structure

 Comparison of 802.11 and 802.16
• Similarity - Provide high-bandwidth wireless
• Differences
   • 802.16 provides service to stationary buildings which can
     have multiple computers. High quality full-duplex link is
     used. Higher cost is affordable. 802.11 provides service to
     individual mobile users.
   • Longer transmission range security/privacy
   • More user in each cell more spectrum is needed, operate in
     10-66 GHz absorbed by water

        The 802.16 Protocol Stack
• Physical layer
   – Physical medium dependent sublayer – narrow-band radio
     (which means that it contains all of its power in a very
     narrow portion of the radio frequency bandwidth, prone to
     interference ) is used with conventional modulation schemes.
   – Transmission convergence sublayer – hide the different
     physical medium technologies from the data link layer.
• Data link layer
   – Security layer – deal with privacy and security
   – MAC sublayer common part – channel management
   – Service specific convergence sublayer – integrate with both
     datagram protocols (PPP, IP, and Ethernet) and ATM.
The 802.16 Protocol Stack

     The 802.16 Protocol Stack.
        The 802.16 Physical Layer
• Physical media:
   – 10-to-66 GHz millimeter waves travel in the straight line.
   – Transfer up to 155 Mbps
   – 30 miles range
• The base station has multiple antennas, each pointing
  at a different sector.
• The signal-to-noise ratio drops sharply with distance.
  Three modulation schemes are used depending on
   – QAM-64 (6 bits/baud)
   – QAM-16 (4 bits/baud)
   – QPSK (2 bits/baud)
The 802.16 Physical Layer

  The 802.16 transmission environment.
Broadband Wireless

       The 802.16 Physical Layer
• Unlike equal bandwidth allocation in the cell phone
  system, more bandwidth is allocated for downstream
  than upstream traffic.
• Two schemes are used to allocate the bandwidth:
   – FDD (frequency division duplex)
   – TDD (time division duplex) – downstream, guard, and
     upstream time slots
• The Hamming code is used for forward error

The 802.16 Physical Layer

Frames and time slots for time division duplexing.
The 802.16 MAC Sublayer Protocol
• Base station sends out frames
• Each frame includes a number of subframes, which
  include a number of time slots
• The first two subframes are the downstream and
  upstream maps, which tell what is in which time slot.
• Downstream subframes (channels) are straight
  forward. The base station decides what to send
• Upstream channel is more complex, due to

 The 802.16 MAC Sublayer Protocol
• Four classes of service are defined and are connection-
   – Constant bit rate service: transmit uncompressed voice
     (similar to a T1 channel). Certain time slots are dedicated to
     each user.
   – Real-time variable bit rate service: compressed multimedia
     and other soft real-time applications (bandwidth needed at
     each instant may vary). Polling is used.
   – Non-real-time variable bit rate service: e.g., file transfer.
      • User can request a poll to send constant rate
      • If a station does not respond to a poll K times, the base station put it in
        a multicast group. The user will content for service.
   – Best efforts service
      • No polling. User compete for channel in the time slots marked inthe
        upstream map for contention.
      • If a request is successful, it will be noticed in the next downstream
        map.                                                                 86
The 802.16 MAC Sublayer Protocol
• Generic frame:
   – EC: tells whether the payroll is encrypted
   – Type: frame type (fragmentation)
   – CI: whether checksum presents (optional as FEC in physical
   – EK: which encryption key is used
   – Length: complete length including header
   – Connection ID: which connection this frame belongs to
   – CRC: checksum over header only
• Bandwidth request frame: indicate bytes needed

 The 802.16 Frame Structure

(a) A generic frame.   (b) A bandwidth request frame.
•   Bluetooth Architecture
•   Bluetooth Applications
•   The Bluetooth Protocol Stack
•   The Bluetooth Radio Layer
•   The Bluetooth Baseband Layer
•   The Bluetooth L2CAP Layer
•   The Bluetooth Frame Structure

• In 1994, Ericsson was interested in connecting mobile
  phone with other devices without cables
• Started by Ericsson, a SIG was formed with four other
  companies (IBM, Intel, Nokia, and Toshiba) to develop
  a wireless standard for short range, low power, and
  inexpensive wireless radios.
• This project was named as Bluetooth, after Harald
  Blaatand (Bluetooth), a Viking king who unified
  Denmark and Norway, also without cable
• In 1999, 1500 page specification of V1.0
• IEEE 802.15 adopts the Bluetooth as a personal area
  network standard.
          Bluetooth Architecture
• The basic unit of Bluetooth is piconet, including a
  master node, and up to seven active slave nodes within
  10 m, and up to 255 parked nodes
• Multiple piconets can coexist. An interconnected
  collection of piconets is called a scatternet.

    Bluetooth Architecture

Two piconets can be connected to form a scatternet.
Bluetooth Applications

     The Bluetooth profiles.
     The Bluetooth Protocol Stack
• The Bluetooth Protocol Stack does not follow OSI
  model and revised by IEEE to fix 802.11.
• The Physical Radio layer corresponds to the ISO
  physical layer and deals with radio transmission and
• The Baseband layer corresponds to MAC sublayer and
  some physical layer and handles how the master
  control time slots.
• The link manager handles establishment of logic
  channel (power management, authentication, QoS).
• The logical link control adaptation protocol (L2CAP)
  shields the upper layer from lower layer
     The Bluetooth Protocol Stack
• Audio and Control protocols deal with audio and
• LLC (Logical Link Control) is inserted by IEEE to
  make it compatible with other 802.
• RFcomm (Radio Frequency communication) emulates
  serial port for connecting mouse, keyboard, modem.
• Telephony protocol real-time speech, call setup and
• The service discovery protocol is used to locate service
  in a network.
• Each application uses a specific subset of protocols
 The Bluetooth Protocol Stack

The 802.15 version of the Bluetooth protocol architecture.
    The Bluetooth Frame Structure
• The address field identifies which of the eight active
  devices the frame is intended for.
• The type field identifies the frame type.
• The flow bit is asserted by a slave when its buffer is
• The acknowledge bit is used to piggyback an ACK.
• The sequence bit is used to number the frames to detect

The Bluetooth Frame Structure

      A typical Bluetooth data frame.
      Data Link Layer Switching
• Bridges from 802.x to 802.y
• Local Internetworking
• Spanning Tree Bridges
• Remote Bridges
• Repeaters, Hubs, Bridges, Switches, Routers,
• Virtual LANs

• Connect LANs
• Do not exam payload of the frames (so it can support
  any L3 protocols, IPv4, IPv6, etc)
• In contrast, routers exam addresses in packets

         Data Link Layer Switching

Multiple LANs connected by a backbone to handle a total load higher
                  than the capacity of a single LAN.         101
Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.
      Bridges from 802.x to 802.y
• Difficulties
  – Different frame format, copying data takes CPU
  – Different data rate (gigabit Ethernet 11 Mbps
  – Different maximum frame length
  – Security: 802.11/802.16 support encryption in L2,
    but Ethernet doesn’t.
  – QoS: 802.11/802.16 support, but Ethernet doesn’t.
   Bridges from 802.x to 802.y

The IEEE 802 frame formats. The drawing is not to scale.
          Local Internetworking
• Connecting multiple LANs (same network)
  – Accept all frame
  – Decide to discard it or forward it, by looking up the
    destination address in a big table
     • If destination is on the same LAN, discards it
     • If destination is in on a different LAN, forward to that
     • If destination is unknown, flooding
  – At beginning, floods all frames
  – Learn the destinations by looking at the source
    address of received frames
  – Each entry in the table has a life time                  105
    Local Internetworking

A configuration with four LANs and two bridges.
Spanning Tree Bridges

  Two parallel transparent bridges.
        Spanning Tree Bridges

(a) Interconnected LANs. (b) A spanning tree covering the
      LANs. The dotted lines are not part of the spanning tree.
                Remote Bridges
• Connect remote LANs by point-to-point line
• PPP can be used on the point-to-point lines

   Remote bridges can be used to interconnect distant LANs.109
Repeaters, Hubs, Bridges, Switches,
      Routers and Gateways

  (a) Which device is in which layer.
  (b) Frames, packets, and headers.
             Repeaters and Hubs
• Repeater
  – An analog device connecting two cable
  – A signal appearing on one cable is amplified and put
    out on the other
  – E.g., classic Ethernet allow four repeaters
• Hub
   – Has a number of input lines that it joins electrically
   – Frames arriving on any of the lines are sent out on all others
   – Entire hub is a collision domain
   – All the lines coming into a hub must operate at the same
   – Don’t amplify signal
   – Both hub and repeat don’t exam MAC addresses
             Bridges and Switches
• Bridge
  – Connects two or more LANs
  – Maintains a table including hosts attached in the LANs
  – When bridge receives frame, software in bridge extracts the
    destination MAC address and forward frames
  – May have different speed lines
• Switch
  –   Similar to bridge, check MAC address
  –   Switch is used to connect individual computers
  –   No frame loss due to collision (if with buffer)
  –   If frame come in faster than it can handle frame loss
  –   Cut-through Switch: start forwarding frames as soon as the
      destination address field comes in                      112
           Routers and Gateways
• Router
  – Payload packet is extracted and passed to routing software
  – A out-going line is chosen based on L3 address
• Gateway
  – Transport gateway: connect two computers using different
    connection-oriented transport protocols (TCP, ATM)
  – Application gateway: translate format and contents of data.
    E.g., email gateway: translate Internet message into SMS for
    mobile phones.

Repeaters, Hubs, Bridges, Switches,
      Routers and Gateways

     (a) A hub. (b) A bridge. (c) a switch.
                Virtual LANs

A building with centralized wiring using hubs and a switch.
                   Virtual LANs
• Network administrators like to group users on LANs to
  reflect organizational structure. Issues are:
   – Security: prevent one group from sniffing the data of another
   – Load: separate load, e.g., prevent the heavy traffic of
     research group to affect the communication in accounting
   – Broadcasting storm
• It can be done by plugging lines to hubs carefully. But
  when one changes office frequently or changes group
  frequently, the overhead comes.
• Solution: Virtual LAN (VLAN) is a way to rewire
  building entirely in software.
• VLAN is based on specially designed VLAN-aware
  switches (or bridges). Administrators determine:
   – How many VLANs needed
   – Which computer on which VLAN
   – What’s the names for VLANs (color)
• Configuration tables have to be set up in the switches (or
   – Tells which VLAN is accessible via which port
   – E.g., a frame from gray VLAN must be forwarded on all the
     ports marked G.
   – A port may be labeled with multiple VLAN colors

                   Virtual LANs

(a) Four physical LANs organized into two VLANs, gray and white,
by two bridges. (b) The same 15 machines organized into two
VLANs by switches.                                         118
• How to know the coming frame in which VLAN
  – Ports - Only works if all machines on a port belong to the same
  – MAC address - Notebook can be docked in any place where
    the dock station has aMAC address MAC address does not
    always reflect the user
  – IP address - Solve the problem. However, has to check the IP
    address violate the fundamental rule in networking:
    independence of layers
• Solution
  – Put a label in MAC header, so that no need to check IP
  – For 802.11 and 802.16, easy
  – For 802.3, difficult because there is no free space in its header
         The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded
            symbols are VLAN aware. The empty ones are not. 120
  The IEEE 802.1Q Standard

The 802.3 (legacy) and 802.1Q Ethernet frame formats.

Channel allocation methods and systems for a common channel.

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