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Link layer (Link Layer (48 bit MAC (or LAN or physical or Ethernet) …

- - - - point-to-point
      - broadcast (shared wire or medium)
  - - - TDMA
      - FDMA
    - - Slotted ALOHA
        
        Pros
        
        • single active node can continuously transmit at full rate of channel
        • highly decentralized
        • simple
        
        Cons
        
        • collisions, wasting slots
        • idle slots
        • once collision happens, entire slot is wasted
        • clock synchronization
        
        Analysis of Collisions
        
        Throughput S:
        
        Offered load G:
        
        efficiency
        
        At best: 37%
      - Pure ALOHA
        
        pros
        
        simpler, no synchronization
        
        cons
        
        increase collision probability
        
        Maximum efficiency = .18
      - CSMA, CSMA/CD, CSMA/CA
        
        CSMA/CD
        
        easy in wired LANs:
        
        difficult in wireless LANs
        
        used in Ethernet
        
        CSMA/CA
        
        used in 802.11
    - - Polling:
      - Token passing:
  - - - unique LAN address
      - allocation administered by IEEE
      - portability
    - - Star topology:active switch in center
      - Ethernet Frame Structure
        
        Preamble:
        
        Addresses: 6 bytes
        
        Type:
        
        CRC:
      - connectionless
      - unreliable
    - - physical-layer
    - - link-layer device: smarter than hubs, take active role
      - transparent
      - plug-and-play, self-learning
      - multiple simultaneous transmissions
  - - - wireless hosts
      - base station
      - wireless link
      - infrastructure mode
      - Ad hoc mode
    - - decreased signal strength
      - interference from other sources
      - multipath propagation
      - Hidden terminal problem
      - Signal fading
    - - Characteristics of wireless LANs
        
        Advantages
        
        – very flexible within the reception area
        – Ad-hoc networks without previous planning possible
        – (almost) no wiring difficulties (e.g. historic buildings, firewalls)
        – more robust against disasters
        
        Disadvantages
        
        – typically low bandwidth compared to wired networks due to shared medium
        – Less reliable
      - Design Goals for Wireless LANs
      - 802.11: Infrastructure
        
        •Station (STA)
        
        •Access Point
        
        •Basic Service Set (BSS)
        
        •Portal
        
        •Distribution System
      - 802.11: Ad hoc mode
        
        Station (STA)
        
        Independent Basic Service Set (IBSS)
      - 802.11 - Layers and functions
        
        Station Management
        
        MAC Management
        
        MAC
        
        PHY Management
        
        PLCP Physical Layer Convergence Protocol
        
        PMD Physical Medium Dependent
      - 802.11 Physical layer
        
        IEEE 802.11b
        
        IEEE 802.11a and g
        
        IEEE 802.11n
        
        IEEE 802.11ac
        
        IEEE 802.11ad
      - 802.11 MAC layer
        
        Background
        
        Uses CSMA/CA
        
        key method :DCF or distributed coordination function
        
        Using acknowledgements
        
        Includes a method based on request
        
        Traffic services
        
        Asynchronous Data Service (mandatory)
        
        Time-Bounded Service (optional)
        
        Enhanced DCF and PCF (802.11e)
        
        Broadcast, multicast, and unicast
        
        Reliability
        
        prone to errors
        
        CSMA
        
        Persistent CSMA
        
        Non-persistent CSMA:used in wifi
        
        Backoff Interval
        
        reduce collision probability
        
        CW is contention window
        
        Count down
        
        Transmit
        
        MAC layer signaling
        
        defined through different inter frame spaces
        
        no guaranteed, hard priorities
        
        SIFS:highest priority, for ACK, CTS, polling response
        
        PIFS
        
        DIFS:lowest priority, for asynchronous data service
        
        802.11 frame: addressing
        
        Address 1: MAC address of wireless host or AP to receive this frame
        
        Address 2: MAC address of wireless host or AP transmitting this frame
        
        Address 3: MAC address of router interface to which AP is attached
        
        Address 4: used only in ad hoc mode
        
        duration of reserved transmission time (RTS/CTS)
        
        frame seq # (for reliable ARQ)
        
        frame type (RTS, CTS, ACK, data)
        
        Frame format
        
        • Types
        
        • Sequence numbers
        
        • Addresses
        
        • Miscellaneous
        
        Distributed and centralized access methods
        
        – DFWMAC-DCF CSMA/CA (mandatory)
        
        – DFWMAC-DCF w/ RTS/CTS (optional)
        
        DFWMAC- PCF (optional)
        
        Binary Exponential Backoff in DCF
      - 802.11 Management
        
        Synchronization
        
        using a Beacon
        
        (ad-hoc)
        
        (infrastructure)
        
        Association/Reassociation
        
        Scanning
        
        Passive scanning
        
        Goal: find a network to connect
        
        Active scanning
        
        Roaming: change networks by changing access points
        
        Power management
        
        Timing Synchronization Function (TSF)
        
        Infrastructure
        
        Traffic Indication Map (TIM)
        
        Delivery Traffic Indication Map (DTIM)
        
        Ad-hoc
        
        Ad-hoc Traffic Indication Map (ATIM)
        
        MIB - Management Information Base
        
        Beacon Frame
- - - - Internet standards
        
        RFC: Request for comments
        
        IETF: Internet Engineering Task Force
      - Internet: “network of networks”
      - protocols control sending,receiving of msgs
    - - communication infrastructure
      - communication services provided to apps:
        
        reliable data delivery from source to destination
        
        “best effort” (unreliable) data delivery
  - - - end systems (hosts):
      - client/server model
      - peer-peer model:
    - - residential access nets: cable modems
        
        HFC: hybrid fiber coax
        
        asymmetric: up to 30Mbps downstream, 2 Mbps upstream
        
        network of cable and fiber attaches homes to ISP router
        
        share access
      - institutional access networks (school, company)
        
        Ethernet Internet access
      - mobile access networks
        
        wireless LANs:
        
        wider-area wireless access
      - Digital Subscriber Line (DSL)
        
        dedicated physical line
        
        up to 1 Mbps upstream
        
        up to 8 Mbps downstream
        
        uses existing telephone infrastruture
      - Typical home network components:
        
        DSL or cable modem
        
        router/firewall/NAT
        
        Ethernet
        
        wireless access point
    - - circuit switching:
        
        FDM
        
        TDM
      - packet-switching:
        
        Statistical Multiplexing
        
        store-and-forward
        
        pros:
        
        great for bursty data
        
        resource sharing
        
        simpler, no call setup
        
        more users
        
        cons:
        
        excessive congestion: packet delay and loss
    - - at center: “tier-1” ISPs
        national/international coverage
      - “Tier-2” ISPs: smaller (often regional) ISPs
        Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
      - “Tier-3” ISPs and local ISPs
        last hop (“access”) network (closest to end systems)