Wireless LAN

  • IEEE 802.11 – standards for Wireless LAN ( WLAN).
  • Every wireless connection is in half-duplex mode because transmitting and receiving stations use the same frequency
  • The IEEE 802.11 standards use the CSMA/CA method to avoid collisions.
  • Frame transmit:
    • If another device is already transmitting a frame, the station must wait until the frame in progress has completed; then it must wait a random amount of time before transmitting its own frame.
    • Frame duration is added to the 802.11 header.
    • All stations has to wait a short amount of time, called the DCF interframe space (DIFS), before transmitting
    • In addition to DIFS, each station must wait for ‘random backoff’ time before transmitting.
    • The whole process of stations avoiding congestion is called distributed coordination function (DCF)
  • WLAN terminologies:
    • In IEEE 802.11 terminology, any group of wireless devices is known as a service set.The devices must share a common service set identifier (SSID)
    • When two or more wireless devices communicate with each other without any external device, it is called ad hoc or Independent basic service set (IBSS).
    • An access point (AP) is the hub of the service set. All wireless device first needs to associate with its AP before using the wireless network. The client should be matching SSID, compatible data rate and authentication.
    • Access points (AP) with its associated clients are called basic service set (BSS).
    • One or more access points can be connected to L2/L3 MLS switches which are called extended service set (ESS).
    • AP is in charge of mapping a VLAN to an SSID. Can use 802.1Q to trunk with uplink.
    • When a client moves from one cell to another, it may be ‘Layer2 roaming’ (IP address not changed) or it can be ‘Layer3 roaming’.
  • Traditional WLAN architecture:
    • BSS interacts with its uplink L2/L3 switch. All operations like security, VLAN-SSID mapping, and bandwidth allocation are performed within an access-point.
    • Cisco calls this as ‘autonomous mode AP’
  • Cisco Unified Wireless Network Architecture (‘split-MAC architecture’)
    • The entire job performed by ‘autonomous mode AP’ is spitted and performed by Lightweight Access point (LAP) and wireless LAN controller (WLC).
    • RF transmit/receive, MAC management, encryptions are performed by LAP.
    • RF management, association and roaming management, client authentication, security management and QOS are performed by WLC.
    • LWAPP (Light weight access point protocol) or CAPWAP (Control and Provisioning Wireless Access Points protocol) tunnel is created between LAP and WLC.
    • Control messages (encrypted) and data messages (already encapsulated from client to AP) are passed via this tunnel.
    • LWAPP uses UDP destination ports 12222 and 12223 on the WLC end. Similarly, CAPWAP uses UDP ports 5246 and 5247.
    • LAP sends join request to the first WLC from its list of WLC (Primary, secondary and tertiary)
    • Traffic pattern from one client to another (even in same BSS) travels via LAP -> WLC -> LAP (via tunnel)
    • The LAP requires an access mode port—not a trunking port.
    • Clients negotiate their associations with the WLC directly via LAP.
    • In Intracontroller roam, the client is moved from one AP to another but still associated with same WLC and traffic is carried via same tunnel.
    • In intercontroller roam, the client association is moved from one WLC (anchor point) to another (foreign agent). WLC communicate with each other via ‘mobile exchange’ messages.
  • Mobility groups:
    • In intercontroller roam, a client can move from one WLC to another without changing its IP address only if the WLCs belong to same mobility group.
    • A mobility group can have up to 24 WLCs
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