The Nature of an ESS

1
The Nature of an ESS

• Darwin Engwer, Nortel NetworksBob OHara,
  Airespace
• dengwer_at_nortelnetworks.combob_at_airespace.com

2
Further thoughts on ESS(triggered by submission
11-04-0614-00)

• Doc 614 1 cites a number quotes directly from
  the standard wrt the definition of an ESS.
• Strictly interpreted these existing definitions
  constrain the implementation options as cited in
  doc 614
• However, other logical (abstract) views are
  possible, which have in fact allowed vendors to
  construct functional WLANs within the constraints
  imposed by existing definitions.
• First, a diagram from doc 614 ...

3
AP DS Portal
Slide 6 from 1 doc 11-04-614-00
HOST
Router
802.3 network
Portal
Distribution System
AP
AP
AP
STA
4

• But real world implementations (to date) look
  like this ...

5
Real World AP Devices
HOST
Router
802.3 network
802.3 L2 Hub or Switch
STA
STA
STA
6
Real World AP Review

• In that common real world AP and WLAN
  implementation, the mini-DSs and portals in each
  AP combined with the 802.3 L2 network can be
  considered as forming a single logical
  Distribution System and Portal, since those
  logical components meet all the criteria
  described in 802.11-1999 Clause 5.
• 5.2.2 The IEEE 802.11 LAN architecture is
  specified independently of the physical
  characteristics of any specific implementation.
• The DS enables mobile device support by
  providing the logical services necessary to
  handle address to destination mapping and
  seamless integration of multiple BSSs.

7
Logical DS and Portal
HOST
Router
802.3 network
Portal
802.3 L2 Hub or Switch
Distribution System
A single logical portal is indistinguishable from
multiple physical portals.
STA
STA
STA
8
Logical DS and Portal

• There is no disagreement between the Logical DS
  and Portal model and the formal definitions
  provided in 802.11-1999 Clause 5.
• Consider 5.2.2.1 The key concept is that the
  ESS network appears the same to an LLC layer as
  an IBSS network. Stations within an ESS may
  communicate and mobile stations may move from one
  BSS to another (within the same ESS)
  transparently to LLC.

9
L2 Connectivity with aLogical DS and Portal
HOST
Router
802.3 network
Portal
802.3 L2 Hub or Switch
Distribution System
STA
STA
STA
10
Extent of the ESS

• Hence, the ESS can span all the APs in such a
  network.

11
ESS with aLogical DS and Portal
HOST
Router
802.3 network
Portal
802.3 L2 Hub or Switch
Distribution System
STA
STA
STA
ESS
L2 link
ESS
L2 link
12
What if the network includes a router?
13
L2 Connectivity with aLogical DS and Portal
using a Router
HOST
Router
802.3 network
Portal
Portal
Distribution System
802.3 L2 Hub or Switch
802.3 L2 Hub or Switch
STA
STA
STA
L2 link
L2 link
14
L2 Connectivity thru a Distribution System that
includes a Router

• At first look it appears as if the two STAs on
  opposite sides of the router do not have L2
  Connectivity.
• However, the DS (or some other component in the
  network) can arrange this to be true.
• e.g. the mini-DS in one AP can transport the MSDU
  to the mini-DS in the AP that is on the other
  side of the router. The second AP can then
  deliver the MSDU to the mobile STA. To the
  mobile STAs it appears as if they have L2
  connectivity.
• e.g. A consistent VLAN can be set up on both
  sides of the router, so that packets on that VLAN
  are switched across the router. In fact, this is
  a common deployment scenario today. Again, to
  the mobile STAs it appears as if they have L2
  connectivity.

15
ESS with aLogical DS and Portal using a Router

• Hence, the ESS concept can apply to WLAN
  deployments that rely on a logical DS/ logical
  portal(s) construct, even when using a router.

16
ESS with aLogical DS and Portal using a Router
HOST
Router
802.3 network
Portal
Portal
Distribution System
802.3 L2 Hub or Switch
802.3 L2 Hub or Switch
STA
STA
STA
ESS
L2 link
ESS
L2 link
17
ESS Summary

• BSSs with the same SSID may be part of the same
  ESS.
• No scenarios are necessarily excluded.
• The 802.11-1999 definition of an ESS is somewhat
  broad and flexible. Therefore, TGr may need to
  specify and explicitly define the BSS-transition
  cases that are in-scope and out-of-scope.
• i.e. If TGr wants something more precise than
  ESS, then need to specify that.

18
References

• 1 11-04-0614-00-frfh-what-ess.ppt
• 2 11-04-0500-01-0mes-need-clarification-definiti
  on-ess-mesh.ppt

19
The End