Transcript Document

When Networking
meets Wireless
Terminology issues
The networking and wireless worlds
sometime use identical words or
expressions that have a different meaning
in their respective environments
Since we are approaching wireless in the
context of networking (or vice versa ? :), we
need to agree on definitions, and thus avoid
misunderstandings.
Confusion
Some of the terms that need clarifying:
- PoE (Power over Ethernet)
- Access point
- Router
- Roaming
- Bridge
- Broadcast domain
PoE (Power over Ethernet)
• In the wireless world, PoE is referred to any
time a category 5/5e/6 cable is used to
carry both the Ethernet signal and the
power. The power can be 12, 24, or 48
volts DC (or any voltage in between).
• In the Networking world, PoE refers to the
IEEE 802.3af standard that provides 48
volts DC over the same cable that carriers
the Ethernet signal.
• The conflict is in the DC voltage. Beware!
Access point
Reminder: in wireless, all equipment that can
connect to a wireless network categoriezed into
one of three categories:
1. access point, master, sometimes referred
to as infrastructure. These are typically
boxes that we’ve been configuring that
have a combination of radios and wired
Ethernet ports.
2. Client. This is typically your laptop.
3. Ad-hoc. A special mode where two
devices act as peers and talk to each other
Access Point continued
When connected to wired networks, an
access point can function as a bridge
(L2), a router (L3), or even both.
In the networking world, the term access
point is almost always used to designate a
device used to bridge traffic between a
wireless network and a wired network, at
Layer 2. This terminology difference is the
root of a large portion of our discussion
yesterday.
Bridge
A bridge is used to connect 2 or more Layer
2 segments together
A segment in this case may be:
- a wired network
- a wireless network
A bridge is essentially a 2 port switch
1
2
Bridge (2)
An access point which connects 2
interfaces: ...
- a wired interface (e.g.: 100/1000baseT)
- a wireless interface (e.g.: 802.1g)
… at Layer 2 is a bridge
1
2
Router
There is not much ambiguity here, but still a
good idea to refresh:
- a router is a device that forwards packets
between distinct Layer 3 networks
- a router usually has at 2 or more physical
network interfaces, though not a strict condition
Classification by role and scale
- core / backbone routers (large scale)
- access routers (medium)
- SOHO/residential GW/CPE/edge (small)
Router (2)
Good to point out that NAT is not an essential
function of an IP router
Small devices usually implement it, including
most access points, which frequently can
be configured to function in router mode
Probably not desirable in larger environments
Broadcast domain
In computer networking, a division of the
network where all nodes (or hosts) within
can reach each other by broadcast at L2
Broadcast is, on ethernet, performed by
sending traffic to MAC address ff:ff:ff:ff:ff:ff
In the context of wireless, the equivalent of a
broadcast domain from the networking
world is implemented as SSIDs, so a single
broadcast domain will be a single SSID
Roaming
Roaming is somewhat harder to define
Wireless world borrowed definition from
cellular telephony world
What do we mean by roaming on a campus?
- Layer 2. Moving between access points that
have the same SSID
- Layer 3. Moving between access points that
are advertising wireless networks that are
separate/different subnets
Does it matter ?
Roaming (2)
No clear definition
Some define it to be the seamless transition
from one wireless network to another
- … how seamless ?
- is it good enough for your Mail ? Web ? VoIP ?
- does it need it to be L2 to be ”roaming” ?
Even with L2-only, other mechanisms will
make the transition visible (e.g.: 802.1x
transition)
Roaming (3)
On smaller networks, it's easy to do L2
roaming
As networks get bigger, best to avoid large L2
(broadcast) domains
IP segmentation/subnetting
- Why sacrifice this architectural principle when
implementing wireless ?
Roaming (4)
Necessary to find the right balance
- Groups of access points in same L2, same
SSID, when closely located (same building,
room, …)
- Different locations, different L3 (IP) networks,
different SSIDs
Roaming matrix
ssA
ssA
ssA
subnet
same SSID
ssA
subnet
ssB
subnet
different SSID
OK (1)
OK (2)
same IP subnet
NO (3)
OK (4)
different IP subnet
ssA
subnet
ssA
subnet
ssB
subnet
(1) same SSID, same IP subnet is considered ”seamless” at L2, but large L2 does not scale
(2) no practical application to have different SSIDs on same subnet (manual balancing ? )
(3) client may not be informed of change of L3 network, and continue to assume that it
is on the still on the network it has dissociated from
(4) 1 SSID = 1 broadcast domain principle, no roaming to speak of, all L3
Networking World
Network engineers may tend to oversimplify /
downplay the complexity of wireless
networks (”it's just ethernet, no big deal”)
- goal is to constrain the role of wireless
equipment to well defined L2 scenarios: edge
access, PtP
- best practice is to avoid L2 networks spanning
multiple broadcast domains (usually an IP
subnet)
Wireless World
Wireless people may tend to oversimplify /
underestimate the architecture of a wired
network (”an access point can route or
bridge, no big deal”)
- somewhat less stringent about what is core
Questions
?