When Networkingmeets Wireless

When Networkingmeets Wireless

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.

Terminology issues

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

same SSID different SSID

OK (1) OK (2) same IP subnet

NO (3) OK (4) different IP 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

ssA ssA

subnet

ssA ssB

subnet

ssA ssA

subnet

ssA ssB

subnet subnet subnet

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

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