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1-what is the diffrence between hdlc and ppp protocol
2- why frame realy technogy called NBMA
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Answer Posted / arno
HDLC
====
Proprietary, vendor-specific. Cannot be used to connect
routers of different vendors.
HDLC is for use over synchronous serial lines (requires a
"heartbeat").
PPP
===
Sort of standardized HDLC, but compatible accross all
vendors. In addition, with PPP you also get authentication
(at layer 2) and data compression for free. Cisco even
provides data encryption over PPP.
PPP also let you cut large IP packets into smaller ones, as
a way to increase the overall throughput of slower PPP
lines. This is a very important feature if your line is also
used for carrying Voice over IP.
Like HDLC, PPP is used over synchronous serial lines
(heartbeat), but PPP supports a general-purpose asynchronous
mode as well. Async mode is used over modems (to replace the
obsolete SLIP), or to do the authentication job as part of
the L2TP and PPTP VPN protocols.
PPP is also replacing the ATM or Frame Relay encapsulations
that are required to carry IP packets over Telecom SONET
backbones (eg. submarine cables). This is POS, "Packet Over
SONET", which uses PPP instead of ATM or FR.
NBMA
====
NBMA is an acronym used only in the world of OSPF, to
point-out a weakness (a feature?) of ATM and Frame Relay
networks.
NBMA is best understood by contrasting it with the other
Layer 2 technologies:
- PPP and HDLC can only connect 2 routers together
point-to-point.
HDLC and PPP do not support broadcasts, but, because the
link has only 2 endpoints, broadcasts work between the two
routers anyway ("if it's not me it must be you").
- Ethernet can connect more than 2 routers using the same
subnet (although this setup doesn't make much sense on a
typical LAN).
Broadcasts would always work between the routers, because
Ethernet has built-in support for transmitting broadcasts.
- Frame-Relay (and ATM) can also connect several routers
sitting on the same subnet, just like Ethernet.
Being a WAN protocol, Frame Relay was even _designed_ to do so !
That's the typical Frame Relay hub and Spoke WAN, with a
multipoint subinterface at the central office and a
point-to-point subinterface at each branch office.
You end up with all offices sitting on the same subnet.
But, contrary to Ethernet, Frame Relay is unable to transmit
broadcasts. Hence the name NBMA subnet (Non Broadcast Multi
Access).
So, why bother about NBMA ?
You will probably be running OSPF over your Frame Relay
hub-and-spoke WAN subnet, because there are not much
alternatives.
The hub-and-spoke mesh may even act as your OSPF backbone
"area 0".
But OSPF uses lots of broadcasts for its own functioning,
that Frame Relay is unable to transmit.
To solve this problem, on an NBMA subnet OSPF elects one
router that will act as the central "spokesperson" on the
behalf of all the other routers of the subnet: that's the
OSPF "Designated Router" (DR).
Which router is that ? preferably the central hub router,
because it has direct Frame Relay connections with every
other spoke router.
MPLS is replacing both ATM and Frame Relay for connecting
corporate offices together. So we may expect to be rid of
the NBMA stuff in a few years..
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