=?iso-8859-1?q?maine=20dude?= wrote:
>
> Hi, I have a couple of queries regarding HDLC and Frame Relay.
> I gather they're both forms of data encapsulation for data and
> basically this means putting the data in headers and trailers
> to identify to the next layer or computer how to deal with the
> data. Please advise whether this is correct.
Both HDLC and Frame Relay have a header and trailer and yes, they do
encapsulate network-layer data and above. But it's a bit of an exaggeration
to say that they "identify to the next layer or computer how to deal with
the data."
HDLC and Frame Relay are data-link layer protocols that provide Wide Area
Networking (WAN) connectivity. Acting at the data-link layer, they are
analagous to Ethernet or Token Ring in a LAN. You wouldn't say that Ethernet
"identifies to the next layer or computer how to deal with the data" and you
shouldn't say this about HDLC or Frame Relay either. They may identify what
the next layer is, but not "how to deal with the data." Think of the OSI
model. Each layer calls on the layer below and depends on the service
provided by the layer below, but not the other way around. Each layer passes
the encapsulated data to the layer above without touching it or
understanding what it does. Sorry if that's picky.
The original HDLC packet format did not have a field to identify the
payload, i.e. the type of network-layer data that is encapsulated. But
modern derivitaves of HDLC, including Cisco HDLC and PPP do have such a
field. The standard Frame Relay packet format doesn't have a field to
identify the next layer either, but Cisco's Frame Relay format does.
As mentioned, HDLC and Frame Relay are WAN protocols. The obvious difference
from LANs is that they connect devices or sites across a relatively long
distance. The other, and possibly more important, difference is that you
need a service provider or telco to implement a WAN. With LANs, you own the
whole thing.
With WANs, you own the routers, but then you lease capacity from a service
provider or telco and get an agreement that the provider will send your data
across its internal network of switches that span the long distance. This
brings with it an entire set of administrative, political, and monetary
issues, and means from an implementation and troubleshooting point of view
that you have to work with the provider's engineers and sales geeks. But
it's worth it. There's no way you can set up your own link between San
Francisco and Los Angeles, for example, without the help of a telco/service
provider. Things like mountains, roads, earthquake faults, and pot farms
would get in your way. Just kidding. :-)
In the olden days, HDLC was sometimes used to connect computers, such as
mainframes and terminal controllers. These days, in a Cisco-oriented
environment, both HDLC and Frame Relay are used to connect routers. That's
another difference from LANs. You wouldn't normally put HDLC or Frame Relay
on an end computer, whereas an end computer does have an Ethernet NIC in it.
HDLC and Frame Relay are built into the Cisco Internetwork Operating System
(IOS) and use a serial interface for the hardware.
Now, for the differences between HDLC and frame Relay. HDLC is used for a
point-to-point link, as you mentioned. It's used on a leased line that you
get from a telco. You could connect a router in Atlanta, for example, to the
local telco and contract with them to get your data to a telco in New York,
for example, where you connect another router to the telco there. The result
is a permanent, real circuit (as opposed to virtual circuit) between Atlanta
and New York that only you can use.
What if you also have sites in Boston, Los Angeles, and Chicago, as well as
Atlanta? Should you lease a point-to-point link to make every connection?
The number of circuits would be n(n-1)/2 where n is the number of sites.
That's expensive. And that's where Frame Relay comes in.
Frame Relay allows you to have virtual circuits to many different sites.
With Frame Relay, you can lease a single line into the service provider's
Frame Relay "cloud" and then contract with them for virtual circuits to
other sites. For example, if New York is your HQ, you could have just one
line into the telco in New York, but a virtual circuit to every other site.
The outlying sites communicate with each other through New York. Each of
them also just has one link into their local telco. Your network traffic
travels across the Frame Relay provider "cloud," which is shared by all the
provider's customers.
Well, I'm running out of steam here and have to get to work. This is covered
in many books and white papers, as you probably know. I'm not sure which
book you are using. Cisco Academy maybe? But if you have some specific
questions, let us know.
I'm wondering too if you could try to get a tour of a company's network and
get a better feel for this? Talk to some network engineers about their
network designs and physical facilities, etc. This is som
