=?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 something that will click with you much better if you can get some real-world exposure, rather than reading text and pictures in books. OK, well I have definitely run out of steam now. :-) _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com > If this is > correct, could you please advise under what circumstances HDLC > and Frame Relay are used in a real world environment? Do they > function in their own right, or are they part of software or > hardware or a protocol? I see that HDLC is for point to point > transfer only; presumably this means that if a router is using > HDLC it can only talk directly to one device out a single > interface but with FR, it can send frames over a LAN-like > structure? I seem to be in some confusion as to where these two > things fit in; the rest of the two chapters (3 and 4) seem > fine, I just keep contradicting myself over these two things. > I'm sure it's one of those blindingly obvious things that will > be really simple to understand once someone sets me straight - > I think I'm interpreting these things differently each time I > read about them. Thanks in advance for your help DJ > > > > --------------------------------- > With Yahoo! Mail you can get a bigger mailbox -- choose a size > that fits your needs > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=66336&t=66324 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
