Date: Fri, 11 Oct 1996 15:09:51 -0400 (EDT) From: Pritish Shah <pritish@iocenter.net> To: nanog@merit.edu Subject: Cisco's AIP vs HSSI

Can anyone tell me which is better and why? Is there any resources out there that will help me in that decision.

The short answer: You will undoubtedly be much happier if you use the Cisco AIP card rather than a HSSI card with an external ATM DSU. A longer answer: HSSI ---- The HSSI card is a fairly dumb interface. It is actually an interface between the Cisco router and an external DSU. As I recall, it has a V.35 interface. I think its top speed is a bit more than a DS-3. Its original function was to connect a DS-3 (45 Mbps) DSU to a Cisco router. ---------- | | ------------ | Router |======| DS-3 DSU |----------> DS-3 leased line | | ------------ ---------- The first available method of connecting a Cisco router to an ATM network was to use an ATM DSU connected to a HSSI interface. The ATM DSU appeared to the Cisco router to be pretty much the same as a traditional DSU. All of the ATM functions, largely segmentation and reassembly, were performed in the ATM DSU. ---------- | | ------------ | Router |======| ATM DSU |----------> DS-3 leased line to ATM Network | | ------------ ---------- The HSSI card generally uses "frame relay" or NLPID encapsulation within the AAL frame. The more standard encapsulation today is the LLC/SNAP encapsulation. I believe that Digital Link built the first ATM DSU. This was a fairly early device, so it implemented AAL3/4, (rather than AAL5 which is the predominant AAL today). The Digital Link cannot use AAL5. The Cisco AIP card generally wants to use AAL5. We have been unsuccessful in connecting a Cisco with a HSSI and Digital Link to a Cisco with an AIP card. The details are another long discussion which I won't bother to type at this time. ADC Kentrox also builds an ATM DSU. It supports AAL5. However, the experience at the ATM NAPs indicate that this device has some performance limitations. Among other things, the DXI specification, (which specifies the interface between the router and the ATM DSU), says that only 16 reassembly buffers are required in the ATM DSU, (if memory serves me...). This limited reassembly space can cause problems. On the other hand, the Kentrox ATM DSU supports AAL5. I think it also interoperates with the Cisco AIP card, although I haven't seen it first hand. Other issues with a HSSI/ATM DSU solution include: o The DXI interface supports only PVCs. In a world which is moving towards SVCs, this is a rather serious limitation. o There are a whole host of new ATM features which the HSSI card/ADSU solution won't be able to support. o Check with Cisco about performance limitations in the HSSI card. o I don't believe that the HSSI card will support anything near OC-3c speeds. OC-3c ATM products are readily available in local area networks. Also note that most people run their wide-area ATM connection through a local ATM switch before connecting it to a router. I would be inclined to make the connection between the local ATM switch and the router OC-3c rather than DS-3 (for a bunch of reasons). Cisco AIP --------- I don't believe that the Cisco AIP card is the ultimate solution, but it is far superior to the HSSI interface. Important advantages include: o The AIP card supports SVCs. o The AIP card supports standard encapsulations, AAL5 and LLC/SNAP. (It might also support NLPID and AAL3/4; check with Cisco.) o The AIP supports a variety of interfaces. I think these include multi-mode OC-3c, single-mode OC-3c and DS-3. Verify with Cisco. o I think the performance of the AIP card may be better than the HSSI card, but I don't have any test results in front of me. Summary ------- Read my short answer and skip the rest. Disclaimer: Verify the facts with your vendor. I typed this in on the fly late at night; the general ideas are most likely correct, but some of the details might be a bit off. -tjs