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Technology Stocks : The *NEW* Frank Coluccio Technology Forum -- Ignore unavailable to you. Want to Upgrade?

To: Frank A. Coluccio who wrote (869)	9/22/2000 10:47:38 AM
From: justone	Respond to of 46821

Frank: Regarding cable upstream traffic capabilities and prioritization schemes for voice. I've looked a bit into cable modems, and found that some of the vender's information easier to read than the www.packetcable.com (from www.cablelabs.com). Ciscos' product paper is very instructive, since it gives DOCSIS 1.0 (broadband data cable modem), and then described the new functions expected for QoS and voice and fax in DOCSIS 1.1. ieng.com I've reproduced some relevant sections from this document below the ... line. My conclusion is that the managed IP link between the home cable termination device and the head end was designed to support a non contention based voice priority communication in DOCSIS 1.1. Traffic engineering of the upstream then can be used to evaluate the likelihood of blocking of voice, and the amount of bandwidth available for data. Still, we need to know what compression algorithm will be used, will IPv6 be used, will RTP be used or compressed RTP, what it the voice compression algorithm. Making a wild guess, if you get a 4:1 compression (which I'm not sure about), and have say, 16 Kbps per voice channel required. Now upstream speed is surprisingly hard to find on the web, but values range between 1 Mbps and 6. Thus, a 1 Mbps upstream could support ~ 20 simultaneous voice calls. At .2 erlangs this is about 100 subscribers. This doesn't include video conferencing, or other priority real time data. Even if you get 6 Mbps upstream, this is only 600 subscribers, with no room left for data. I can see why a target of 50 homes is a useful worst case. I know this is a "back of the envelope" type calculation, but I expect some such thinking is behind ATT's push for lightwire. When you can't calculate traffic, you can only architect deployment. It also accounts for the cable companies, as you note in recent posts, are going towards a tiered, or multi-service pricing model- the 'bandwidth hogs' must pay more. .............................................................................................................................. The CMTS provides time slots and controls the usage for each upstream interval. The CMTS sends regular mappings of minislot structure in downstream broadcast MAP messages. The CMTS allocates contention broadcast slots that all CMs can use, and also allocates upstream minislots for unicast or non-contention data from specific CMs. ...> They expect to add for DOCSIS 1.1 the following extra features to help support vocie and fax: Concatenation Support---DOCSIS Concatenation combines multiple upstream packets into one packet to reduce packet overhead and overall latency, as well as increase transmission efficiency. Using concatenation, a DOCSIS cable modem needs to make only one bandwidth request for a concatenated packet, as opposed to making a different bandwidth request for each individual packet; this technique is especially effective for bursty real-time traffic, such as voice calls. ... Embeddedlient Signaling (dynamic SIDs)---Supports the dynamic creation, configuration, and deletion of Service Identifiers (SIDs) to accommodate different classes of service. This allows cable modems to request high-priority or high-bandwidth data streams as needed, such as when a VoIP call is made. ... IP Precedence-Based Rate Limiting---In addition to the currently supported traffic shaping techniques, Cisco IOS Release 12.1(1a)T1 supports a new configuration field that associates a maximum bandwidth (in kbps) with a particular setting of the IP type of service (ToS) bits. This can be used to ensure that certain traffic, such as data, does not exceed a pre-set rate limit and thereby interfere with higher-priority real-time traffic, such as VoIP calls. Support for Unsolicited Grants---New fields in the DOCSIS configuration file can be used so that when a cable modem requests a voice or fax SID, the MAC scheduler on the Cisco uBR7200 series router schedules fixed periodic slots on the upstream for that traffic flow. The cable modem does not have to contend for these slots, and because the Cisco uBR7200 series router controls the timing of the slots, it has a very precise control over potential delay and jitter. This provides a Constant Bit Rate (CBR) traffic flow for real-time traffic such as voice and fax calls.