Quality of Service issues in Modular Cable Modem Termination Systems -Glenn McGuire April 12, 2005

Today's Cable Modem Network ● CMTS – Cable Modem Termination System – The device which connects IP to Cable Modems over an HFC* network ● Performs per-flow token-bucket rate shaping ● Asymmetric upstream/downstream 38 Mbps/10Mbps ● Hundreds of CM nodes per segment (also asymmetric: Downstream =4x Upstream) *HFC – Hybrid Fiber Coax. The cable television distribution medium

Today's Cable TV Network ● Video Server – A source of (MPEG-2 or MPEG-4) digital video ● Edge QAM – A device which converts a digital video stream into a QAM* encoded stream (MPEG/QAM/HFC) ● Current hardware uses IP/ethernet between server & modulator *QAM – Quadrature Amplitude Modulation. An efficient digital modulation technique

HFC network characteristics ● 700MHz to 800MHz raw bandwidth ● North American networks have 6MHz channel spacing ● Hard limit of ~ analog channels ● several channels used for Cable Modems (1 or more for DS, 1 or more upstream) ● Nodes can be 100mi from central office but usually within 15

Packet Cable Voice ● Voice over IP integrated with cable modem network ● MTA – Multimedia Terminal Adapter – VoIP handset adapter with embedded cable modem. ● CMTS is aware of calls to/from MTA, enforces QoS ● Service is latency tolerant but jitter sensitive ( mS)

EQAM ● Relatively inexpensive modulator device ● Usually take 1-Gbps Ethernet input ● Converts MPEG/UDP/IP transport into MPEG/QAM ● Can fit 24 channels over 1 GigE ● Can fit >10 programs per channel or >2 High-def programs using digital MPEG compression

Moduar CMTS ● Cable companies want to combine data and digital video networks ● Multiport Edge QAM less $$ than integrated CMTS QAM ● Use off the shelf edge QAM with CMTS using IP/GigEth interconnect

Moduar CMTS – standalone diagram

Moduar CMTS – combined diagram

M-CMTS complications ● Packet scheduling for upstream is dependent upon instantaneous round-trip time ● Packet Cable voice jitter sensitivity ● CMTS data now mixed with digital video on shared network ● Extra queueing points at EQAM and each LAN switch. Switches may have variable buffering – 10mS - 200mS ● Jitter & latency inserted at each hop ● Use prioritization of jitter sensitive traffic to reduce impact

Simulation ● Cable companies looking to provide more multimedia services ● Will this work in M-CMTS environment ● Size of GigE LAN undefined ● Simulate it using ns-2, analyse jitter& latency ● Focus on downstream direction

Simulation – CMTS traffic ● Mostly HTTP with some PC-VoIP ● Not simulating P2P ● PackMime – simulates many sessions of HTTP traffic ● Developed by UNC and Bell Labs (Jeffay, et. al.) ● Uses Bruce Mah packet size distribution ● DelayBox – simulates uniform distribution of delay inserted by multihop network ● Voice using CBR 189B packets at 75Kb/s ● Simulate G.711 codec w/ 20mS granularity

Simulation – Video traffic ● Using CBR traffic streams with random delay enabled (assume video traffic had been “groomed”) ● 23 QAMs per link shared w/ data traffic (assume no overprovisioning) ● 1360B packets – UDP/IP containing 7 MPEG 188B each – de-facto standard VOD format. ● Not simulating MPEG4 burstiness – too long to simulate at gigabit speeds, technical difficulties

Simulation network ● 3 hop network between CMTS core (3) and EQAM (7) ● Send 960Mb over each Gb link (13-15, 12-14, 11-7) ● Needed to represent HFC (8) ● HTTP (0-9) ● Voice (2-10)

Initial results ● First overprovisioned link dominates jitter. Without rate shaping at CMTS core, jitter exceeds 35mS. ● With rate-shaping, jitter generally very low ~2mS ● Queue depth of intermediate nodes has little impact for well behaved system

Future Work ● Make Digital Video model more sophisticated – More delay insertion (DelayBox not Random) – MPEG4/TES traffic model – 24 channels of video on each link ● More sophisticated voice modeling – Different Codecs – Dynamic call generation ● More sophisticated upstream traffic modeling – Model P2P traffic on upstream – DOCSIS signaling on upstream