Transcript ppt

Quality of Service issues in Modular Cable
Modem Termination Systems
-Glenn McGuire April 12, 2005
Today's Cable Modem Network
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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
Today's Cable TV Network
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R egional
area
netw ork
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
LAN Switch
LAN Switch
24-port
Edge24-port
QAM
Edge24-port
QAM
Edge QAM .
D igital
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plant
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Server
Server
Server
LAN Switch
HFC network characteristics
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700MHz to 800MHz raw bandwidth
North American networks have 6MHz channel
spacing
Hard limit of ~115-130 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
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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 (10 20mS)
EQAM
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Relatively inexpensive modulator device
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Usually take 1-Gbps Ethernet input
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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
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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
Provider
network,
Public
Internet
24-port
Edge
QAM
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Ethernet L A N
M-CMTS
Packet
shelf
U pstream
H FC
interfaces
LAN Switch
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M TA
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Moduar CMTS – combined diagram
R egional
area
netw ork
Provider
network,
Public
Internet
M-CMTS
Packet
shelf
LAN Switch
LAN Switch
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Server
Server
Server
LAN Switch
U pstream
H FC
interfaces
24-port
Edge24-port
QAM
Edge24-port
QAM
Edge QAM
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24-port
Edge
QAM
M-CMTS complications
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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
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Cable companies looking to provide more multimedia
services
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Will this work in M-CMTS environment
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Size of GigE LAN undefined
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Simulate it using ns-2, analyse jitter& latency
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Focus on downstream direction
Simulation – CMTS traffic
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Mostly HTTP with some PC-VoIP
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Not simulating P2P
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PackMime – simulates many sessions of HTTP traffic
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Developed by UNC and Bell Labs (Jeffay, et. al.)
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Uses Bruce Mah packet size distribution
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DelayBox – simulates uniform distribution of delay
inserted by multihop network
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Voice using CBR 189B packets at 75Kb/s
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Simulate G.711 codec w/ 20mS granularity
Simulation – Video traffic
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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 frames @
188B each – de-facto standard VOD format.
Not simulating MPEG4 burstiness – too long to simulate
at gigabit speeds, technical difficulties
Simulation network
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3 hop network between CMTS core (3) and EQAM (7)
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Send 960Mb over each Gb link (13-15, 12-14, 11-7)
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Needed to represent HFC (8)
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HTTP (0-9)
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Voice (2-10)
Initial results
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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
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Make Digital Video model more sophisticated
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More delay insertion (DelayBox not Random)
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MPEG4/TES traffic model
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24 channels of video on each link
More sophisticated voice modeling
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Different Codecs
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Dynamic call generation
More sophisticated upstream traffic modeling
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Model P2P traffic on upstream
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DOCSIS signaling on upstream