Tuesday, December 3, 2013

AT&T U-Verse vs. Comcast Xfinity

Technology used

U-Verse: FTTN (Fiber-to-the Node) and VDSL
Comcast: DOCSIS (DataOver Cable Service Interface Specification) and HFC


CMTS = Cable Modem Termination System  (usually at curbside of a neighborhood serving house)
VRAD = Video-Ready Access Device (usually at curbside of a neighborhood serving house)
VDSL = Very-high-bit-rate Digital Subscriber Line
CPE = Customer Premise Equipment (e.g, U-Verse Residential Gateway)
FTTN = Fiber To The Node
HFC = Hybrid Fiber Coax
RG = Residential Gateway

How They Work

The central office above is a simplified of interwork of switches, edge routers (facing customers), and core routers (facing the Internet cloud, where Tier-1 backbone connections are interconnected).


The top network is AT&T network, while the bottom one is Comcast network.  Off course, the diagram above is oversimplified.  There are many other components of the Internet (web servers, mail servers, dhcp servers, dns servers, etc.  They will be discussed some day in separate blog).

AT&T's U-Verse network system consists of CPE sitting inside customer's home.  The upstream connection most of the time use existing coax cable that are usually pre-installed inside most houses for cable tv to minimize cross-talk and noise.  The Layer-1 protocol of this connection to VRAD is VDSL (or VDSL2 for higher speed [24 Mbps], or even VDSL2 bonding for even faster speed up to 45 Mbps.  Using VDSL2 vectoring, theoretically we can achieve even 100 Mbps).  

VRAD is an equipment (a rack equipment) sitting at the curb aggregating traffic from premises (homes).  It acts mostly like a layer-2 switch (with some layer-3 capability, such as DHCP, IGMP, some filtering).  The uplink connection to C.O is FTTN (Fiber To The Node) via optical fiber using GPON technology (or other optical technologies), while downlink connections to premises using existing regular phone's twisted pairs.  VRAD usually is equipped with backup batteries, so even when there is power outage, customers still can make phone call (if the customer uses VOIP, he also needs backup battery for his/her CPE).

The AT&T's CPE usually has multiple downlink ethernet ports and one (or two) POTS for VOIP (optional).  One of the Ethernet port is connected to setop-box (in case the customer subscribes to video as part of dual-play or triple-play).  Internet packets and IPTV packet streams are separated over separate VLAN.  For example, VLAN=100 for the Internet, while VLAN=999 for IPTV.  

When customer wants to watch a TV program by selecting a certain channel via remote, the setop-box sends IGMP join packet to VRAD.  VRAD, with its IGMP snooping capability, then requests C.O's router to send multicast packets containing the program.  This multicast packets are then forwarded to the customer's CPE as unicast packets.When other customers watch the same channel, they just join the multicast group and VRAD then forward the stream to them as unicast packets, so there is single multicast video stream coming from central office to VRAD.

All AT&T's residential gateway /CPE support Wi-Fi.  Wireless connection is treated like other wired connection in a sense it is bridged logically.  Once a CPE is up, as usual it sends DHCP (if it is set for automatic IP assignment).  This DHCP is snooped by VRAD and forwarded to C.O.  Once the client device has been assigned an IP address (public IP address), everything is the same as normal wired connection.  It is up to the CPE/RG to assign a local private IP address to any device connected to it (see previous posting for more detail about how it works).

Cable Network

Front Panel of a cable modem

Back panel of a cable modem

Cost Comparison

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