Computer Networking Lecture 25: Last Mile Technologies Peter Steenkiste. Fall







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15-441 Computer Networking

Lecture 25: Last Mile Technologies

Peter Steenkiste

Fall 2013




Classic view: different types of wires

Copper: telephone, modem, xDSL

Cable: TV driven

Fiber: future proofing

(Wireless: satellite and terrestrial)

Media encoding and streaming


Many a type of access

F.O. Copper or F.O. Copper Broadcast SAT Long haul Distribution Network Access Network Cable Home Network UNI SNI


• Modem offers a bit stream.

• Aggressive signal processing has dramatically increased the available throughput - beats the Nyquist limit!

• SLIP: Serial Line IP.

• Protocol to sent IP packets with minimum framing

• Lacks authentication, error detection, non-IP support, .. • PPP: Point-to-Point Packets.

• Better framing, error control, and testing support

• Can negotiate choice of higher layer protocols, IP address

• Can support unreliable and reliable transmission

Home PC Modem Bank Telephone Network


Example: Modem Rates

















Integrated Services Digital Network (ISDN)

• ISDN integrates voice and data services.

• Provides a set of bit pipes that can be used for voice, data, signaling.

• Implemented by using time multiplexing

• Basic rate ISDN offers to 64Kbs data bit pipes and one 16 Kbs signaling channel. Home PC ISDN Exchange Telephone Network Public Branch Exchange Phone LAN


Digital Subscriber Line

• Squeeze more bandwidth out of the telephone line using advanced signal processing.

• Asymmetric digital subscriber line (ADSL).

• More “download” bandwidth, e.g. video on demand or web surfing

• Initially: T1 incoming path, 64 Kbs outgoing path – now much higher bandwidths

• (Symmetric) digital subscriber line (DSL).

• Same bandwidth both ways, e.g. 768 Kbs

Home PC ADSL Network Unit Telephone Network ADSL Subscription Unit Phone

DSL: Physical Layer Matters

• Telephone wiring was design to carry a telephone signal

• Carry analog voice signal in 0 – 4 KHz

• 1-pair of voice-grade unshielded twisted pair (UTP)

• Ends of wiring were conditioned for optimizing low frequencies – cuts off higher frequencies

• Changes needed for higher frequencies

• Change conditioning at the end points

• Better coding and modulations

• Bandwidth depends on the distance

• Can upgrade to better wiring, e.g., 2-pair DG UTP


Workshop on standardization in E-health – ITU-T , Geneva, 23-25 May 2003 ADSL2+ G.992.5 Video/ voice/ data/copper 16 – 25 Mb/s Down Up to 800 Kb up Higher-optional video capabilities. Dist 1.5 km/BW ADSL2

With and without splitter – G.992.3 / 4 Video/ voice/ data/copper 8 Mb/s down 800 Kb/s up Higher-optional video capab. Dist + 200m ADSL ADSL

With and without splitter – G.992.1 / 2

Voice/data /copper

Up to 8 Mb/s down Up to 1.5 Mb/s up

Full use existing copper. Web brows/Voice 2.7- 5.4 km VDSL Symmetric Asymmetric G.993.1 Video/ voice/ data (V/V/D) over copper Up to 52 Mb/s down asymm Up to 26 Mb/s Symm. Broadcast video, VoD, internet TV, 1.5 km - 300m SHDSL Symmetric G.991.2 Voice/data/ Video 1/2 pairs 192 K to 4.6 M Steps 8/16 Kb/s Poss amplif. V/V/D Services mainly for business appl. HDSL Symmetric G.991.1

Voice data and video 1/2/3 pairs 784 Kbit/s to 2320 Kbit/s V/V/D Services mainly for business appl.

DSL Speeds


Cable Modem

• Use cable infrastructure for data service.

• Inherently has more bandwidth

• The last mile is a shared infrastructure that was designed for broadcasting.

• Meaning: the bandwidth is shared by users

• Example: 27 Mbs shared incoming path; 768 Kbs common outgoing path

Fiber Infrastructure

Fiber – “FTTX”

Traditionally, fiber only used in network “core”

• More expensive technology – higher capacity

Reach of fiber has expanded over time, i.e., fiber

reaches closer to the consumer

• Fiber to the cabinet

• Fiber to the curb

• Fiber to the home

• Options include “active” and “passive”

Trend applies to all copper technologies


Optical Access Network Architectures

O N U O N U O N U O L T N T N T Fibre Fibre Fibre Copper Copper HOME Access network SNI FTTB/C FTTCab FTTH

Optical Distribution Network


Cable versus Fiber

• Cable Modem Network

• Simplex 6 MHz downstream channels

• Simplex 200 KHz to 6+ MHz upstream channels

• All traffic traverses the Headend

Headend AMP AMP





Copper Coax


2nd Generation

Hybrid Fiber Coax

FTT curb


PON Access System


Axs Node



OPT Splitter ONU ONU ONU ONU 1530 nm 1310nm +/- 50 nm


• Modems use “worst case” technology.

• Has to fit within any voice channel so encoding suboptimal

• Wires can be very long (end-to-end)

• ISDN can be more aggressive – dated quickly

• DSL is highly optimized for the transmission medium

• But there are some constraints on distance

• Cable modem uses a transmission medium that

has inherently a higher bandwidth, but the network architecture will limit throughput.

• Designed for broadcasting, not for point-point connections


Access technological evolution

Year 9.6 kbit/s 28.8 kbit/s 56.6 kbit/s 128 kbit/s 2 Mbit/s 640 kbit/s 8 Mbit/s 25 Mbit/s 50 Mbit/s 1989 1997 2000 Analog modems ISDN ADSL2+ ADSL2 HDSL/ADSL VDSL OPTICAL ACCESS 622 Mbit/s G-PON 2.5 Gbit/s 2003


Change in User Demand: Triple Play

• Cable companies already offer TV and data service – natural add phone service

• Very low bandwidth

• Plays well in market – convenient and cost effective

• Services emerged as a fourth component

• What about telephone companies

• How do you deliver TV service over a phone wire?

• Necessary to be competitive

• How about incumbents?

• They start with zero wires



Classic view: different types of wires

Media encoding and streaming

Big Picture

Voice and video compression

Protocols: SIP and RTP

Triple play: IPTV, cable, fiber to the home


Different Classes of Streaming

• Multimedia streaming covers audio and video

• Playback: you play back stored content

• Full content is available up front

• Flexibility in whencontent is transmitted

• Broadcast: transmission of live content

• Content generated on the fly but flexibility in playback delay – seconds in practice

• VCR/DVR functionality adds a twist

• Interactive: voice and video conferencing

• Latency is very critical


Options When Using a Streaming Server

Compensating for Jitter:

VoIP with Fixed Playout Delay


Adaptive Video Streaming

Source Feedback Playback

Receiver reports dropped packets.

Sender reduces frame size/rate if loss rate is too high.

» Also uses probing to detect additional bandwidth

Similar to TCP, except that lost data is typically not retransmitted.

Voice over IP (VoIP)

In voice over IP (VoIP), calls are digitized, packetized, and transported over an IP network: either an internal IP network or the Internet.


Voice over IP (VoIP)

A media gateway connects a VoIP network to the PSTN. This gives VoIP users access to PSTN users.

The media gateway must translate between both signaling technology and transport technology.

VoIP Signaling and Transport

Signaling is the transmission of control messages.


VoIP Signaling and Transport

VoIP transport consists of a stream of VoIP packets.

Each VoIP packet contains a short amount codec-encoded voice. There is no time to wait for error correction, so UDP is used. The Real Time Protocol (RTP) header “fixes” weaknesses of UDP. First, the RTP has a sequence number to place packets in order. Second, RTP has a time stamp so that the voice steam

can be played back at the correct time.


Classic view: different types of wires

Media encoding and streaming

Big Picture

Voice and video compression

Protocols: SIP and RTP


Steps in Encoding and Decoding

• Process has three steps on each side

• Digitize: represent information in bits

• Sample, quantize, (eg. PCM)

• Compress: reduce number of bits

• Audio: GSM, G.729, G.723.3, MP3, …

• Video: MPEG 1/2/4, H.261, …

• Send over the network

• Reverse process on receive side: uncompress, convert, play – need to match!

Audio Encoding

• Traditional telephone quality encoding: 8KHz samples of 8 bits each – 64 Kbps

• CD quality encoding: 44.1 KHz of 16 bits

• 1.41 Mbs uncompressed

• MP3 compression similar to MPEG

• Frequency ranges that are divided in blocks, which are converted using DCT, quantized, and encoded

Range Ratio

Layer 1 384 kbps 4


VoIP Codecs

Codec Transmission Rate

G.711 64 kbps G.721 32 kbps G.722 48, 56, 64 kbps G.722.1 24, 32 kbps G.723 5.33, 6.4 kbps G.723.1A 5.3, 6.3 kbps G.726 16, 24, 32, 40 kbps G.728 16 kbps G.729AB 8 kbps

Two phones must use the same codec to encode and decode voice. They must agree on one of several standard codec protocols

through negotiation. Generally, more Compression gives lower sound quality but lowers transmission cost

Video Encoding

• Once frames are captured ("raw" video) resulting file is very large:

• 320 x 240 x 24-bit color = 230,400 bytes/frame

• 15 frames/second = 3,456,000 bytes/second

• 10 seconds takes around 30 Mbytes! (no audio)

• Commonly-used encoding “tricks”:

• Per-frame versus inter-frame encoding

• Leverage fact that successive frames tend to be similar

• Impacts latency

• Layered encoding


Representative Video Bit Rates


Lo Quality)

• 1.2 Gbps Uncompressed HDTV • 19.4 Mbps ATSC ( ≈ HDTV quality) • 8 - 9 Mbps MPEG4 ( ≈ HDTV quality) • 90 Mbps Uncompressed NTSC (SDTV) • 3 - 6 Mbps MPEG2 ( ≈ SDTV quality) • 1.5 Mbps MPEG4 ( ≈ SDTV quality) • 1.5 Mbps MPEG1 ( ≈ VHS < SDTV quality)

Note: ATSC, MPEG2, & MPEG4 support a


variety of formats (SDTV ↔ HDTV)

Image Encoding: JPEG

• Divide digitized image in 8 x 8 pixel blocks.

• The DCT phase converts the pixel block into a block of frequency coefficients.

• Discrete Cosine Transform – similar to FFT

• The quantization phase limits the precision of the frequency coefficient.

• This is based on a quantization table, which controls the degree of compression

• The encoding phase packs this information in a dense fashion.


MPEG Encoding of Video

• MPEG uses inter-frame encoding.

• Exploits the similarity between consecutive frames

• Three frame types:

• I frame: independent encoding of the frame (JPEG)

• P frame: encodes difference relative to I-frame (predicted)

• B frame: encodes difference relative to interpolated frame

• Note that frames will have different sizes

• Complex encoding, e.g. motion of pixel blocks, scene changes, ..

• Decoding is easier than encoding.

• MPEG often uses fixed-rate encoding.


MPEG System Streams

• Combine one more MPEG video and audio streams in a

single synchronized stream.

• Consists of a hierarchy with meta data at every level describing the data.

• System level contains synchronization information

• Video level is organized as a stream of group of pictures

• Group of pictures consists of pictures

• Pictures are organized in slices


MPEG Standards

• MPEG 1 (1992)

• Compression of motion video & audio at about 1.5 Mbps (VHS Quality)

• Targeted at digital playback & storage

• Has Random Access capabilities

• Divides picture up into 8x8 pixel blocks - Converts blocks to bit stream

• MPEG 2 (1994)

• Targets higher quality compression, typically at 3-6 Mbps bit rates

• Being used for Direct Broadcast TV

• Large chunks of MPEG2 used in U.S. HDTV standard


• Aimed at Multimedia Coding

• Bit rates from 8 Kbps - 40+ Mbps

• Can codes objects as opposed to NxN blocks

• Ability to interact & manipulate objects

• Standard in 1999


H.261, H.263, & H.264

Target real time videoconferencing

Subset of MPEG

Wide variety of bit rates

64 Kbps - 128 Kbps: Face shot (video phone)

384 Kbps: considered to be minimum speed for decent full screen videoconferencing

We are using H.263/4, mostly @ 768 Kbps

H.264 quality > H.263 > H.261

Newer protocols require more processing power

Older protocols become less popular over time

1 10 100 1000 10000 100000 1970 1975 1980 1985 1990 1995 2000 2005 2010

voice HiFi audio TV qual video

Kbit/ sec G.711 G.726 G.728 G.729 G.723.1 Audio CD M P3 M P3Pro, AAC

Alca tel Propr. Codec M - JPEG


Source: Alcatel




Classic view: different types of wires

Media encoding and streaming

Big Picture

Voice and video compression

Protocols: SIP and RTP

Triple play: IPTV, cable, fiber to the home


Transport Protocol Properties

• Reliability.

• Some lost data may be acceptable – depends on

encoding and user expectations

• Timeouts typically result in unacceptable delay – there may not be enough time to retransmit data

• Congestion control.

• Nature of the flow fundamentally limits its bandwidth

• Reduction of rate in response to congestion should reduce data size, not transmit rate for samples

• E.g. change frame rate or frame size

• Flow control: natural pacing.

• Samples should be paced at the rate of the data

• Too slow --> underflow and missed deadlines


Real Time Transport Protocol (RTP)

• Multimedia senders append header fields before

passing to transport layer

• Format, sequence numbers, timestamps, …

• RTP logically extends UDP: application layer between UDP and application

• RTP does not guarantee timely data delivery.

• Simply helps applications with formatting and the collection of session information

• Guarantees can only be provided at lower level

• The protocol has two parts: Real-time Transport Protocol (carry data) and Real-Time Control protocol (monitor quality, participant info, ..)

RTP Packet Format

• Source/Payload type

• Different formats assigned different codes

• Eg. GSM -> 3, MPEG Audio -> 14

• Sequence numbers

• Time stamps

• Synchronization source ID


Real Time Control Protocol (RTCP)

• RTCP packets transmitted by each participant in RTP session to all others using multicast

• Distinct port number from RTP

• Reports on:

• Loss rate

• Inter-arrival jitter

• Identity of receivers

• Delay, (indirectly)

• Control bandwidth sharing

• Needed for scalability

• E.g., 5% of data bandwidth

SIP Introduction

• SIP is an application level signaling protocol for initiating, managing and terminating sessions in the Internet

• Registrations, invitations, acceptations, and disconnections

• Sessions may include text, voice, video

• Can use unicast or multicast communication

• Client-server model: request-reply transaction

• Common headers in plain text, similar to MIME/HTTP

• request/response line (e.g., INVITE SIP/2.0)

• message headers (identification, routing, etc)


SIP Entities

• User agent (UA)

• UA client (UAC) and UA server (UAS)

• Proxy server

• relay calls; chaining; forking

• Redirect server

• redirect calls

• Registrar server

• UA registration (UA whereabouts)

SDP: Session Description Protocol

• Media type, network/transport parameters

• e.g., media: media, port, protocol, format_list

• m=audio 2000/2 RTP/AVP 0 98

• a=rtpmap:0 PCMU/8000

• connection: net_type, add_type, address

• c=IN IP4


SIP Session Establishment and

Call Termination

From the RADVISION whitepaper on SIP


Call Proxying

From the RADVISION whitepaper on SIP


Classic view: different types of wires

Media encoding and streaming

Triple play: IPTV, cable, fiber to the home

Multicast, traffic management


Change in User Demand: Triple Play

• Cable companies already offer TV and data service – natural add phone service

• Very low bandwidth

• Plays well in market – convenient and cost effective

• Services emerged as a fourth component

• What about telephone companies

• How do you deliver TV service over a phone wire?

• Necessary to be competitive

• How about incumbents?

• They start with zero wires


Delivering “Triple Play” to Consumers

• Can be viewed at the 1990’s dream of integrated

services networks: voice, video, and data

• IPTV delivery over DSL optimized to work around last mile bottleneck

• Rest of the lecture

• Cable and fiber have plenty of bandwidth – channels can be dedicated to specific uses

• Take “TV” channels and use for Internet service and voice

• Demand for large numbers of channels is starting to stress even cable capacity: deliver less unpopular channels “on demand”, DSL style (switched video)


IPTV - Internet Protocol television

• Deliver TV broadcast to users using Internet topology

• Also, services, data, and voice, i.e. triple play

• Design only delivers channels of interest to the home

• Contrast with broadcast nature of cable design

• Reduce load on the last mile link

• Uses IP multicast and RTP for distribution of broadcast TV

• More on this later

• Other traffic is unicast, using different higher level protocols

IPTV Components

• Streaming server: encoding of live content

• Also, encryption (DRM), protocol support, e.g., for DVR, PIP, …

• IP network with multcast support

• DSLAM - Digital Subscriber Line Access Multiplexer

• Mux/demux from/to multiple subscribers

• CPE - Customer Premises Equipment

• Receives IP streams from DSLAM and distributes them throughout the home

• STB - Set Top Box


Delivering video services over DSL


TV over IP using DSL

Streaming Server

• DSL has limited bandwidth (few to few 10s of Mbps)

• Depends on distance, technology


TV over IP using FTTH

Pushing fiber to the home dramatically increases

bandwidth, e.g., channels

• Easy to justify for, e.g., apartment buildgin

Multicast – Efficient Data Distribution



IP Multicast Service Model (rfc1112)

• Each group identified by a single IP address

• Groups may be of any size

• Members of groups may be located anywhere in the


• Members of groups can join and leave at will

• Senders need not be members

• Group membership not known explicitly

• Analogy:

• Each multicast address is like a radio frequency, on which anyone can transmit, and to which anyone can tune-in.

• For IPTV membership is known and access is controlled end-to-end

IP Multicast Addresses

• Class D IP addresses

• –

• How to allocated these addresses?

• Well-known multicast addresses, assigned by IANA

• Transient multicast addresses, assigned and reclaimed dynamically

• Addresses can be centrally assigned in IPTV networks



IP Multicast Service

• Sending – source sends one packet

• Receiving – two new operations

• Join-IP-Multicast-Group(group-address, interface)

• Leave-IP-Multicast-Group(group-address, interface)

• Receive multicast packets for joined groups via normal IP-Receive operation

• Routers must replicate packets

Multicast Router Responsibilities

• Learn of the existence of multicast

groups (through advertisement)

• Identify links with group members

• Group management - next

• Establish state to route packets

• Replicate packets on appropriate interfaces

• Routing entry:

Src, incoming interface List of outgoing interfaces



IP Multicast Architecture

Hosts Routers Service model Host-to-router protocol (IGMP)

Multicast routing protocols (various)

Internet Group Management Protocol

• End system to router protocol is IGMP

• Host who wants to joint multicast message send IGMP join request to its router

• Router will add host’s port to the multicast tree

• Each host keeps track of which mcast groups are subscribed to

• Socket API informs IGMP process of all joins

• Hosts periodically contact router with groups of interest

• Objective is to keep router up-to-date with group membership

• Routers need not know who all the members are, only that members exist


Lecture 24: 11-20-200270

Source-based Trees

Router Source Receiver S R R R R R S S

Multicast Routing Protocols

• Many routing protocols proposed for IP multicast

• Flood and prune: broadcast pruned to multicast

• Link state: routers broadcast information on its MC receivers

• Distance vector: traditional protocol that generates tree that connects all possible members

• Not used in public internet due to security and public concerns

• IPTV network requirement are a good fit for multicast

• Private network: full control over networks, endpoints

• Single source and all destinations are known – routing is easy!


Controlling Quality of Service

• IPTV and voice have very high QoS requirements

• Need to compete with traditional telephone and cable TV delivery based on circuits

• Bandwidth requirements are known

• Data and interactive TV services have traditional web-like requirements

• Optimize response time, e.g., browsing

• Maintain bandwidth, e.g., Netflix

• Bursty bandwidth requirements

• QoS control is based on careful bandwidth allocation and enforcing bandwidth limits using traffic shapers



• Lecture includes slides from several sources:


• ppt





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