Satellite Networking - University of Surrey

© Dr Z SUN, University of Surrey1Satellite Networking

Satellite Networkingas a component of Satellite Communications B (EEM.scmB)

Dr Zhili SUN Centre for Communication Systems Research

School of Electronic Engineering, Information Technology & MathematicsUniversity of Surrey

GuildfordSurrey

GU2 7XHTel: 01483 68 9493Fax: 01483 68 6011

Email: [email protected]


Contents

� Network Protocols basics and reference models� Satellite networks and network services� PDH and SDH transmissions technology� SDH - Intelsat scenarios� ISDN� ATM and B-ISDN over satellite� TCP/IP over satellite� IP QoS over satellite


Satellite Networking Review

� Network protocol basics and reference models� Telecommunication Services� Network Description and Architecture� Basic Technical Issues� Digital Transmission (PDH & SDH)� SDH over Satellite - Intelsat scenarios� Satellite system performance related service requirement� Issues on ISDN and B-ISDN� ATM and broadband networks over Satellite� Internet over Satellite and QoS� Standards - ITU-T and ITU-R


Protocol basics


Protocol architecture

� Layers, protocols and interfaces

� Connection-oriented and connectionless services


The ISO reference model


Data Transmission in the OSI model


Internet - the TCP/IP reference model


The B-ISDN ATM reference model


Satellite networks and network services


Custom Designed Networks

� Telecommunication networks� Custom Designed Networks

• Broadcast TV• TV distribution• Small Dish / VSAT type date network


Basic Technical Problems

� Propagation Delay� Limited bandwidth� Transmission Errors� Transmission Power

GEO

MEO

LEO

terrestrial


Error Control Mechanisms

� Re-transmission for non-real time applications� Forward Error Control (FEC)

• such as Adaptive Reed-Solomon Coding� Interleaving Techniques to randomise burst errors as it is

easier to correct random errors than burst errors• such as cell based interleaving technique used in

COMSAT equipment


Main network services

� Voice (bandwidth 300 - 3.4 kHz)� Voice band data (Facsimile, etc.) via 3.1 kHz channel up to

9.6 kbit/s� 64 kbit/s “digital” data (ISDN and leased network)� Broadband (64 kbit/s -> 2 Mbit/s -> 155 Mbit/s -> ...)� Satellite usage must take into account the end-to-end

customer requirements as well as signalling/routeing constrains of particular network configuration

� The requirements of these services may also differ depending on whether they are carried on a dedicated (leased) circuit within the main network or a switched connection.


Network architecture

International Node

Main Network

Node

Local Network

Node

Customer’s terminal

International Node

Main Network

Node

Local Network

Node

Customer’s terminal

Switching function

Switching function

Main Network

Node

Local Network

Node

Main Network

Node

Local Network

Node


Circuit switched main networkInternational Transmission Network

(Cable, satellite and radio)

Analogue International

exchange

Analogue Main Network

Exchange

Analogue local

exchangePhone

Fax

Voicebanddata

Cordless phone

Mobile

PBX

Modem

Digital International

exchange

Digital Main Network

Exchange

Digital local exchange

Phone

Fax

Voicebanddata

Cordless phone

Mobile

ISDN

Modem

PBX

Notes: Satellites can in principle be used on any section (or combination of sections) of the network. In Europe they are mainly used in connections of international gateways worldwide. Need to carefully control circuit routeing to void picking up 2 satellite hops on a particular call.


Main network transmission � Local Access

• Analogue: standard 2-wire, 3.1 kHz local line• 64 kbit/s: leased line for access using ITU-T X-series

interfaces• 144 kbit/s: ISDN two 64 kbit/s information channels

plus a 16 kbit/s signalling link to control these channels • 2 Mbit/s: used for wideband leased circuit access or to

connect a PBX with 30x64 kbit/s information channels plus a 64 kbit/s signalling channel.

� Main Network• Analogue transmission (this is being replaced by digital

transmission• Digital transmission (120 Mbit/s TDMA, IDR 2 Mbit/s)


Analogue transmission hierarchy

Single Channel (3100 Hz)

Group (12 or 16 channels)

Super-Group (60 channels)

Master-Group (300 Channels)

12 MHz (2700 Channels)

16 Super-Group (960 channels)



Super-Master-Group (900 Channel)

Hyper-Group (900 Channels)

nLower order systems from a single channel up to 60 channels.nHigher order systems from 300 up to 10800 channels


PDH and SDH transmissions technology


History of Digital Transmission Systems

� Until 1970 achievement in long-haul routes: • Frequency Division Multiplexing (FDM)

� Early 1970s begin to appear:• Digital Transmission Systems

� Pulse Code Modulation (PCM) technique• Represent standard 4 kHz analogue telephone signal as a 64

Kbit/s digital bit stream


Transmission Hierarchies

64 Kbit/s64 Kbit/s

15441544

20482048

North American

Europe

X24

63126312 4473644736X4 X7 X6

X4 X4 X4 X4X30

274176274176

84488448 3436834368 139264139264 564992564992

X3X3


Principles of Plesiochronous Operation

" Greek meaning of plesiochronous: almost synchronous"

1234

123

"fast" incoming bitsat 2 Mbit/s channel

"slow" incoming bitsat 2 Mbit/s channel

Bit rate adaptor

1234JJ

Bit rate adaptor

123JJJ

Masteroscillator

Less justification bit added

More justification bit added


The Synchronous Digital Hierarchy (SDH)

� 1989 CCITT Blue Book covering SDH: • Recommendation G707, G708 & G709

� Basic transmission rate STM-1• (Synchronous Transport Module): 155.520 Mbit/s

� Higher Transmission rates STM-4 & STM-16:• 622.080 Mbit/s and 2.488320 Gbit/s

� Suggested higher rate STM-8 & STM-12:• 1.224160 & 1.86624 Gbit/s

� Introducing Operations Administration and Maintenance (OAM)


Digital transmission hierarchy (SDH)

� The “primary rate” STM-1 (synchronous transport module - 1) has a bit rate of 155.520 Mbit/s

� Each frame consists of “payload” space of carrying a PDH 140 Mbit/s signal completely, with extra capacity for error-checking and management channels.

� The current defined higher SDH levels are STM-4 (4 STM-1s) and STM-16 (16 STM-1s).

� The proposed STM-R, the reduced bitrate STM-1 is an attempt to design STM with a bit rate of 51.84 Mbit/s.

� The satellite community should note that all levels of the SDH contain a considerable percentage of overhead (3.33%) much of which is at present undefinded.


Mapping PDH to SDH

STM-NXN

AUGX1

AU-4

AU-3

VC-4

VC-3

TUG-3

TUG-2 TU-2X1

VC-2

C-4

TU-12

X3

VC-12 C-12

TU-11

X4

VC-11 C-11

C-2

C-3

140 Mb/s

45/34 Mb/s

TU-3 VC-3X3

X3

6 Mb/s

2 Mb/s

1.5 Mb/s

X7

X7

s

s

s

s

e

e

e

X1 e

s: ANSI SONET specific optione: Europe ETSI specific option

AUG: Administrative Unit GroupTUG: Tributary Unit GroupVC: Virtual Container

multiplexingmappingaligning


Simplification of PDH Add-Drop principle

140 Mbit/s line terminator

140

34

140

34

140

34

140 Mbit/s line terminator

140

34

140

34

140

34

Customer site

PDH

Customer site

SDH

SDH

Multipleter

SDH

Multipleter

SDH

Multipleter


Synchronous Operation

Example: European mapping route for primary rate service

STM-1X1

AU-4 VC-4 TUG-3 TUG-2

TU-12

X3

VC-12 C-12

VC-11 C-11

2 Mb/s

1.5 Mb/s

s: ANSI SONET specific optione: Europe ETSI specific option

AUG: Administrative Unit GroupTUG: Tributary Unit GroupVC: Virtual Container

multiplexingmappingaligning

AUGX1 X3 X7

s


Transmission rates

Levels Referring to SDH:

STM-1: 155.520 Mbit/s

STM-4: 622.080 Mbit/s

STM-8: 1224.160 Mbit/s

Sugested high rates:

STM-12: 1866.240 Mbit/s

STM-16: 2488.320 Mbit/s

Levels Referring to PDH:

11 1.544 Mbit/s

12 2.048 Mbit/s

21 6.312 Mbit/s

22 8.488 Mbit/s

31 34.368 Mbit/s

32 44.736 Mbit/s

4 139.264 Mbit/s


The STM-1 Frame270 bytes

11

2

3

4

5

6

7

8

9

Section overhead

AU ptr

Section overhead

9 10 270

9 bytes

125 microseconds

STM-1 Payload

J1

B3

C2

G1

F2

H4

Z3

Z4

Z5

VC-4

POH


STM-1 Section Overhead

Bytes reserved for future use. For example, these are proposed by whin ITU-T to be used for media specific applications, e.g. Forward error correction in radio systems.

A1 A1 A1 A2 A2

B1

D1

AU pointers

A2 C1

B2 B2 B2 K1 K2

D4

E1

D5 D6

D7 D8 D9

D10

Z1 Z1 Z1 Z2 Z2 Z2 E2

D11 D12

D2 D3

F1

Regenerator section overhead

Multiplex section overhead

STM-1 Payload


SDH over satellite - Intelsat Scenarios (1/2)

� Full STM-1 transmission (point to point) through a standard 70 MHz transponder.

� STM-R uplink with STM-1 downlink (point to multipoint)


SDH over satellite - Intelsat Scenarios (2/2)

� Intermediate data rate (IDR) of 2 Mbit/s � PDH IDR link with SDH to PDH conversion at the earth

station


Satellite system performance related to service requirement

� Echo: some form of echo control is always advisable on satellitebased networks carrying voice traffic, irrespective of the associated delay.

� Delay: the one way propagation delay between satellite earth station via a geostationary satellite is approximately 260 ms - see ITU-T G.114

� Digital transmission error performance objective:• G.821 based on 64 Kbit/s circuit switched connection:

– Bit Error Ratio (BER): is the ratio of the number of bits in error to the total number of bits transmitted during a measurement period. Objective is to get BER < 10 E-6.

– Errored second (ES) - BER > 10E-6– Severely errored second (SES) - BER > 10E-3

• G.826: define ES and SES differently at high bit rates


Error Performance Objectives for G.826

� Hypothetical reference path (HRP)

Path end point (PEP)

Path end point (PEP)

Intermediate countries (4 assumed)

IG IG IG IG IG IG

Intercountrypath (e.g. cable, sat.)

International portion

Hypothetical reference path: 27,500 km

Terminating country

Terminating country

IG: International Gateway

2% 2% 2% 2%

17%

17%

1% 1%

1% per 500 km


ISDN over Satellite


Issues on ISDN?

� The ITU definition of an Integrated Services Digital Network (ISDN) is:

A network evolved from the telephony IDN that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by a limited set of standard multipurpose customer interfaces.


ISDN Access

� Two customer access schemes - the basic rate access and the primary rate access.

� Large business customers will access an ISDN network via a digital PABX at the primary (or possibly higher) PCM multiplex rates of 1.544 (US) or 2.048 Mbits/s (European)

� This corresponds to a TDM group of 30 B-channels in Europe (or 23 in US) plus 1 D-channel operating at 64 kbits.

� The signalling over this D-channel will be handled using an extension to the No 7 signalling scheme.

� The small business or domestic customer will access at 2 B-channels of b4 Kbit/s plus a D-channel of 16 Kbit/s


ISDN over satellite

Satellite links can easily support ISDN services with� Basic rate:

144 Kbit/s (2 x 64 Kbit/s B-channel + 16 Kbit/s D-channel)� Primary rate:

• 1.544 Mbit/s = 23B + D = 23x64 + 64 (for North America configuration)

• 2.048 Mbit/s = 30B + D = 30x64 + 2x64 (for Europe) -where one time slot is used for framing and one general network maintenance

� Routeing Plan - no hierarchical, no more than 2 hops


ATM and B-ISDN over Satellite


Broadband ?

� B-ISDN or Broadband ISDN: Broadband Integrated Services Digital Network

� ITU-T definition: • A service or system requiring transmission channels

capable of supporting rates greater than the primary rate.


Relationship Between ATM and B-ISDN

� ATM evolved from the standardization efforts for B-ISDN. � ATM is the technology upon which B-ISDN is based.


ATM Technology

� Cell Switching and fixed-length cells• 53 bytes cells• 48 payload and 5 byte header

� Negotiated Service Contract• Connection Oriented • end-to-end Quality of Services.

Head

5 Octets

Payload

48 Octets

Why 53 bytes?


Mapping ATM into STM-1

270 bytes

11

2

3

4

5

6

7

8

9

Section overhead

AU ptr

Section overhead

9 10 270

9 bytes

125 microseconds

STM-1 Payload

J1

B3

C2

G1

F2

H4

Z3

Z4

Z5

VC-4

POH

...... ......

ATM Cells


Mapping ATM into Cell Based Transmission

......1 2 26 27 28

......1 2 26 27 28

29

OMA Cell

ATM layer: 149.760 Mbit/s

Physical Layer: 155.520 Mbit/s


ATM Layer - Head Structure

GFC

CLPPTVCI

VCI

VPI

VPI VCI

HEC

1 2 3 4 5 6 7 8

1

2

3

4

5

at the UNI

CLPPTVCI

VCI

VPI

VPI VCI

HEC

1 2 3 4 5 6 7 8

at the NNI


VPs and VCs

Physical LayerVirtual Path (VP)

Virtual Channel (VC)

Each VP within the Physical Layer has itsown distinct VPI; each VC within a VP has itsown distinct VCI


B-ISDN ATM Adaptation Layer (AAL) Types (363)

Higher Layer Functions

Convergence

Generic Flow ControlCell header generation/extractionCell VPI/VCI TranslationCell Multiplexing and Demultiplexing

Cell rate decouplingHEC header generation/verificationCell delineationTransmission frame adaptationTransmission frame generation/recovery

Segmentation and Reassembly

Bit timingPhysical Media

Layermanagement

CS

SARAAL

ATM

Physicallayer

TC

PM


B-ISDN ATM Adaptation Layer (AAL) Service Classification(362)

Timing relationBitrate

Connectionmode

Class A Class B Class C Class D

required not required

constant variable

connection-oriented connection-less

Examples: A - Circuit emulation, CBR VideoB - VBR video and audioC - CO data transferD - CL data transfer


AAL1 for Class A

Header functions include:

• Lost cell detect: used by Adaptive Clock Method

• Byte alignment: allows channelise circuit emulation, e.g.,channelised DS 1

• Time stamp: used for end-to-end clock synchronisation, e.g., Synchronous Residual Time Stamp method

AAL1header

Payload

1 byte 47 bytes


AAL2 for Class B

PayloadIT

48 bytes

SN CRCLI

SN - Sequence Nubmber, IT - Information Type

LI - Length Indicator, CRC - Cyclic Redundancy Check


AAL3/4 for Class C&D

44 bytes of data per cellCyclic Redundancy Check (CRC) per cellMessage Identifier (MID) allows muitipleinterleaved packets on a virtual connection

MID CRC

ErrorChecking

User Data

2 2

4

44

Data

ErrorChecking

4 or 8 bytes

0 - 65535 bytes

MID CRC User Data

2 244

MID CRCUser Data

2 244

PAD


AAL5 for Class C&D

48 bytes of data per cellUse PTI bit to indicate last cellOnly one packet at a time on a virtual connection

0 User Data

48

Data

0 - 65535 bytes

0 User Data

48

148

PAD

Last cell flag

0-47

0 LEN

CRC

2 2 4 bytes

Error Detection Fields


ATM Networks and Interfaces

ATMSwitch

ATMSwitch

PrivateUNI

Terminal

Terminal

ATMSwitch

ATMSwitch

ATMSwitch

PublicNNI

MetropolisData ServiceInc.

ATMSwitch

ATMSwitch

ATMSwitch

PublicNNI

CountryWide CarrierServices

Terminal

Terminal

ATMDXI

PublicUNI

B-ICI

PrivateNNI


ATM Networks and Interfaces (cont.)

� Public and private networks� User network interface (UNI)� Network node interface (NNI)� ATM DXI� B-ICI


ATM over satellite

ATMSwitch

ATMSwitch

ATMSwitch

PublicNNI

Public Network

ATMSwitch

ATMSwitch

ATMSwitch

PublicNNI

Public Network

B-ICIATMSwitch

ATMSwitch

PrivateUNI

Terminal

PublicUNI

PrivateNNI

ATM

Switch

ATM

Switch

ATM

Switch

Public

NNI

PublicNetwork


ATM Forum Scenarios

Major work items identified for consideration by the WATM working group.

(A) "Radio access layer" protocols including (but not limited to):A.1 Radio physical layer. A.2 Medium access control for wireless channel (with QoS, etc.).A.3 Data link control for wireless channel errors.A.4 Wireless control protocol for radio resource management.

(B) "Mobile ATM" protocol extensions including (but not limited to):B.1 Handoff control (signaling/NNI extensions, etc.)B.2 Location management for mobile terminalsB.3 Routing considerations for mobile connections.B.4 Traffic/QoS control for mobile connections.B.5 Wireless Network Management


Reference Model for ‘W’UNI to ‘M’NNI usage via ATM Switch enabled Satellite

Radio Access Segment Fixed Network SegmentRadio Access SegmentSatelliteSegment

MobilityEnabledATMSwitch

ATMNetwork

ATMHost

WATMRadio Port

WATMTerminal

WATMTA

WATMRadioPort

Satellite-Based

M.E.Switch

WATMRadioPort


WATM‘R’ RAL

ATMNNI

ATMUNI

ATM‘M’NNI

ATM‘W’ UNI

WATM‘R’ RAL

ATM‘W’NNI

User Process

AAL

ATM

WATM

RAL

User Process

AAL

ATM

ATM

PHY

ATM

ATM ATM

PHY PHYU-PLANE

ATM

WATM ATM

RAL PHY

ATM

WATM WATM

RAL RAL

ATM

ATM ATM

PHY PHY

C-PLANE

W-CTL

W-CTL

SIG,NNI + M

SAAL

ATM

ATM ATM

RAL RAL

SIG, UNI

SAAL

ATM

ATM PHY

SIG,NNI

SAAL

ATM

ATM ATM

PHY PHY

SIG,NNI + M

SAAL

ATM

WATM ATM

RAL PHY

W-CTL

SIG,NNI + M

SAAL

ATM

ATM ATM

PHY PHY

SIG, UNI

SAAL

ATM

WATM RAL

W-CTL


Reference Model for ‘M’NNI to ‘M’NNI usage via ATM Switch enabled Satellite

Radio Access Segment Fixed Network SegmentRadio Access Segment Satellite SegmentMobile Multi-User Platform

WATM‘R’ RAL

WATM‘R’ RAL

WATMRadioPort

Satellite-Based

M.E.Switch

WATMRadioPort

ATM‘W’NNI

ATMNNI

ATMUNI


WATMRadio Port

ATMNetwork

ATMHost

ATMNNI

ATMUNI

ATM‘M’NNI


ATMNetwork

ATMHost

WATMRadio Port


ATM‘W’NNI

U-PLANE

User Process

AAL

ATM

ATM

PHY

ATM

ATM ATM

PHY PHY

ATM

WATM ATM

RAL PHY

ATM

WATM WATM

RAL RAL

User Process

AAL

ATM

ATM

PHY

ATM

ATM WATM

PHY RAL

ATM

ATM ATM

PHY PHY

ATM

ATM ATM

PHY PHY

C-PLANE

IG, UNI

SAAL

ATM

ATM PHY

SIG,UNI

SAAL

ATM

ATM PHY

SIG,NNI

SAAL

ATM

ATM ATMPHY PHY

SIG,NNI + M

SAAL

ATM

ATM ATMRAL PHY

W-CTL

W-CTL

W-CTL

SIG,NNI + M

SAAL

ATM

WATM WATMRAL RAL

SIG,NNI + M

SAAL

ATM

ATM ATMPHY PHY

SIG,NNI + M

SAAL

ATM

ATM ATMPHY RAL

W-CTL

SIG, NNI

SAAL

ATM

ATM ATMPHY PHY


Reference Model for ‘W’UNI to ‘M’NNI usage via Relay Satellite

Radio Access Segment Fixed Network SegmentRadio Access SegmentSatellite Segment

WATM‘R’ RAL

ATMNNI

ATMUNI

ATM‘M’NNI

ATM‘W’ UNI

WATM‘R’ RAL


ATMNetwork

ATMHost

WATMRadio Port

WATMTerminal

WATMTA

RelaySatellite

WATMRadio Port

WATMRadio Port

User Process

AAL

ATM

WATM

RAL

User Process

AAL

ATM

ATM

PHY

ATM

ATM ATM

PHY PHYU-PLANE

ATM

WATM ATM

RAL PHY

ATM

ATM ATM

PHY PHY

WATM WATM

RAL RAL

C-PLANE

SIG, UNI

AAL

ATM

WATM RAL

SIG, UNI

AAL

ATM

ATM PHY

SIG,NNI

SAAL

ATM

ATM ATM

PHY PHY

W-CTL

SIG,NNI + M

SAAL

ATM

ATM ATM

RAL PHY

W-CTL

SIG,NNI + M

SAAL

ATM

ATM ATM

PHY PHY

WATM WATM

RAL RAL


Reference Model for ‘M’NNI to ‘M’NNI usage via Relay Satellite

Radio Access Segment Fixed Network SegmentRadio Access Segment Satellite SegmentMobile Multi-UserPlatform

WATM‘R’ RAL

WATM‘R’ RAL

ATMNNI

ATMUNI

ATM‘M’NNI

ATM‘M’NNI

ATMNNI

ATMUNI


ATMNetwork

ATMHost

WATMRadio Port


WATMRadio Port

ATMNetwork

ATMHost

RelaySatellite

WATMRadio Port

WATM Radio Port

U-PLANE

User Process

AAL

ATM

ATM

PHY

AT

ATM

PHY

ATM

WATM ATM

RAL

AT

ATM

PHY PHY

User Process

AAL

ATM

WAT

PHY

ATM

ATM

PHY RAL

AT

ATM

PHY PHY

WATM

RAL

C-

User Process

AAL

ATM

ATM

SIG, UNI

AAL

ATM

ATM PHY

SIG,NNI

SAAL

ATM

ATMPHY

SIG,NNI + M

SAAL

AT

ATMRAL

W-CTL

SIG,NNI + M

SAAL

AT

ATMPHY PHY

SIG,NNI + M

SAAL

ATM

ATM ATMPHY RAL

WCTL

SIG, NNI

SAAL

AT

ATMPHY PHY

WATM

RAL RAL


Internet over satellite


Protocol reference architectures

PLATM

TDMA

Satellite ATM Architecture

AAL

Services & Applications

PLEther FDDI DQDB ATM

IP (Internetwork)TCP UDP

XDR

RPC

YPftp, mail , rcp, rlogin , rsh, telnet , talk, name

voice , video , multimedia

Existing Network Architecture

PL PL PL

NFS

Satellite


IP Throughput Issues

Version Header Len. Type of service

Total length

D M Fragment offset

Time-to-live Protocol

Header checksum

Identification

Source address

Destination address

Options

Data(<=65536 octets)

Bit order1 16H

ead

er


Time to live (8 bits, about 4.26 minutes)

� The field is decremented at least once at every router thedatagram encounters and when the TTL reaches zero, thedatagram is discarded.

� Specifications for higher layer protocols like TCP usually assume that the maximum time a datagram can live in the network is only two minutes.

� The significance of the maximum datagram lifetime is that it means higher layer protocols must be careful not to send two similar datagrams within a few minutes of each other.

� This limitation is particularly important for sequence numbers.


IP Fragmentation

� Different network media have different limits on the maximum datagram - Maximum Transmission Unit (MTU).

� IP supports fragmentation and reassembly� Fragments are identified using a fragment offset field .� Datagrams are uniquely identified by their source,

destination, higher layer protocol type, and a 16 bit IP identifier.

� there’s a clear link between the TTL field and the IP identifier

� MTU Discovery is a mechanism that allows hosts to determine the MTU of a path reliably.


TCP Segment Header

Data offset Reserved Flags

Urgent pointer

Header checksum

Sequence number

Options & Padding

Data Size (Optional)

Bit order1 16H

ead

er

Source port

Destination port

Acknowledgment number

Window size


Throughput Expectations

� TCP throughput determines how fast most applications can move data across a network. such as HTTP, FTP

� TCP performance directly impacts application performance.

� No formal TCP performance standards� A TCP connection should be able to fill the available

bandwidth of a path and to share the bandwidth with other users.


TCP Sequence Numbers

� TCP keeps track of all data in transit by assigning each byte a unique sequence number. The receiver acknowledges received data up to a particular byte number.

� TCP allocates its sequence numbers from a 32-bit wraparound sequence space.

� For a given sequence number uniquely identifies a particular byte, TCP requires that no two bytes with the same sequence number be active at the same time.

� Timestamps using an algorithm called PAWS (Protection Against Wrapped Sequence numbers) to distinguish between two identical sequence numbers sent less than two minutes apart.


TCP Transmission Window

� To allow the receiving TCP to control how much data is being sent, by advertising a window size to the sender.

� The window measures, in bytes, the amount of unacknowledged data that the sender can have in transit .

� The distinction between the sequence numbers and the window is that sequence numbers are designed to allow the sender to keep track of the data in flight, while the window is to allow the receiver to control the receiving rate.

� The standard TCP window size cannot exceed 64 KB, with the window of 16 bits wide.

� IETF enhanced TCP to negotiate a window scaling option.


Slow Start Algorithm

44 40 36 32 28 24 201612840

0 2 4 6 8 10 12 14 16 18 20 22 24

Timeout

Threshold

Threshold


Slow Start Algorithm (continue)

� The slow start algorithm is based on data sent per round trip. � This loss is interpreted as indicating congestion and the

connection scales back to a more conservative approach.� There are two problems

• The probing algorithm can take a long time to get up to speed. The time is R(1 + log2 (DB/L)) , where, R: round trip time, DB: delay bandwidth product, L: average segment size.

• The second problem is interpreting loss as indicating congestion.

� There is no easy way to distinguish losses due to transmission errors from losses due to congestion (assuming that all losses are due to congestion.)


Congestion Avoidance

� The sending TCP maintains a congestion window � Every round trip, the sending TCP increases its estimate of

the available bandwidth by one maximum-sized segment. Whenever the sender either finds a segment was lost or receives an indication from the network (e.g., an ICMP Source Quench) that congestion exists, the sender halves its estimate of the available bandwidth.

� The major issue with this algorithm is that over high-delay-bandwidth links and the linear probing algorithm

� Another issue is that the rate of improvement under congestion avoidance is a function of the delay-bandwidth product.


Selective Acknowledgments

� An extension to TCP by IETF� SACKs have two major benefits.

• improve the efficiency of TCP retransmissions by reducing the retransmission period.

• better evaluate the available path bandwidth in a period of successive losses and avoid doing a slow start.

� Inter-Relations - It is important to keep in mind that all the various TCP mechanisms are interrelated• the sequence space, window size, ... • More broadly, tinkering with TCP algorithms tends to

show odd interrelations.


Satellites and TCP/IP Throughput

� The need to implement the extensions to the TCP sequence space and window size.

� The relationship between slow start and performance over satellite links and some possible solutions.

� Satellites offer a range of channel bandwidths with relatively small delays of LEO and much larger delays of GEO satellites.

� The satellite performance problems are due to high-delay-bandwidth paths.


Slow Start Revisited

� The initial slow start period can be quite long and involve large quantities of data. Even at 1.5 Mb/s a GEO link must carry nearly 200 KB (5.6 seconds) before slow start ends.

� Interestingly enough, long-distance terrestrial links will also look slow and are comparable to those of LEO links.

� Short data transfers will never achieve full link rate. � Obviously some sort of solution to reduce the slow start

transient would be desirable. � But finding a solution isn’t easy.


Improving Slow Start

� Protect the Internet from congestion collapse. One of the important problems is that a sending TCP has no idea how much bandwidth a particular transmission path has.

� In the absence of knowledge, a TCP should be conservative: slow start by sending just one datagram in the first round trip.

� If the TCP had more information about the path, it could presumably skip at least some of the slow start process possibly by starting the slow start at a somewhat higher rate than one datagram.

� But actually learning the properties of the path is hard. IP keeps no path bandwidth information.


Spoofing

� To have router near the satellite link to send back acknowledgements for the TCP data to give the sender the illusion of a short delay path. The router then suppresses acknowledgements returning from the receiver, and takes responsibility for retransmitting any segments lost downstream of the router.

� There are a number of problems with this scheme. • Must buffer the data segment. • Requires symmetric paths.• Vulnerable to unexpected failures. • Doesn’t work if the data in the IP datagram is encrypted

unable to read the TCP header.


Cascading

� A TCP connection is divided into multiple TCP connections, with a special TCP connection running over the satellite link. Also know as split TCP.

� Because each TCP connection is terminated, cascading TCP is not vulnerable to asymmetric paths. In cases where applications actively participate in TCP connection management (e.g. Web caching) it works well. But otherwise cascading TCP has the same problems as spoofing.

� Higher error rates cause retransmissions and typically interpreted as a sign of congestion (goto slow start).

� Need to either reduce the error rate or to let TCP know that the datagram loss is due to transmission errors, not congestion.


Acceptable Error Rates

� There is no hard and fast answer to this problem. � The established TCP connection with data to send will

alternate between two modes:• congestion avoidance • slow start when loss becomes severe.

� To reach full capacity lasts p round-trip times, where p is the largest value such that the following inequality is true:

pΣ j < b or p(1+p)/2 < b j=1

� where b is the buffering in segments at the bottleneck in the path.


Teach TCP to Ignore Transmission Errors

� A link should have an effective error rate sufficiently low that it is very unlikely that the congestion avoidance phase will be prematurely ended by a transmission error.

� As an alternative to, or in conjunction with, reducing satellite error rates we might wish to teach TCP to be more intelligent about handling transmission errors.

� There are basically two approaches: either TCP can explicitly be told that link errors are occurring or TCP can infer that link errors are occurring.

� NASA has funded some experiments with explicit error notification as part of a broader study on very long space links.


Summary of Internet over satellite

� Satellite links are today’s high-delay-bandwidth paths. Tomorrow high-delay-bandwidth paths will be everywhere.

� Most of the problems described in this article need to be solved not just for satellites but for high-delay paths in general.

� TCP implementations should contain all the modern features (large windows, PAWS, and SACK).

� The TCP window space is larger than the delay-bandwidth product of the path.

� Reduce the impact of slow start.


Internet Quality of Service (QoS) over Satellite


Internet Applications and QoS

Elastic

Interactive e.g. Telnet, X-windows

Interactive bulk e.g. FTP, HTTP

Asynchronous e.g. E-mail, voice-mail


Inelastic applications

Inelastic(real-time)

Tolerant

In-tolerant

Rate Adaptive

Non-adaptive

Adaptive

Non-adaptive

Delay adaptive

Rate Adaptive

traditionalreal-timeapplications

newerreal-timeapplications


Integrated Services (Inteserv) Architecture

The InteServ architecture uses the following function to manage congestion and provide QoS transport:

� Admission Control� Routing Algorithm� Queuing discipline� Discard policy


Service categories

� Guaranteed• Assured data rate• Upper bound on queuing delay• No queuing loss• Real time playback

� Controlled load• Approximates behavior to best efforts on unloaded

network• No specific upper bound on queuing delay• Very high delivery success

� Best Effort


Call admission in Intserv

� Traffic characterisation and specification of desired QoS• Rspec defines specific QoS being requested• Tspec characterises traffic being sent by source, or

being received by destination� Signalling for call setup

• RSVP protocol carries Tspec and Rspec to routers on source-destination path

� Per-element call admission• Determine whether or not to admit call, depending on

existing commitments as well as requested Tspec and Rspec


RSVP and soft state

� Reservations are maintained with what is called “soft state” in Inteserv• each reservation has an associated timer• if timer expires, then reservation is removed• to maintain reservation, need periodic refresh

messages� Contrast with hard state

• explicit action required to establish and remove connections


Differentiated services

� Challenges of per-flow Intserv resource reservation• Scalability – maintaining per-flow state information for

each flow through router is a significant overhead• Pre-specified classes limit flexibility of service models

� Hence Diffserv, which is:• Scalable, through provision of simple functionality in

core, with more complex functions at edge• Flexible, in that it provides functional components to

define services� Diffserv provides bulk QoS


DifferServ architecture

� Packets are labelled for differing QoS using existing IPv4 Type of Service or IPv6 Traffic class.

� Service level agreement is established between provider and customer prior to use of Differentiated Service (DS).

� DS provides a built-in aggregation mechanism.� It is implemented by queuing and forwarding based on DS

octet.� DS services are defined within DS domain (contiguous

portion of internet) including: • Consistent set of DS policies are administered• Typically under control of one organization• Defined by service level agreements (SLA)


Core function: forwarding

� Per-hop behaviour (PHB) is “a description of the externally observable forwarding behaviour of a Diffserv node applied to a particular Diffserv behaviour aggregate, i.e. forwarding behaviour depends on DS field “mark”

� So, a PHB allows different traffic to receive different QoS� A PHB does NOT specify any particular mechanism � Differences in behaviour must be observable


Edge functions

� Packet classification• packets are classified, depending on IP address, port

number, protocol ID, etc• they are then marked, using the DS field• the mark identifies behaviour aggregate

� Traffic conditioning• after being marked, a packet may be forwarded into the

network immediately, delayed, or discarded• uses shaping/dropping functions, such as leaky bucket,

to do conditioning


IP QoS over satellite

� Satellite links can be integrated into IP networks� With Inteserv, QoS can be achieved by resource

reservation:• Guaranteed service• Controlled load service• Best effort services

� With Diffserv, QoS can be achieved by Per-Hop-Behaviour (PHB) at all routers and traffic conditioning function at edge router:• Classifier, Meter, Marker, Shaper and Dropper


ITU-T standards

� Those of most interest to Main Network Satellite operation are as follows:• E-series: Telephone Network and ISDN• G-series: Transmission• I-series: N-ISDN, B-ISDN and ATM• M-series: Administration, operation and Maintenance• Q-series: Switching and signalling• V-series: data communication over the telephone

network• X-series: data communication network


ITU-R standards

� The ITU-R is currently reorganising its documentation to more closely align with the ITU-T practice of putting more detail in its recommendations. Those of most relevant to Main Network Satellite operation are as follows:• Recommendation R.614: Allowable Error Performance

for a Hypothetical Reference Digital Path in the Fixed Satellite Service Operating below 15 GHz when forming part of an international connection in an ISDN.

• Report 997: Characteristics of a Fixed Satellite Service Hypothetical Reference Digital Path forming part of an an ISDN.


Internet Standards

� DoD IAB (1983) - Internet Activities/Architecture Board� Technical report RFCs (Request for Comments)� IRTF/IETF (1989) - Internet Research/Engineering Task

Force (long term research / short term engineering)� Internet society (1992)� More formal standardisation processing (learn from ISO)� RFC -> Proposed standard -> Draft standard -> Internet

standard � Rough consensus and running code


Satellites versus Fibre

� As recently as 20 years ago, data transmit was based 1200-bps modems over telephone lines.

� Since 1980s, long-haul networks have been replaced with optical fibre and high-bandwidth services like SMDS and B-ISDN.

� Communication satellites have some major niche markets that fibre does not (and sometimes, cannot) address.

� Many users still trust old twisted pair local loop with only 28.8 Kbit/s or 36.4 Kbit/s for fast modem.

� Some may wish to bypass local loop.


Satellites versus Fibre (continue)

� Fibre is not available everywhere, but satellite service is.� A second niche is for mobile communication. It is possible

to combine cellular radio and fibre for most users (but probably not for those airborne or at sea).

� A third niche is for situations in which broadcasting is essential. as in transmitting a stream of stock prices .

� A fourth niche is for communication in places with hostile terrain or a poorly developed terrestrial infrastructure.

� A fifth niche market for satellites is where obtaining the right of way for laying fibre is difficult or unduly expensive.

� Sixth, when rapid deployment is critical, as in military communication systems in time of war.


Summary

� Protocol basics and reference models� Network Description and Architecture� SDH over Satellite - Intelsat scenarios� Satellite system performance related service requirement� Issues on ISDN and B-ISDN� ATM over Satellite� Internet over Satellite � Standards - ITU-T, ITU-R, Internet

Documents

Satellite Networking - University of Surrey