An up-to-date overview on QoS for Satellite IP networks

An up-to-date overview on QoS for Satellite IP networks 1st Petre-Daniel Matasaru1, 2nd Luminita Scripcariu1, 3rd Felix Diaconu1 1

Telecommunications Department, Faculty of Electronics, Telecommunications and Information Technology, Technical University “Gh. Asachi” Iasi, Romania [email protected]

Abstract—This paper is an up-to-date short overview of the QoS for Satellite IP networks. Some definitions, parameters and requirements for QoS are given. IP QoS Architectures are presented and various contributions to end-to-end QoS at each layer are included. DVB-RCS and evolution to standard DVB-S2 RCS to support emerging Internet services via satellite is also briefly approached. Possible integration with Next Generation Network (NGN) architectures in order to meet their QoS requirements is presented. The horizontal integration of the satellite segment into an IMS network may provide a consistent QoS management across the entire network, while mechanisms defined in the IMS framework bring flexible and performant QoS support to the session and network.

I.

INTRODUCTION

Satellite networks represent a powerful resource in providing global Internet access and insure electronic connectivity, being able to meet a great variety of data communication needs for different beneficiaries. The feature of wide area coverage plus the ability to deliver wide bandwidths with a consistent quality of service makes satellite links attractive alternatives for both emerging and developing markets and geographically large developed countries and regions. Nevertheless, the point to multipoint connectivity feature of geostationary satellites represents a better option than fiber optic networks for the distribution of IP-based multimedia content (Internet websites, radio, television, movies etc.) in the case of unpopulated, hardly accessible territories [1]. II. A.

QOS: DEFINITIONS, PARAMETERS, REQUIREMENTS

Definitions of QoS The term “Quality of Service (QoS)” has multiple meanings, ranging from the final user’s perception of a certain service, warranty of a certain level of performance to a data flow and ending to a set of parameters required to achieve a given service quality. According to S. Kota and M. Marchese [2], the end-to-end Quality of Service (QoS) is defined as the ability of a network component to have a certain assurance level that its traffic and service requirements can be satisfied. According to ISO 8402 [3], QoS represents the totality of characteristics of an entity that bear on its ability to satisfy stated and implied needs. ITU-T Recommendation E.800 states that QoS is the collective effect of service performance which determines the degree of satisfaction of a user of the service [4]. Depending on the point

of view, QoS may be different - that is, a user’s perception of QoS is not identical to the QoS offered and achieved by the service provider (e.g. the availability of a satellite service is 99.5% in a year, the degree of satisfaction for a costumer ranges from 1 - unsatisfactory to 5 - excellent). B. QoS Parameters The digital data that is contained within a satellite transmission may take many forms, a standard 36 MHz transponder used in C-bands and Ku-bands worldwide being able to transfer up to 80Mbps, enough for wide-band applications and multimedia. Thus, satellite data networks employing VSAT’s become alternative to terrestrial networks composed of fiber optic and microwave radio [1]. However, in any packet-switched networks, QoS is affected by human-related factors like stability of service, availability of service, delays, user information and technical factors like reliability, scalability, effectiveness, maintainability, grade of service, etc.[5] Some of the initial relevant QoS parameters are shortly presented in the following lines [2]. Bandwidth represents the maximal bit-rate (throughput) to be sustained for end-to-end communication and it is limited by both physical components of the traffic path within transit networks and the varying load from different users sharing the same network resources. Real time multimedia services may be affected by a low bandwidth. Packet loss represents the ratio of the number of undelivered packets to the total number of sent packets. Packets are not delivered when their data load is corrupted due to bit errors caused by noise and interference (wireless communications), when data packets are delivered out-of-order or when routers buffers are already full and packets are dropped. TCP requests for the information to be re-transmitted, thus causing delays and lower bandwidth. Video and VoIP streams may be affected. Reliability represents the percentage of network availability depending on weather and environmental conditions. In satellite networks availability also depends on the operational frequency band, power levels, antenna size and the amount of traffic required for the provided service. Delay represents the time taken by a packet to be transported from sender to destination. Sometimes it may take a long time due to being held in queues or taking a less direct route to avoid traffic congestion. Large delays make the Transmission Control Protocol (TCP) insensitive to dynamic short-term changes, thus

interactive VoIP, video applications or online gaming become unusable. Jitter represents the variation in end-to-end transit delay, meaning that packets from the sender will reach the destination with different unpredictable delays. TCP protocol operates inefficiently when it tries to re-establish a data flow, thus realtime applications like audio and video streaming quality may be diminished down to unsatisfactory level. However, with the continuous development of technology and increased demand for efficient, reliable and low-cost equipment, starting from the standardization of Digital Video Broadcasting - Returned Channel via Satellite (DVB-RCS) and the long-term efforts to evolving the standard to DVB-S2/RCS to support emerging Internet services via satellite, a lot of work on QoS in IP over satellite has been done, targeting an improved encapsulation mechanism for IP packets and a new adaptive transmission scheme. Furthermore, fully IP-based solutions represent a necessary, yet an intermediate step in the complete integration of satellite networks in Next Generation Networks [7]. C. QoS Requirements The International Telecommunication Union (ITU) defined in 2001 a model for multimedia Quality of Service (QoS) categories from an end-user viewpoint [9]. Eight different categories have been identified considering user expectations for various multimedia applications and tolerance in terms of delay and packet loss. This model can be used as basis for deriving realistic QoS classes for satellite networks and associated QoS control mechanisms. Long term evolution of IP QoS architectures (starting with IETF Intserv [8] and IETF Diffserv [9], continuing with Multiprotocol Label Switching - MPLS based architectures [10] and further) and the introducing of Adaptive and Coding Modulation techniques (ACM) that increase network efficiency according to weather conditions brought impact and new perspectives to the QoS management systems. Bandwidth allocation capabilities are insured by two algorithms - Connection Admission Control (CAC) and Bandwidth-on-Demand (BoD). The Condition of Service (CoS) requirements can be categorized based on constraints of bandwidth, delay and jitter. For each identified MPLS traffic category, Tabel I maps the CoS reqirements, the adequate DVB class of priority, the Quality of Service quarantees and the functional modules that help meeting the QoS requirements [7] TABLE I. IP classes GS IntServ EF DiffServ CS IntServ AF DiffServ

INTSERV / DIFFSERV QOS CLASSES MAPPING ONTO DVB-S / RCS [7] CoS requirements

DVB priority class

QoS guarantees

CAC

BoD

Bandwidth Delay Jitter

Real Time

Bandwidth Delay Jitter

yes

no

Bandwidth

Critical Data

Bandwidth

yes

optional

IP classes

CoS requirements

Best Effort

-

DVB priority class Best Effort

QoS guarantees

CAC

BoD

-

no

yes

In July 2016, a generic IP performance Model, IP pachet transfer performance parameters and IP service availability function and parameters have been defined by ITU as Recommendation ITU-T Y.1540. QoS requirements for performance of speed, acuracy, dependability and availability of IP packet transfer of Internet Protocol version 4 (IPv4) and version 6 (IPv6) data communication services have been set. Other recommendations for the performance of point-tomultipoint IP service are currently in study and development before release. [11] III.

IP QOS ARCHITECTURES. IP MULTIMEDIA SUBSYSTEM

Many satellite communication systems support Internet applications, but QoS supplying architecture standards to support multimedia services over satellite IP still have to be developed. Considering the user point-of-view in a satellite network, end-to-end QoS depends on the quality of services targeted and acquired in every layer of the network, meaning that all layers and all network elements must properly work together to minimize performance lowering per each individual layer. The satellite dependent and satellite independent functionality may be separated, as ETSI suggests in their Technical Report on a protocol architecture for a broadband satellite multimedia (BSM) proposal [10] QoS functions of the satellite network components and QoS parameters may define QoS classes to be mapped with IP QoS from the lower to the higher level. IT applications using broadband multimedia or modern client-server computer networks require efficient data transfer among different users in different locations, thus by insuring a good IP QoS satellite links maintain an undisputed position as part of the backbone of the Internet. At the physical layer, satellite network designers apply new and advanced technologies to grow data capacity of satellite transponders, aiming to either improve the power performance (Eb/N0), or bandwidth efficiency, or both. Modulation and coding, adaptive coding, power control and diversity are some of the most important techniques. Modulation types determine the transmitter power, the required bandwidth, considering interference from adjacent channels and transponder linearity. Techniques like QPSK, 8PSK, 16-QAM, 16-APSK, 32-APSK are most used in satellite systems. Turbo coding significantly enhances performance for digital video broadcasting - return channel via satellite systems (DVB-RCS). Adaptive coding provides efficient throughput and saves electrical power aboard the satellite. Power control techniques allocate extra power to the transmit carriers to compensate for rain attenuation. [11]

At the link-layer, for broadband IP-based satellite networks a medium access control (MAC) has been standardized and adapted to provide QoS. Multi-frequency-time division multiple access (MF-TDMA) implies dynamic bandwidth allocation and this is a challenge when it comes to QoS for various classes of service. An alternative to MF-DAMA DVB-RCS may be a CDMA-based spread ALOHA single code multiple access scheme. [12] At the IP-layer, IP-packets are treated equally in terms of best-effort, but in terms of QoS things are changed, because if in the case of delay-non sensitive data applications performance may be satisfactory, for delay sensitive data applications performance may turn unacceptable. There are four QoS models for an IP network: IntServ – Integrated Services, DiffServ – Differentiated Services, IntServ/DiffServ – IntServ at the margin and DiffServ at the core of the network and MPLS – Multiprotocol Label Switching. Integrated Services distinguish every source-destination flow, using resource reservation protocol (RSVP), packet scheduling and buffer management, similar to ATM technology. Differentiated Services do not distinguish each traffic-flow, marking the packets when they enter the network and aggregating more flows in a traffic class of services. MPLS represents a convergence of ATM and IP technologies, incoming packets being classified and wrapped in a header that carries the adequate label for forwarding. [13] Internet security standards (IPSec) provide encrypted security services like confidentiality, authentication and integrity and became mandatory for IPv6. [14] In order to develop specific QoS architectural models for global satellite networks, efficient queue-management and dedicated algorithms for packet scheduling, application layer security, end-to-end encryption rules and performance enhancing proxies need to be considered. Last, but not least, interconnecting satellite IP networks and mobile IP wireless 3G, 4G (LTE) and 5G must be achieved. At the transport-layer, when packets are lost due to errors, the TCP sender may react different if Explicit Transport Error Notification (ETEN) for satellite environments is used [15]. Satellite Transport Protocol (STP) is a particular type of transport protocol developed for satellite links that uses buffers of the size of estimated bandwidth delay [16]. Space Communications Protocol Specifications – Transport Protocol (SCPS-TP) was developed for both delay and multiple sources of packet loss, using a congestion control algorithm independent of signaling network congestion on data loss. [17]

The appearance of the IP Multimedia Subsystem (IMS) architecture [18] for terrestrial managed networks opened a valid basis for real integration of the satellite access systems into wider networks [19]. It was designed initially as an upgrade of GPRS mobile networks to deliver IP services, suggesting a standardized service architecture based on IP protocols and enabling end-to-end QoS oriented services between different types of networks (e.g. mobile, fixed and IP networks) independent from underlying technologies. Later on, IMS was intended to provide Internet access for 3G networks [20], so the network layer has IP as the core protocol and relies on PolicyBased Network Management principles. Baudoin et al [21] proposed a new QoS architecture for IP satellite systems to introduce optimization paths between layers and supply advanced end-to-end QoS support. Their focus is on transparent satellite access systems (e.g. DVB-RCS) leading to a star topology via a satellite gateway, but the approach may be completed up to systems with mesh connectivity where the satellite terminal can communicate directly without going through the gateway, the standardized access point to IMS (proxy) being co-located with the satellite terminal. The QoS architecture presented in Fig. 2 is derived from the terrestrial IMS QoS architecture, with a Policy Decision Point in charge of the satellite access domain connected to the access point and 2 Policy Enforcement Points - one in charge of QoS enforcement at IP level, located in the satellite terminal and another with the same role for the edge router of the satellite access domain [21]. Furthermore, this approach may be extended to mesh connectivity, a Connection Control Protocol designated to establish a MAC connection between satellite terminals [22] indicating mechanisms for QoS management in link layer similarly to QoS model used in terrestrial networks. Emulation of GPRS and UMTS networks that rely on IP traffic in Policy Decision Points is also available with this Connection Control Protocol in the context of IMS architecture [21]. However, Next Generation Networks (NGN) does not rely just on IP Multimedia Subsystem (IMS) architecture. End-toEnd QoS Over Heterogeneous Networks (EuQoS) is another NGN architecture compliant with the ITU NGN recommendation [23], targeting end-to-end QoS warranty for final users and sharing several network elements and protocols with IMS networks. A clear separation between applications, services and transport is made, with standardized interfaces and communications on network independent functions and specific QoS enforcement Policy on functions and elements dependent of the network.

Figure 1. Satellite IMS QoS architecture [21]

IV.

CONCLUSIONS

This paper is an up-to-date short overview of the QoS for Satellite IP networks. Some definitions, parameters and requirements for QoS according to ITU-T G.1010 and ITU-T Y.1540 are given. IP QoS Architectures are presented and various contributions to end-to-end QoS at each layer are included in the overview. Possible integration with Next Generation Network (NGN) architectures in order to meet their QoS requirements and specific optimization schemes to improve QoS is also approached. For satellite IP networks, more research on access protocols, application QoS models and interconnectivity and operability with mobile networks and endto-end security is required. The horizontal integration of the satellite segment into an IMS network may provide a consistent QoS management across the entire network, while mechanisms defined in the IMS framework bring flexible and performant QoS support at the session and network layers of the network architecture. REFERENCES [1]

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