Very high-speed Backbone Network Service

Transcription

In other lessons, we introduced three key network elements: the DSL Modem, DSLAM and B-RAS. Network elements are devices that make up a digital network. The arrangement and inter-linking of various network elements that make up a network is ëTopologyí. The DSL Modem, DSLAM and B-RAS cover the ëlast mileí or ëlast several milesí of the network which connects the subscriberís home or office. But thereís more in the network that extends beyond the DSLAM and B-RAS and towards the worldwide internet. In order to better understand Triple Play on DSL, we will take a detailed look at topology of a typical backward compatible DSL Broadband Network that connects a DSL customerís home or office through the CO and the Communication Service Providerís network all the way to the internet. So why are we interested in a ëbackward compatibleí network? DSL Network elements have advanced over many generations; however, worldwide, investment has been made in older generations of devices while newer ones evolved. Thus, in current DSL networks, elements with newer technology must remain compatible with older ones. This is ëbackward compatibilityí. Before we get into the networking details, let us briefly discuss some backward compatibility requirements of DSL networks. In earlier implementations of DSL, digital data streams between the DSLAM and the DSL Modem were transmitted via Asynchronous Transmission Mode or ATM links. These links were built on top of Broadband DSL. Although DSLAMs and DSL Modems have moved away from ATM towards Internet Protocol or IP, many DSLAMs still retain ATM capability for backward compatibility with older local loops and modems. In other lessons, we discussed that the DSLAM terminates multiple local loops connecting various subscribers. The DSLAM aggregates digital data on these multiple local loops and places the data on one or more ëupstream linksí. Upstream links are high capacity links usually built on Gigabit Ethernet or, in older implementations, Asynchronous Transmission Mode (ATM) technologies. Through these links, the DSLAM connects to an element called ëBroadband Remote Access Serverí (B-RAS) that sits at the ëedgeí of the Communication Service Providerís network. One key function of the B-RAS is to authenticate each subscriber to prevent fraudulent or unauthorized use of DSL services. In the earliest days, the DSLAM used to connect to the B-RAS on a single ATM link. Later, it was found that a single ATM link met only the performance needs of internet access, whereas the Video and Telephony parts of Triple Play required more stringent performance that was not met by this single ATM link. Internet access could be ëbest effortí, which means that internet browsing and download related data streams could be slowed down or stopped for short intervals when the DSL network was not performing at its peak, before recovering the streams again. Yet, the subscriber would not mind such short intervals of performance degradation most of the time because as they were browsing or downloading from the internet in bursts, these degradations didnít cause appreciable issues. However, Video and Telephony over DSL are not forgiving of such performance degradations. So, later generation DSL implementations used separate ATM links for internet access, TV and Telephony data streams between the DSLAM and B-RAS. These links were possible due to ATMís Switched Virtual Circuit (SVC) architecture. This reduced contention over network resources by multiple services and established priority for Video and Telephony data streams such that these streams would not slow down or stop when running. Beyond the B-RAS and towards the Communications Service Provider network, the data that the DSLAM aggregated from all subscriber local loops were ëroutedí to ëEdge Routersí over Gigabit Ethernet links. The Edge Routers carried subscribersí data into and from the internet. In more recent implementations, the B-RAS has been merged with another entity called ëBroadband Network Gatewayí (BNG) because the BNG has more functionality over and above B-RAS. In our subsequent discussions, we will refer to the BNG rather than B-RAS as BNG is a more contemporary entity. So, to summarize, one of the main backward compatibility requirements for DSL Broadband networks is support for both ATM and Ethernet technologies. In the next chapter, we will discuss details of a DSL Broadband network designed for Triple Play. ** Here is a typical topology of DSL Broadband network designed for Triple Play services, i.e., Data, Video and Telephony. In our example network, the link between DSLAM and DSL Modem is Asynchronous Transmission Mode or ATM based, and the uplink between DSLAM and BNG is Gigabit Ethernet based. We chose this example since it is a typical scenario in many existing implementations around the world. The DSL local loop between DSLAM and the Modem implements ATM as a Data Link Layer Protocol over the DSL Physical Layer. The BNG and the DSL Modem use a Point to Point protocol or PPP to transmit subscriber traffic. Since the subscriber ultimately connects to the internet, the protocol that the subscriberís data, video and telephony traffic uses is Internet Protocol or IP. Thus, the subscriber traffic consists of IP packets. PPP is used to encapsulate these IP packets and ëtunnelí them across the path connecting the DSL Modem, DSLAM and BNG. Note that BNG is typically connected by Gigabit Ethernet links, so the PPP frames themselves are encoded to Ethernet Frames. We call this arrangement PPP over Ethernet or PPPoE. The link layer protocol between the DSL Modem and DSLAM is Ethernet frames, which carry PPP as their payload, encoded within ATM cells. We will cover more on ATM cells shortly. That apart, what we have discussed so far in our example network is a protocol ëstackí. If we map this stack to the OSI seven-layer protocol stack, for the link between DSL Modem and DSLAM, DSL is at the Physical layer and ATM and Ethernet at the Data Link Layer. For that between the DSLAM and BNG, Resilient Packet Ring or RPR is at the Data Link Layer. Let us take a brief look at DSL which is at the physical layer between DSL Modem and DSLAM. DSL technology can be classified into three groups, according to standards laid down by American National Standards Institute (ANSI) and European Telecommunications Standards Institute (ETSI). Each group of DSL technology has several variants. Each variant is designed for most optimal performance with a certain loop length (distance of the customerís premises from the CO), maximum data transmission rate, identical or different data transmission rates in upstream and downstream directions and the type of application, i.e. home or business. Symmetric DSL: this kind of DSL allows for the same data transmission rate in both upstream and downstream directions. Some variants in this group are: HDSL, SDSL, SHDSL Asymmetric DSL: this kind of DSL provides higher downstream data transmission rate than upstream data transmission rate. This design is a common application because many users need higher downstream data transmission rate to allow for faster downloads and page loads in their internet browsers. Following are some variants in this group: ADSL, ADSL 2, ADSL 2+ Symmetric & Asymmetric DSL: This kind of DSL provides both symmetric and asymmetric data transmissions. Some variants in this group are: VDSL, VDSL2 It is common for modern DSL Modems to double up as a Modem and a Router. This device functions as a Modem on the WAN side and as a Router on the Local Area Network or LAN side. While the WAN side connects the BNG, the LAN side is the Subscriberís home or office network where the Subscriber connects their computers and other related devices such as a printer. Such devices in the network are generally called ëhostsí. Let us come back to the protocol stack we described for the WAN. The ATM layer in the stack is a connection oriented protocol that creates a virtual connection or ëcircuití between two endpoints before data communication commences between the endpoints. ATM utilizes small, fixed size ëcellsí into which subscriber data is encoded. These cells are transmitted in virtual circuits or VCs set up between the DSL Modem and DSLAM. A Virtual Circuit is characterized by its Virtual Path Identifier or VPI and Virtual Channel Identifier or VCI pair. Thus, an ATM based DSL Modem has at least one VC specified by a VPI/VCI pair. This is an important parameter from the point of view of configuring the DSL Modem, since modern DSL Modems that are designed for Triple Play can support separate VCs for data, video and telephony. Since separate VCs can be dedicated to data, video and telephony services, each VC can be configured with a ëQuality of Serviceí or QoS contract that is needed for delivery of the concerned service at its optimal quality. Every contract comes mainly in the form of traffic engineering to achieve the desired bit rate demanded by a service. Four basic types of traffic contracts in use are Constant Bit Rate or CBR, Variable Bit Rate or VBR, Available Bit Rate or ABR and Unspecified Bit Rate or UBR. The protocol stack on the WAN side of DSL Modem establishes an IP network between the DSL Modem and BNG. We can see that it is logical that BNG, being PPP termination for the DSL Modem, manages subscriber authentication based on user id/password credentials exchanged via PPP. Authentication of subscriber is needed before authorizing service access to the subscriber. After BNG performs authentication of subscriber and authorizes for service access, the DSL Modem typically receives an IP address on its WAN interface via DHCP. The DSL Modem can also receive an IP address by other means such as from a static pre-configured list. Recall that the modern DSL Modem also has a router function that manages the LAN side. For data (internet) access, the WAN IP address that gets assigned to the DSL Modem is typically Network Address Translated (NATed) to the private IP addresses of hosts on the LAN side, according to RFC 1483 Routing. The router attached to the DSL Modem acts as the default gateway for the LAN, and it allocates private IP addresses to each host connected to the LAN. In our example, the Modem and Router are integrated as one device. In this arrangement, the Modem and Router are two electronic modules integrated via an internal bus. Thus both the Modem and Router modules are inside the same box. In this case, the NAT between WAN IP address and private LAN IP addresses and RFC 1483 Routing are done via the internal bus. The IP packets are encapsulated by PPP. PPP frames are in turn encapsulated by Ethernet frames and the Ethernet frames are encoded into ATM cells for transmission between DSM Modem-Router and DSLAM. This is designated by Point to Point Protocol over Ethernet over ATM or ëPPPoEoAí. For Video service such as TV over Internet Protocol or IPTV, NAT is not done in our exmple, instead, the WAN IP address is ëbridgedí according to RFC 1483 Bridging by the router attached DSL Modem to a TV host such as Set Top Box. In this case, the Set Top Box is assigned the WAN IP address itself. In this case, since the DSL Modem-Router is only acting as a bridging device for the WAN IP, the actual IP path is between the Set Top Box, DSLAM and BNG. So, IP packets from the Set Top Box need to be encapsulated by PPP for transmission along this route. But the Set Top Box is connected to the DSL Modem via Ethernet. Therefore, PPP frames must be encoded into Ethernet Frames so that the PPP frames can be transmitted between the Set Top Box and BNG via the bridging DSL Modem. Note, on the WAN side, the DSL Modem is capable of receiving a different WAN IP address per Virtual Circuit since each VC ëseesí a separate IP network established via PPPoEoA. Since each VC is dedicated to a service, every service is accessed on a separate WAN IP network. Because every VC is engineered to provide QoS appropriate for the service it is delivering, the services can be delivered without too much resource contention or quality degradation. The topology of the network between DSLAM and BNG is not DSL based. In our example, it is based on Resilient Packet Ring or RPR. DSLAM extracts Ethernet payload from the PPP frames and transmits the extracted Ethernet frames on the RPR ring. Note that the Ethernet ëpayloadí is nothing but the subscriberís traffic, i.e. IP packets originating from the subscriberís LAN via DSL Modem-Router. RPR attempts to take the reliability of connection oriented Digital Backbone network communication and combine it with efficiencies of packet oriented communication. Thus it is suitable for reliable transmission of Ethernet frames to BNG. Since RPR is not a DSL standard, we will discuss RPR in another lesson. To summarize, we discussed a typical DSL Broadband network in which we walked through some key design and topology considerations for delivering Triple Play services. Fulfillment of Triple Play with such a network requires allocation and provisioning of network resources, configuration of related parameters and planning for network capacity and performance. Information Technology plays a central role in performing these functions, implementing and managing DSL Broadband Networks for Triple Play. With OSSnextTM, we provide training on design, analysis and architecture of IT systems that perform these functions. Our aim is to provide high quality industry readiness and domain experience in Information and Communications Technology (ICT) industry. We invite both fresh and experienced IT professionals who aspire to grow their career in the Telecom Operations industry to talk to us about how we can help.