Encapsulation at higher layers

ATM in a nutshell

ATM meets the requirements for a personal broadband service include handling timing-sensitive transmission of video and audio services while efficiently transferring large and variable blocks of data involved in traditional data communications tasks such as web browsing and file downloads.

ATM uses a short and fixed length cells consisting 53 byte of information, and these properties of the cells allow ATM to transport both data and real time information and efficient multiplexing of data from multiple sources. ATM also offers the advantage of making it easy to interleave different user data streams into a common connection with no impact on the individual users. ATM is responsible for the definition of logical connections through the network that are known as virtual circuits (VCs).
more info on ATM can be found here.

The Architecture

Specific ADSL Reference Model for ATM Mode consists of two primary  parts:  Access Node (AN) and Broadband Network Termination (B-NT), connected by an interface called U-interface.  We will discuss the main components of both parts, which are detailed in the following figure:

Figure 1. ATM over ADSL architecture

• Access Node :  the ATM core network is connected via the V interface to the Core ATM Network Interface Element (for example: SONET), which supplies several PHY-ATM functions (such as congestion control, cell rate decoupling, traffic shaping, etc.).
  The VPI/VCI Translation and Higher-Layer Functions is in charge of the multiplexing/demultiplexing process:  in the downstream direction the cells are divided between the ATU-Cs on a VPI/VCI basis.  In the upstream direction the opposite happens - the cells from each ATU-C are combined to form a single cell stream for the ATM network.  In any case, both fast (TC-F) and interleave (TC-I) channels are supported (provided the ATU-C supports them both).
  The ATU-C is the ADSL Transceiver Unit — Central Office, as specified in the ADSL PHY Recommendation.  This Recommendation also specifies to include the ATM-TC (Transmission Convergence) functions within the ATU-C as specified in the ATM  transport protocol, and the logical interface (called V-C) between those and the corresponding ATM layer functions .  Those functions include: header error control verification, idle cell insertion, bit timing ordering, and more.  For ATM layers description and info please check the ATM site.
  
• Broadband Network Termination:  The ATU-R is the ADSL Transceiver Unit — Remote, as specified in the ADSL PHY Recommendation.  This Recommendation also specifies to include the ATM-TC (Transmission Convergence) functions within the ATU-R as specified in the ATM  transport protocol, and the logical interface (called T-R) between those and the corresponding ATM layer functions.
  The VPI/VCI multiplexing process is similar to the access node.
  The optional ATM access module may perform some PHI-ATM functions as in the AN, as well as VPI/VCI translation.
  The PDN/TE (terminal equipment) interface element function contains the PHY layer interface to the PDN/TE via the T or S interface.
  Finally, an optional module terminating the ATM provides ATM adaptation layer functionality (SAR and PDN functions).  The S or R interface performs the final connection of the BN-T to the non-ATM premises PDN or TE.

Transport of ATM over ADSL and QoS

In order to implement a medium that can transfer ATM traffic, we require at least one full duplex capable communication media. To implement this minimum requirement, we will need to define at least one upstream channel and one downstream channel. We can add another pair of channels to implement dual latency traffic.

ATM systems use channelization using different Virtual Paths and Circuits. This mechanism is also used to provide QoS (quality of service).  ADSL can be used to support ATMs QoS requirements in the following ways:

• The underlying ADSL transport layer for ATM may support some optional functions such as Dynamic Rate Repartitioning (DRR). This function once implemented in the ADSL layer, can allow the ATM layer on top greater control on quality of service. As we recall, ADSL bandwidth is determined by an aggregate number of channels. The ATM layer, once it determines there is an application that needs lower latency, can repartition the ADSL channels, and ask the ADSL layer to change one of the channels to a fast (non-interleaved channel).
• Since ADSL aggregate link speed is dynamic, and can change over time due to physical reasons (external interference for example), it can employ an option named Dynamic Rate Change to notify the ATM layer of the aggregate link capability. The ATM layer receives information about the minimum link capability, maximum link capability, and current capability. The minimum capability should be the guaranteed resources provided by ATM, while any extra bandwidth could be provided as non-guaranteed resources. Should the extra bandwidth suddenly drop for example, some ATM traffic with low priority using the non-guaranteed resources can be dropped.

• The BER(bit error ratios)/ Latency/ Range are fully configurable by the Network Operator to meet the required service quality for the network. BER of 10^-7 is recommended.

Most of the Traffic management functions as specified by ATM Forum Traffic Management 4.0 and ITU-T I.371 are not supported by the underlying ADSL transport layer, and thus have to be supported by the U and V interfaces. An operator having control of control of the traffic endpoint, can shape the traffic to conform to the agreed upon contract.

ADSL-based protocol stacks

ADSL requires no dial-up. When ATM connection is set up between the ATU-R and the ATU-C, this connection is named a permanent virtual circuit (PVC). Each PVC is connecting between the DSL line and the ATM network, and to an ISP. Therefore, the user has a connection to the internet through the ISP as long as the ATU-R is turned on.

ADSL infrastructure  is build upon the current dial-up system, which uses point-to-point protocol (PPP) to support network services such client addressing. PPP-over-ATM-over-ADSL model has become the standardized method for accessing data networks over ADSL.

The question of choosing the most suitable protocol stack for the higher layers above the ADSL is not a simple one and depends on the existing  network, the services we expect to receive, and more.  Figure2 summarizes the most common schemes for ADSL based protocol stacks.

Figure 2.  ADSL based protocol stacks.

Ethernet frames over ATM (EoA)

Ethernet frames are encapsulated into the ATM Adaptation Layer 5 (AAL5) using RFC1483, the "Multi-protocol Encapsulation over AAL5".  Both routing (i.e. forwarding messsages using IP addresses)  and bridging (i.e. forwarding  messages using MAC address) are supported.

IP packets over ATM (IPoA)

Uses the same type of encapsulation as EoA, only adds address resolution function to the ATM PVCs, in order to enable the IP stack obtain the IP address for another IP host connected to its local ATM subnet (RFC 2225 - "Classical IP and ARP over ATM").

PPP over ATM (PPPoA)

Based on the standard RFC 2364 "PPP over AAL5", along with RFC1483, this is the most logical choice to connect between the individual customer's CPE and the service provider over existing ATM network.  The combination of the PPP layer makes its great features available for use.  Those include:

• Authentication - an important feature to suplly security for the connection.  Identification is controlled with a user name and password, varified by the service provider servers.
• IP address management - Static, Pseudo, and Dynamic (most common) address assignment.
• Link monitoring
• DNS address assignment

PPP over ETHERNET (PPPoE)

Similiar to PPPoA, this is the most logical choice to connect between the individual customer's CPE and the service provider over existing Ethernet-based network.  The only difference involves the encapsulation technology used.  PPPoE uses Ethernet networking with PPP in an encapsulation scheme which enables multiple PCs to connect to multiple destinations through a single, shared CPE using only one PVC, based on RFC 2516
PPPoE also enjoys the benefits of PPP as PPPoA, but pay with a higher packet overhead.

Native ATM

A method which uses ATM all the way, without the use of any other layers which increses overhead.  Ideal for voice and video applications.

Next: Variants of ADSL