Advantages and Disadvantages of PPPoA Architecture

PPP over ATM adaptation layer 5 (AAL5) (RFC 2364) uses AAL5 as the framed protocol, which supports both PVC and SVC. PPPoA was primarily implemented as part of ADSL. It relies on RFC1483, operating in either Logical Link Control-Subnetwork Access Protocol (LLC-SNAP) or VC-Mux mode. A customer premises equipment (CPE) device encapsulates the PPP session based on this RFC for transport across the ADSL loop and the digital subscriber line access multiplexer (DSLAM).

Advantages and Disadvantages of PPPoA Architecture

PPPoA architecture inherits most of the advantages of PPP used in the Dial model. Some of the key points are listed below.

• Advantages

- Per session authentication based on Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP). This is the greatest advantage of PPPoA as authentication overcomes the security hole in a bridging architecture.

- Per session accounting is possible, which allows the service provider to charge the subscriber based on session time for various services offered. Per session accounting enables a service provider to offer a minimum access level for minimal charge and then charge subscribers for additional services used.

- IP address conservation at the CPE. This allows the service provider to assign only one IP address for a CPE, with the CPE configured for network address translation (NAT). All users behind one CPE can use a single IP address to reach different destinations. IP management overhead for the Network Access Provider/Network Services Provider (NAP/NSP) for each individual user is reduced while conserving IP addresses. Additionally, the service provider can provide a small subnet of IP addresses to overcome the limitations of port address translation (PAT) and NAT.

- NAPs/NSPs provide secure access to corporate gateways without managing end-to-end PVCs and using Layer 3 routing or Layer 2 Forwarding/Layer 2 Tunneling Protocol (L2F/L2TP) tunnels. Hence, they can scale their business models for selling wholesale services.

- Troubleshooting individual subscribers. The NSP can easily identify which subscribers are on or off based on active PPP sessions, rather than troubleshooting entire groups as is the case with bridging architecture.

- The NSP can oversubscribe by deploying idle and session timeouts using an industry standard Remote Authentication Dial-In User Service (RADIUS) server for each subscriber.

- Highly scalable as we can terminate a very high number of PPP sessions on an aggregation router. Authentication, authorization, and accounting can be handled for each user using external RADIUS servers.

- Optimal use of features on the Service Selection Gateway (SSG).

• Disadvantages

- Only a single session per CPE on one virtual channel (VC). Since the username and password are configured on the CPE, all users behind the CPE for that particular VC can access only one set of services . Users cannot select different sets of services, although using multiple VCs and establishing different PPP sessions on different VCs is possible.

- Increased complexity of the CPE setup. Help desk personnel at the service provider need to be more knowledgeable. Since the username and password are configured on the CPE, the subscriber or the CPE vendor will need to make setup changes. Using multiple VCs increases configuration complexity. This, however, can be overcome by an autoconfiguration feature which is not yet released.

- The service provider needs to maintain a database of usernames and passwords for all subscribers. If tunnels or proxy services are used, then the authentication can be done on the basis of the domain name and the user authentication is done at the corporate gateway. This reduces the size of the database that the service provider has to maintain.

- If a single IP address is provided to the CPE and NAT/PAT is implemented, certain applications such as IPTV, which embed IP information in the payload, will not work. Additionally, if an IP subnet feature is used, an IP address also has to be reserved for the CPE.
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