encapsulation ADSL modem-router

[albatros]

I know what is encapsulation but in relation to ADSLmodem/router where does it fit?
The router's protocol is PPPoA.

Can somebody explain that to me?

[SomeJoe]

I know what is encapsulation but in relation to ADSLmodem/router where does it fit?
The router's protocol is PPPoA.

Can somebody explain that to me?

In terms of the layer 7 OSI model, an internet browser running on your computer would roughly communicate like this:

Internet browser, protocol HTTP (layer 7 - Application) -->
encapsulated in protocol TCP (layer 4 - Transport) -->
encapsulated in protocol IP (layer 3 - Network) -->
encapsulated in protocol Ethernet2 (layer 2 - Data Link) -->
100Mb 4B/5B encoded electrical/voltage (layer 1 - Physical) -->
sent via ethernet wire from computer to your router -->
unencapsulated back to protocol IP (layer 3 - Network) -->
network address translation (NAT) -->
encapsulated in protocol PPP (layer 3 - Network) -->
fragmented and encapsulated in protocol ATM (layer 2 - Data Link) -->
ADSL signaling electrical/voltage (layer 1 - Physical) -->
sent via DSL signal to telco

- HTTP protocol provides communication between HTTP server and HTTP browser using GET/PUT and other HTTP commands.
- TCP protocol provides reliable data delivery on a particular port. It is connection-oriented with positive acknowledgement of each packet, automatic retransmission of lost packets, and ensures packets arrive in order.
- IP protocol provides addressing and delivery from logical network node A to logical network node B. Provides core services like address resolution (DNS), automatic configuration (DHCP), and network maintenance/status (ICMP).
- Ethernet2 protocol provides delivery from physical ethernet node A to physical ethernet node B using Media Access Control (MAC) addresses. Provides core services for the Ethernet medium like broadcast packets and Address Resolution Protocol (ARP).
- Network Address Translation (NAT) is a mechanism used by some routers to provide access to many IP addresses on the local side of the router using only one IP address on the global side of the router.
- Point-To-Point Protocol (PPP) provides point-to-point transport of any other protocol encapsulated inside it. PPP also provides some core services including logon, authentication, link control/multilink management, and IP address assignment.
- Asynchronous Transfer Mode (ATM) protocol is used to transport data over ATM networks. ATM networks get their speed from simplicity - each packet is a fixed size and only has one 5-byte header. These cells are 53 bytes, with a 5 byte header and 48 byte payload.

As you can see here, the PPPoA mechanism places the PPP logon/authentication information as the next layer inside the ATM cells. The IP packets are inside the PPP packets. This protocol is typically used in small business ADSL implementations where a small range of IP addresses is assigned, and those IP addresses are given to the computers via standard DHCP.

Contrast that mechanism with the more popular residential ADSL service, PPPoE:

Internet browser, protocol HTTP (layer 7 - Application) -->
encapsulated in protocol TCP (layer 4 - Transport) -->
encapsulated in protocol IP (layer 3 - Network) -->
encapsulated in protocol Ethernet2 (layer 2 - Data Link) -->
100Mb 4B/5B encoded electrical/voltage (layer 1 - Physical) -->
sent via ethernet wire from computer to your router -->
unencapsulated back to protocol IP (layer 3 - Network) -->
network address translation (NAT) -->
encapsulated in protocol PPP (layer 3 - Network) -->
encapsulated in protocol Ethernet2 (layer 2 - Data Link) -->
10Mb Manchester encoded electrical/voltage (layer 1 - Physical) -->
sent via ethernet wire from router to modem -->
unencapsulated back to protocol Ethernet2 (layer 2 - Data Link) -->
fragmented and encapsulated in protocol ATM (layer 2 - Data Link) -->
ADSL signaling electrical/voltage (layer 1 - Physical) -->
sent via DSL signal to telco

Here you can see the PPP layer is done at the router and is inside an Ethernet2 packet before being broken into ATM cells and sent to the telco by the modem. The router in this case performs PPP negotiation with the telco and gets an IP address via PPP, not DHCP. Only 1 IP address can be handed out via PPP, and the router uses this as it's outside global address which all internal addresses are NAT'd to.