components of an ADSL network. Most DSL networks
use ATM as the data-link layer protocol. In basic terms, a
DSLAM is an ATM switch with DSL interface cards (ATU-Cs). The
DSLAM terminates the ADSL connections and then switches the
traffic over an ATM network to an aggregation router. The
aggregation router is the Layer 3 device where IP connections
from the subscriber terminate. There are three ways to
encapsulate IP packets over an ATM and DSL connection:
- RFC 1483/2684 Bridged
- PPP over Ethernet
(PPPoE)
- PPP over ATM (PPPoA)
In RFC 2684
Bridging, the ADSL CPE bridges the Ethernet frame from the end
user’s PC to the aggregation router, where integrated routing
and bridging (IRB) provides connectivity to the IP cloud. RFC
2684 Bridging has security and scalability issues, making it
unpopular. ISPs are now opting for PPPoE and PPPoA, both of
which are much more scalable and secure but involve a more
complex implementation.
Note that the connection from
the ATU-R to the ATU-C is using DSL at Layer 1 to send and
receive ATM cells. The connection from the DSLAM to the
aggregation router is over an ATM-based network that may
include a fiber ring at Layer 1. This allows an ATM PVC to be
established between the DSL modem and the aggregation router
that can be used to carry Ethernet frames (RFC 2684 Bridging).
The DSLAM essentially performs as an ATM switch. If the CPE
equipment has an ATM interface this connection still uses DSL
as layer-1 but at layer-2 it establishes an ATM PVC directly to
the aggregate router (PPPoA).
Content 2.5
Deploying ADSL 2.5.5 PPPoE Point-to-Point
Protocol over Ethernet (PPPoE) is a network protocol for
encapsulating PPP frames in Ethernet frames. It is used mainly
with ADSL services. It offers standard PPP features such as
authentication, encryption, and compression. Figure summarizes
these points and shows the PPPoE and TCP/IP protocol stack. In
an ADSL deployment, the CPE bridges the Ethernet frames from
the end user’s PC to an aggregation router over ATM with an
Ethernet frame carrying a PPP frame. A PPP session is
established between the subscriber device with PPPoE client
support (either an end user PC with PPPoE client software or
the CPE router configured as the PPPoE client) and the
aggregation router. Figure shows a router as a client.
Depending on the deployment you choose, either the PC or the
router can be the PPPoE client. In the PPPoE architecture, the
PPPoE client functionality connects to the ADSL service. The
PPPoE client first encapsulates the end-user data into a PPP
frame, and then the PPP frame becomes encapsulated inside an
Ethernet frame. The IP address allocation for the PPPoE client
uses the same principle as PPP in dial mode, which is through
IP Control Protocol (IPCP) negotiation with Password
Authentication Protocol (PAP) or Challenge Handshake
Authentication Protocol (CHAP) authentication. The aggregation
router that authenticates the users can use either a local
database on the aggregation router or an authentication,
authorization, and accounting (AAA) server.PPPoE client
functionality can be available as a software PPPoE client
application on the end-user PC. With this model, PPPoE provides
the ability to connect a host over a simple bridging CPE to an
aggregation router. A host uses its own PPP stack, and the user
is presented with a familiar user interface (using the PPPoE
client software) similar to establishing a dialup connection.
Unlike PPPoA, access control, billing, and type of service can
be controlled on a per-user, rather than a per-site, basis.
Content 2.5 Deploying ADSL 2.5.6
DSL and PPPoE Deployment Options When deploying PPPoE
and DSL, three options are available based on the equipment
used, DSL termination, and PPPoE client functionality:
- A router with an internal modem and PPPoE client
functionality can terminate a DSL line and establish a PPPoE
session. The router can also act as a DHCP server and provide
NAT and PAT functionality to connect multiple users behind a
single ADSL connection with a single PPP username and
password.
- An external modem can terminate a DSL line,
and a router with PPPoE client functionality can establish a
PPPoE session. A router can also act as a DHCP server and
provide NAT and PAT functionality.
- An external modem
can terminate a DSL line. An end-user PC encompasses the PPPoE
client for establishing a PPPoE session.
Content 2.5 Deploying ADSL 2.5.7
PPPoE Session Establishment Usually, PPP only works over a
point-to-point connection. Using PPP over an Ethernet
multiaccess environment requires additional enhancements. PPPoE
has two distinct stages as defined in RFC 2516:
- Discovery stage
- PPP session stage
When
a PPPoE client (end-user PC or router) initiates a PPPoE
session, the client must first complete a discovery process to
identify which PPPoE server can meet the client request. Then,
the host must identify the Ethernet MAC address of the peer and
establish a PPPoE session ID. Although PPP defines a
peer-to-peer relationship, discovery is inherently a
client-server relationship. In the discovery process, the PPPoE
client discovers an aggregation router (the PPPoE server).
Based on the network topology, there can be more than one PPPoE
server, with which the PPPoE client can communicate. The
discovery stage allows the PPPoE client to discover all PPPoE
servers and then select one to use. There are four steps to the
discovery stage : Step 1 The PPPoE client (end-user PC
or router) broadcasts a PPPoE Active Discovery Initiation
(PADI) packet. This packet includes the service type that the
client is requesting. The destination MAC address is set to
broadcast. Step 2 The PPPoE server (aggregation router)
sends a PPPoE Active Discovery Offer (PADO) packet that
describes which service the server can offer. The destination
MAC address is the unicast address of the client (end-user PC
or router). The source MAC is the unicast address of the PPPoE
server. Step 3 The PPPoE client sends a unicast PPPoE
Active Discovery Request (PADR) packet to the PPPoE server.
Step 4 The PPPoE server sends a unicast PPPoE
Active Discovery Session-confirmation (PADS) packet to the
client. When discovery is successfully completed, both the
PPPoE client and the selected PPPoE server have the
information that they will use to build their point-to-point
connection over the Ethernet. After the PPPoE session begins,
PPP goes through the normal link control protocol (LCP) and
Network Control Protocol (NCP) process. A PPPoE Active
Discovery Terminate (PADT) packet can be sent anytime after a
session has been established to indicate that a PPPoE session
has been terminated. Either the PPPoE client or the PPPoE
server can send the packet. More information on the PPPoE
specification can be obtained in RFC 2516. As specified by RFC
2516, the maximum receive unit (MRU) option must not be larger
than 1492 bytes because Ethernet has a maximum payload size of
1500 octets. The PPPoE header is 6 octets and the PPP protocol
ID is 2 octets, so the PPP maximum transmission unit (MTU) must
not be greater than (1500 – 8 =) 1492 bytes. An Ethernet and
PPPoE frame contains one of these Ethertypes: - 0x8863
Ethertype = PPPoE control packets
- 0x8864 Ethertype =
PPPoE data packets
Content 2.5
Deploying ADSL 2.5.8 Data over ADSL: PPPoA
Point-to-Point Protocol over ATM (PPPoA), is a network protocol
for encapsulating PPP frames in ATM AAL5. It is used mainly
with cable modem, DSL and ADSL services. PPPoA provides
authentication, encryption, and compression. If it is used as
the connection encapsulation method on an ATM based network it
has slightly overhead than PPPoE. PPPoA also avoids issues
related to having a MTU lower than that of standard Ethernet
transmission protocols that affect PPPoE. PPPoA is a routed
solution, unlike RFC 1483 Bridged and PPPoE. Figure shows the