5.2.3 EXPLANATION OF WAN SERVICE OPTIONS
PPPoA – Point to Point over ATM
Point-to-Point Protocol over ATM (PPPoA) is a network protocol for encapsulating PPP frames in
AAL5. It is used mainly with DOCSIS and DSL carriers.
It offers standard PPP features such as authentication, encryption, and compression. If it is used as the
connection encapsulation method on an ATM based network it can reduce overhead slightly (around
0.58%) in comparison to PPPoE. It also avoids the issues that PPPoE suffers from, related to having a
MTU lower than that of standard Ethernet transmission protocols. It also supports the encapsulation
types: VC-MUX and LLC based (as does PPPoE)
IPoA – IP over ATM
The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an
adaptation layer in order to support information transfer protocols, which are not based on ATM. This
adaptation layer defines how to segment and reassemble higher-layer packets into ATM cells, and
how to handle various transmission aspects in the ATM layer.
The following ATM Adaptation Layer protocols (AALs) have been defined by the ITU-T. It is meant
that these AALs will meet a variety of needs. The classification is based on whether a timing
relationship must be maintained between source and destination, whether the application requires a
constant bit rate, and whether the transfer is connection oriented or connectionless.
AAL Type 1- supports constant bit rate (CBR), synchronous, connection oriented traffic. Examples
include T1 (DS1), E1, and x64 kbit/s emulation.
AAL Type 2 - supports time-dependent Variable Bit Rate (VBR-RT) of connection-oriented,
synchronous traffic. Examples include Voice over ATM. AAL2 is also widely used in wireless
applications due to the capability of multiplexing voice packets from different users on a single ATM
connection.
AAL Type 3/4 - supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25
data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the
information payload of the cell. Examples include Frame Relay and X.25.
AAL Type 5 - is similar to AAL 3/4 with a simplified information header scheme. This AAL
assumes that the data is sequential from the end user and uses the Payload Type Indicator (PTI) bit to
indicate the last cell in a transmission. Examples of services that use AAL 5 are classic IP over ATM.
Ethernet Over ATM Services.
PPPoE – Point-to-Point Protocol over Ethernet
The Point-to-Point Protocol over Ethernet is a network protocol for encapsulating Point-to-Point
Protocol (PPP) frames inside Ethernet frames. It is used mainly with DSL services where individual
users connect to the DSL modem over Ethernet and in plain Metro Ethernet networks.
IPoE – IP Over Ethernet
The most commonly used link layer protocol for Local Area Networks (LANs) is Ethernet and this is
frequently used to support a range of network layer protocols, including IP. The IP datagrams are
transmitted by encapsulation in Medium Access Control (MAC) frames (or LLC frames using MAC
encapsulation). IPoE is also sometimes referred to as “DHCP” since that protocol plays a key role in
the overall IPoE session.
Bridging
Bridging allows the network to appear as one flat network with devices at the remote site appearing
on the local LAN.
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