SUBJECT D12-1)
Question:What is the ATM Forum's LAN Emulation all about?
Answer:
The ATM Forum has published their LAN Emulation (LANE) V1.0 specification.
Reference that spec for complete details. Here's the basics
on the requirements and general approach.
The organizations who worked on it thought LANE
would be needed for two key reasons
LANE defines the three main areas required to
emulate 802 LANs (connectionless, broadcast/multicast, 802 hardwired MAC
addresses) over ATM networks (connection-oriented, point-to-point,
network-defined telephone-like addresses).
LANE specifies:
LANE also defines how an ATM adapter in a
host can present an Ethernet or Token Ring logical interface to the
protocol stack above. This enables applications and LAN protocols which
were implemented to run above the aforesaid Ethernet or TR LANs to operate
without change over an ATM network.
Surf the ATM Forum's WEB site
http://www.atmforum.com for the
January 1995 back issue of their "53 Bytes" publication. That issue contains
a helpful LANE tutorial.
SUBJECT D12-2)
Question:How does LANE work?
Answer:
Here is a brief spew on how LANE works with ATM:
At this point the 2 ATM adresses are known so the LEC establishes a virtual
circuit connection (VCC) with the LES.
The LEC Registers its ATM/IP/MAC Address with the LES and joins the Emulated
Lan. The LES adds the new LEC to the ARP distribution tree.
The LEC now queries the LES for the Broadcast/Unknown Server (BUS) for multi-
cast.
LES provides BUS address. LEC establishes VCC with BUS and registers its
ATM/IP/MAC Address to mcast distribution tree.
Now we can talk to other end systems by arping for the ATM address to the LES.
LES does a lookup and upon hit returns the address. On a miss the LES
broadcasts the ARP in hopes that some LEC will answer. The response is
returned by the LES to the orignating LEC.
A VCC can now be established between the two LEC's and Data is moved.
SUBJECT D12-3)
Question:What is DXI?
Answer:
The ATM DXI (Data Exchange Interface)is basically the functional
equivalent of the SMDS DXI. Routers will handle frames and packets but not
typically fragment them into cells; DSUs will fragment frames into cells as
the information is mapped to the digital transmission facility.
The DXI, then, provides the standard interface between routers and DSUs
without requiring a bunch of proprietary agreements. The SMDS DXI is
simple because the router does the frame (SMDS level 3) and the DSU does
the cells (SMDS level 2). The ATM DXI is a little more complicated
since it has to accomomodate AAL3/4 and/or AAL5 (possibly concurrently).
SUBJECT D12-4)
Question:Specs on how Frame Relay frames gets mapped to ATM cells.
Answer:
There are at least four. One is the mapping defined
for Frame Relay/ATM network interworking as defined in Version 1.1 of the
ATM Forum's B-ICI spec (network interworking allows Frame Relay end users
to communicate with each other over an ATM network). In this case frames
are mapped using AAL 5 and the FR-SSCS (Frame Relay specific service-specific
convergence sublayer). Despite the long-winded name, the essentials of the
mapping are quite simple to describe: remove the flags and FCS from a Frame
Relay frame, add the AAL-5 CPCS trailer, and segment the result into ATM
cells using AAL 5 SAR rules. The spec defines additional details such as
the mapping between FECN/BECN/DE in the Frame Relay header and EFCI/CLP
bits in the ATM cell headers.
A second mapping is ATM DXI (data exchange interface) mode 1a. This is not
strictly a Frame Relay to ATM mapping but rather uses an HDLC frame structure
identical to that of Frame Relay frames with a two-byte address field (i.e.
a 10-bit DLCI). The HDLC DXI frame address (called DFA in the spec) gets
stripped off and the 10 bits of the "DLCI" get mapped in a funny way to the
VPI and VCI of the ATM cells. The remainder of the DXI frame gets an AAL 5
CPCS trailer and is chopped up into cells by standard AAL 5 rules.
A third mapping is used for ATM/Frame Relay service interworking. This
version allows for conversion between the RFC 1490 multiprotocol encapsulation
and the RFC 1483 multiprotocol encapsulation. It uses AAL5 with the
RFC 1483 encapsulation within the network. It allows a Frame Relay user
to communicate with a user of a different service (e.g. SMDS/CBDS) across
the ATM network.
A fourth mapping is the FUNI which is completely separate
standard ratified by the ATM Forum. It is an extension of the ATM-DXI
standard. However instead of being a local serial interface, it is extended across the
wide area. For more information reference "From Frames to Cells: Low Speed Access to ATM"
in the May 1995 issue of Data Communications.
SUBJECT D12-5)
Question:What is MPOA?
Answer:
The ATM Forum's Multiprotocol Over ATM (MPOA) subworking group is developing an approach
to support seamless transport of layer 3 protocols across ATM networks. Layer 3 protocols
meaning things like IP and IPX. MPOA, operating at layer 2 and 3, will use the ATM Forum LAN Emulation
for its layer 2 forwarding. As such, MPOA can be seen as an evolution beyond LANE.
LANE basically connects together a single legacy LAN subnet across ATM. MPOA will take this
further by allowing direct ATM connectivity between hosts in different subnets.
The proposed architecture consists of edge devices and route servers. An edge device (not
necessarily user equipment) would forward packets between the LAN and ATM networks,
establishing ATM connections when needed, but would not be involved directly in routing. Edge
devices would query a Route Server when an unknown host address is encountered. Route Servers
would be able to map a host address into the information needed by the edge device to
establish a connection across the ATM network. That would be the layer
3 address of the optimal exit point from the ATM network as well as the ATM address of
that exit point. Route servers would also be able to forward packets on to the exit point
on behalf of the edge device while they are establishing their own ATM virtual circuits.
(This last part is LANE.)
Some folks will notice that the Route Server address mapping function is basically the same
problem that the Next Hop Resolution Protocol (NHRP) is addressing.
SUBJECT D12-6)
Question:Info about classical IP over ATM
Answer:
RFC1483 defines the encapsulation of IP datagrams (or other protocols) directly in AAL5.
Classical IP and ARP over ATM, defined in RFC1577, is targetted towards making
IP run over ATM in the most efficient manner utilizing as many of the
facilities of ATM as possible. It considers the application of ATM as a
direct replacement for the "wires" and local LAN segments connection IP
end-stations and routers operating in the "classical" LAN-based paradigm.
A comprehensive document, RFC1577 defines the ATMARP protocol for logical
IP subnets (LISs). Within an LIS, IP addresses map directly into ATM Forum UNI
3.0 addresses. For communicating out a LIS, an IP router must be used -
following the classical IP routing mode. Reference RFC1577 for a full
description of this approach.
SUBJECT D12-7)
Question: What is a Logical IP Subnet (LIS) and how does it differ from any other subnet?
Answer:
RFC1577 is the document which defines LIS, but it doesn't make the concept as obvious as
one might wish, although the info is in there in section 3.
The short answer is that Logical IP subnets are identical, in all
"protocol" aspects, to conventional LAN etc media subnets. The key
aspects that matter in this context are that ATM-attached systems in
the same LIS have the same network numbers and subnet masks, just as on
an Ethernet or other conventional media. Also, two ATM-attached
systems not in the same LIS cannot communicate via RFC1577 except
through a router, even though they are both attached to the same ATM
physical network, with ATM-level connectivity available (PVC or SVC)
between them.
This second limitation was a significant factor in the creation of
RFC1577. The issues of "cut-through routing", or communications
between two systems in different IP subnets on a common ATM network (as
well as other connection-oriented networks) were found to be complex,
and there was a desire to define at least the standard or "Classical"
means of running IP over ATM before all those issues were resolved.
RFC 1932, the IP over ATM: A
Framework Document, has more overview info on these basic issues.
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LANE does not replace routers or routing, but provides a complementary
MAC-level service which matches the trend to MAC-layer switching in the hubs
and wire closets of large LANs.
LANE also works for Token Ring so substitute Token Ring for Ethernet in
the above.
On boot the ATM adapter registers with the local switch and exchanges
management
information. Switch provides a prefix to the ATM adapter which in combination
with the MAC address of the adapter becomes the ATM address
of the adapter. Switch also provides its ATM address.
Last Changed 24 November 2002