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Method for High-Speed Swapping of Asynchronous Transfer Mode Virtual Path Identifier/Virtual Channel Identifier Headers

IP.com Disclosure Number: IPCOM000117230D
Original Publication Date: 1996-Jan-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 159K

Publishing Venue

IBM

Related People

Schindler, HR: AUTHOR

Abstract

Asynchronous Transfer Mode (ATM) is the preferred concept for high-speed data transmission and switching in future public and customer premises networks (1). These networks are based an links and switches. Before data can be transmitted, a virtual circuit is set up. It consists of table entries in every switching node. In ATM, all data is transmitted in form of cells, each cell being 53 bytes long. The first four bytes contain, among others, a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI). VPI is 8 or 12 bits long (depending on the interface) and VCI is 16 bits long.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 45% of the total text.

Method for High-Speed Swapping of Asynchronous Transfer Mode Virtual
Path Identifier/Virtual Channel Identifier Headers

      Asynchronous Transfer Mode (ATM) is the preferred concept
for high-speed data transmission and switching in future public and
customer premises networks (1).  These networks are based an links
and switches.  Before data can be transmitted, a virtual circuit is
set up.  It consists of table entries in every switching node.  In
ATM, all data is transmitted in form of cells, each cell being 53
bytes long.  The first four bytes contain, among others, a Virtual
Path Identifier (VPI) and a Virtual Channel Identifier (VCI).  VPI is
8 or 12 bits long (depending on the interface) and VCI is 16 bits
long.  At the input of a switching node, the VPI/VCI field is
replaced by a new one (swapped) and a routing header is attached,
such that the cell can be properly routed through the switch; the
routing header is then removed and the cell is transmitted on the
proper outgoing link.  High-speed swapping of the VPI/VCI field is a
difficult problem, since (assuming 12 bits for VPI) 2(28), i.e., more
than 268 million VPI/VCI combinations are possible.  In a realistic
implementation, between 1000 and 8000 virtual paths (entries) must be
stored in the swap table of a node.  This is a very small fraction of
all possible entries.  The use of a RAM with 258 million entries is
out of question for cost reasons.  A second approach to the problem
consists in the use of a Content Addressable Memory (CAM).  Such
CAMs are available (Ref. 2), however their size and speed is quite
limited.  In particular, interleaving CAM search and CAM update
operations is time consuming.  At a serial bit rate on a fiber link
of 622 Mb/s, i.e., a cell interval time of 681 ns, interleaving
update and search within a cell interval is not possible for the CAM
of Ref. 2.  The solution below overcomes these problems.

      Fig. 1 shows the a solution to high-speed swapping for 8192
entries based on conventional RAMs.  Since in ATM, pure VPI swapping
(the VCI field is ignored in the search process and the new VCI is a
copy of the old one) is required, a test is made by means of a first
RAM called VPI table.  A condition code (cc) in the table indicates
if a full VPI/VCI swapping or a pure VPI swapping has to be
performed.  In case of pure VPI swapping, the output from the VPI
table points to an entry in the Control Block RAM.  The output from
this RAM consists of a new VPI and a routing tag.  The field 'new
VCI' is ignored; instead the old VCI is used.  A multiplexer (not
shown in the figure) is used to perform this function.  In case the
condition code indicates full VPI/VCI swapping, the data output from
the VPI table is fed to an Programmable Logic Device (EPLD), which is
capable of masking certain fields of the VCI; this may be required
for the so-called LAN-emulation, but is not relevant here.  The full
VPI/VCI field (28 bits) is fed t...