BBN Notes for Lecture 11 (3/5/97)

The Batcher-Banyan Switch

Recall that the Banyan switch has one good property: self-routing. Each Beta element makes its own decision by reading the header. This makes it good for high speed switching.

But, the Banyan switch also has one bad property: blocking.

One early attempt to fix the blocking problem was the Benes switch (pronounced "ben-ish"). It consists of cascaded Banyans to avoid blocking at the early stages. The Benes switch is a non-blocking switch. Unfortunately, it looses the self-routing property.

The Batcher-Banyan is not only self-routing but also non-blocking. The basic idea is that the Banyan fabric is not blocking if the inputs are sorted. To do so, the Batcher is added to sort the inputs first.

FIG.1 The Batcher Banyan Switch

The Tandem Banyan Switch

It combines the good property of the Banyan with the Knockout principle. If there is no conflict in the first Banyan, the packets are correctly routed to the corresponding concentrators for the destined outputs. If conflicts occurs, the packets that loose in the contentions will still continue to route in the consecutive Banyan until correctly routed. Or, the remaining will be discarded after the last Banyan.

FIG.2 The Tandem Banyan Switch

Begin Chapter 5 on ATM

Before starting, recall that B-ISDN is still under construction.
Also, two important properties are assumed, cell-switching and virtual circuit switching.
For more intro, see CCN notes in postscript.

ATM interfaces

UNI = User Network Interface

Connect user equipment to an ATM node.
Allow MUX to multiplex small bandwidth users.
There are two types of UNI

- Public UNI: Connects public network to a private ATM switch, e.g. a big pipe from RPI to NYNEX.
- Private UNI: Connects private ATM switch to an end user. For example, connecting a network interface card at a workstation to a private ATM switch.

NNI = Network Node Interface, also called Network-Network Interface

Connect two ATM switches that trust each other.
There are two types of NNI

- Public NNI: For example, connections within Sprint's backbone.
- Private NNI (PNNI): For example, connections within ECSE's LANs.

ICI = Inter-Carrier Interface

Connect two public networks, for example, AT&T's to Sprint's.

DXI = Data Exchange Interface

- An interface of ATM to legacy networks, such as Ethernet.

The ATM Cell Header

There are two types, one for UNI and the other for NNI. We start with the simpler one for the NNI.

FIG.3 ATM Cell Header Format at the NNI.

The VPI and VCI together identify the virtual circuit -- recall that cells are always part of a virtual circuit.

The Payload Type Indicator (PTI) serves several functions

If the first bit = 0, it means that the payload is data, "totally user's stuff".
- In this case, the second bit usually set at 0, but can be set to 1 in the network to indicate congestion.
- The third bit here is available to user, except in AAL 5 which uses this bit.
- More on these last two later.
If the first bit = 1, it means that the cell itself is an OAM cell (OAM = Operation and Management). In this case, the payload is a message for the network; for example, user to network or network to network.

- Here, if the second and the third bit are "00", the cell payload is for link level OAM.
- If the second and the third bit are "01", the cell payload is for end-to-end OAM.
- If the second and the third bit are "10", then this is a resource management cell (RM) used for flow control. (More on this in the Traffic Management section.)
- The case where the second and the third bit are set to "11" is reserved for future use.

Notes taken by Taweesak Opasvorarat, opasvt@rpi.edu, from the lecture by Prof. K.S. Vastola, vastola@ecse.rpi.edu, on March 5, 1997.