Subject: unidirectional MAC layer

From: "Daniel Russo" <[email-masked]>
Date: Fri, 19 Nov 1999 11:56:19 -0600

I've been searching for a MAC layer protocol that will support
unidirectional links. Protocols like 802.11 do not as they require RTS, CTS,
data and its ACK to come reply over the same link.
If any work as been done, any experimental protocols, any pointers to this
work would be appreciated.

thanks,

daniel russo
[email-masked]

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From: Andrew D Myers <[email-masked]>
Date: Fri, 19 Nov 1999 13:59:25 -0600 (CST)

Mr. Russo,

There are several MAC protocols that can support unidirectional links.  If
you are interested in an asynchronous protocol, then Aloha, slotted Aloha,
and CSMA come to mind.  If you are interested in a synchronous protocol,
then TDMA, TSMA, T-TSMA, and ML-PRS may be your answer.  For your
convenience, I have included a short bibliography below. 

If you have any questions, or need more information, feel free to ask!

Andrew Myers
Center for Advanced Telecommunications Systems and Services (CATSS)
The University of Texas at Dallas

%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Bibliography
%%%%%%%%%%%%%%%%%%%%%%%%%%%%

N. Abramson. "The ALOHA System -- Another Alternative for Computer
Communications," Proceedings of the Fall Joint Computer Conference, 1970.

S. Boztas. "A Robust Multi-Priority Topology Independent Transmission
Schedule for Packet Radio Networks," Information Processing Letters, vol.
55, no. 5, 1995.

I. Chlamtac and A. Farago. "Making Transmission Schedules Immune to
Topology Changes in Multihop Radio Networks," IEEE/ACM Transactions on
Networking, vol. 2, no. 1, 1994.

I. Chlamtac, A. Farago, and H. Zhang. "Time--Spread Multiple-Access (TSMA)
Protocols for Multihop Networks," IEEE/ACM Transactions on Networking,
vol. 5, no. 6, 1997.

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From: Andrew D Myers <[email-masked]>
Date: Fri, 19 Nov 1999 14:32:20 -0600 (CST)

Whoops!

Slotted Aloha is a synchronous protocol.  I apologize for any confusion.

Andrew Myers
Center for Advanced Telecommunications Systems and Services
The University of Texas at Dallas

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From: Joe Macker <[email-masked]>
Date: Fri, 19 Nov 1999 16:49:01 -0500

Daniel:

FYI, perusing the  802.11spec, the RTS/CTS is specified as an optional
CSMA/CA enhancement and has quite a number of variable modes.  So 802.11,as
written, should still support a CSMA/CA vanilla contention mode.  It,
however, seems to require the ACK use for all directed traffic (unicast,
but not multicast and broadcast).  So your point is still valid for unicast
traffic usage even without RTS/CTS, due to the ACK requirement.

-Joe

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From: Jose Garcia-Luna <[email-masked]>
Date: Fri, 19 Nov 1999 14:27:04 -0800

Well, ALOHA and CSMA are not quite unidirectional; thehe implementation
details are to be mentioned.  ALOHA and CSMA require the sender to
know that the receiver has the pkt in order to decide whether or not
to retransmit.

The analysis reported by Kleinrock and Tobagi assumes an ideal channel
in which ACKs are received correctly in 0 time, and the ALOHA papers
apply to networks with a transponder or menehune that retransmits
everything it receives (hence the sender has feedback on the downlink
of whether or not it shows the same that it is sending in th uplink).
Even if we say we can do the ACKs at the LLC sublayer that still needs
a bidirectional link.

An ad hoc network cannot be said to support unidirectional links
by using ALOHA or CSMA. There has to be multihop feedback to the sender
to permit retransmissions and proper backoff strategies.

JJ

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From: Jose Garcia-Luna <[email-masked]>
Date: Fri, 19 Nov 1999 14:44:10 -0800

don't expect to find a MAC for unidirectional links so easily.
Use of unidirectional links is a net-level issue

jj

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From: [email-masked]
Date: Mon, 22 Nov 1999 10:46:11 +0100

It looks like there is a confusion between CSMA and CSMA-CD. In the first
case the transmitter waits for idle channel to transmit the packet for
once, waiting for ACK at higher level. In the second case, the MAC detects
collisions and can schedule retransmissions by itself. 

I think that CSMA should be appropriate for unidirectional links, but of
course with the cost of far less reliable and performant transmissions
because higher level protocols, like TCP, are not tuned to support packet
loss due to other causes than channel overflow.

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From: [email-masked]
Date: Mon, 22 Nov 1999 10:36:36 +0100

Some versions of Lucent Wavelan are only CSMA/CA (i.e. CSMA without
Collision Detection) without the RTS/CTS/ACK feature described in IEEE
802.11. Thus it seems to be a suitable MAC for asymmetric links. 

Philippe Jacquet

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From: Miguel Sanchez <[email-masked]>
Date: Mon, 22 Nov 1999 13:02:33 +0100

Hi Daniel,

I want to point out two aspects of your question:

1) Where do the unidirectional links come from?

Unidirectional links can appear when there is a strong difference in the two
directions  (i.e., different transmission power level)

2) Are your network terminals moving or fixed?

For fixed terminals, the cause of links being undirectional can stay along time.
But because nodes stay at a fixed location, then you can have permanent problems
with hidden terminals.
If terminals can move, then links can change from unidirectional to bidirectional
(sometimes). And hidden terminal problems can be only temporal.

As JJ pointed out,  ACKs will have to be routed and won't be at the MAC layer.
But in this case, you end up with a MAC that needs routing to be working. It is
something weird. On the other hand, performance of such a system will be
interesting to evaluate.

Best,

Miguel Sanchez

--
Miguel Sanchez Lopez                            |       [email-masked]
Universidad Politecnica de Valencia             | voice:+3496 387 9700
Camino de Vera, 14                              | fax:  +3496 387 7579
46071 VALENCIA (Spain)                          |

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From: "M. Scott Corson" <[email-masked]>
Date: Mon, 22 Nov 1999 11:04:36 -0500 (EST)

> Use of unidirectional links is a net-level issue

agreed, if by net-level issue, you mean that reliability must be conveyed
over some *other* link, itself perhaps being unidirectional as well.
this generally would imply net layer processing, as today's MAC layers
do not support this functionality.  But this need not be the case 
indefinitely, as future MAC layers may be sophisticated enough to handle
limited multi-link knowledge and processing.

in our current IMEP implemenation, we compose logical bi-directional links
between adjacent neighbors from two or more oppositely-directed 
unidirectional links.  we generally have two different link layers
running on separate interfaces which can create opposing unidirectionality.
since this occurs a top separate link layers, it is net-layer processing.
however, i imagine future multi-channel MAC layers could evolve which 
compose bi-directional links from uni-directional channels solving this
simple case at the MAC layer.  more sophisticated localized multi-hop
uni-directional link resolution might best be left at the net layer,
something IP-level routing algorithms should be good at... ;-)

-scott

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From: Jose Garcia-Luna <[email-masked]>
Date: Mon, 22 Nov 1999 09:17:13 -0800

Scott,

I think we agree.

I consider multiband operation as a bidirectional link.  If we have
multiband operation or FDD, we are still talking about ACKs being sent
between neighbors, i.e., bidirectionality. Just as in the original
ALOHA with one uplink channel to the menehune and onw downlink channel
to the stations.

Unidirectional links are those in which my neighbor cannot hear me on
any band eventhough I hear my neighbor. Then we need to employ a path
to provide feedback.

My main point was to state the role of ACKs in ALOHA and CSMA....
BTW, for the MANET kids out there (older MANETters have read the paper
:), the impact of ACKs in CSMA was published by Kleinrock and Tobagi
in Trans on Communications 1978.  Unless ACKs are given priority (with
a listening gap), ACKs clobber CSMA's performance.....it follows that
real CSMA performance with hidden terminals can be *really* bad
because we cannot give good listening priorities under hidden terminal
interference.

....back to work now :)

JJ

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From: "M. Scott Corson" <[email-masked]>
Date: Mon, 22 Nov 1999 12:29:52 -0500 (EST)

> any band eventhough I hear my neighbor. Then we need to employ a path

yeah we agree.

-scott