TTM1: ”Burst, packet and hybrid switching in the optical core network”

Steinar Bjørnstad et al.

Content

• Introduction /definitions of OBS and OPS• Hybrid switching: OpMiGua• Comparison of schemes (OCS/OBS/OPS/Hybrid)• OPS/OBS packet/burst handling schemes• Contention resolution• (One possible) OPS node design• Hybrid (OpMiGua) node design• Conclusions

Introduction

Wanted: High capacity optical layer network with the following requirements:

• Support high utilization of resources

• Support high granularity

• Support quality needed for strict real-time services

• Support variable length packets

OBS/OPS definitions• Both based on idea of separating forwarding from switching:

• Traditionally OBS is used to denote ”reservation” type of transport, i.e. T(offset) > 0, but T(offset) = 0 is also used (e.g. in STOLAS).

• If T(offset) > 0 there are many different schemes (partly) depending on information available in BCP.

• A ”burst” (in OBS) will contain many smaller information units, e.g. many IP packets or Ethernet frames.

• A ”packet” (in OPS) is normally one fixed or variable length information unit.

OBS: OPS:

(BCP)

Examples of OBS ”reservation” schemes

• (Simple:) ”Tell-and-Go” (TAG): T(offset)=0, but BCP on separate wavelength. Payload must be ”buffered” (FDLs) while BCP is processed and switching is done. Payload is lost if output is not available. Explicit ”release” control packet after transmission.

• (More advanced:) ”Just-enough-time” (JET): ”Reserve a fixed duration” (RFD) type protocol. T(offset) > 0 and BCP contains arrival time and duration of payload. T(offset) must be large enough to allow processing of BCP end-to-end through network. No buffering of payload needed. - Different algorithms available for different utilisation of channels/wavelengths. (Trade-off: processing time vs. Channel utilisation)

• NB! No ACKs before sending payload; one-way ”reservations” only.

Trade-off: processing time vs. Utilisation of channels/wavelengths

Examples of OBS ”reservation” schemes (2)

Some more OBS ”reservation” schemes

Reserve a Fixed Duration(RFD)

- reservation knows burst start and end time

One-way burst reservation schemes(No transfer acknowledgement before sending data)

Just EnoughTime (JET)

In-Band Termination(IBT)

- burst contains burst"terminator"

Tell-And-Go(TAG)

- separate burst tear-down

Just-In-Time(JIT)

Horizon

Reserve a Limited Duration(RLD)

- reservation contains burstend time

LAUC LAUC-VFJumpStart

OpMiGua (”Optical packet switched Migration capable network with service Guarantees”)

• Static or Dynamic Wavelength routed optical network, S- or D-WRON) used to establish lightpaths through network.

• GST = ”Guaranteed Service Transport” via S/D-WRON cross-connects.

• SM = ”Statistically multiplexed” (or BE=”Best effort”) packets can use any free wavelength (link by link).

• Packet switches must be used for SM header processing and routing/forwarding of packets.

• Packets may be lost if no buffering available. Electronic buffering (RAM) also possible in first generation.

GST lightpath from A→D

A DCB

D

D

D

D

Pure Circuit switched system

Incoming GST packets destined for node D

- Traffic is routed according to circuit e.g. wavelength → minimal delay

- Traffic to different destinations must use different circuits or wavelengths

– Ensures no collisions between the two traffic streams

– Requires one wavelength per stream

–Streams may e.g. be one circuit and one packet: Classic hybrid

C

C

C

C

Pure packet switched system

Ingr

ess

queu

e

Ingr

ess

queu

e

Ingr

ess

queu

e

Egr

ess

que

ue

Egr

ess

que

ue

Egr

ess

que

ue

Transit queue Transit queueTransit queue

A B C

•Single circuit (may be several wavelengths) for transmission•All packets are processed electronically: Header address lookup, queuing, forwarding•Packets from Ingress queue and Transit queue are statistically multiplexed (SM)

Pure packet switched system

A B C

•Packets from A and B are destined to C•Different streams have different color•Interleaving of simultaneously arriving packets gives variable packet delays

C

C

C C

C

Pure packet switched system

A B C

•Packets from A and B are destined to C•Different streams have different color•Interleaving of simultaneously arriving packets gives variable packet delays

CCCCC

C

Pure packet switched system

A B C

•New arrivals at B destined for C•Transmit queue is full => packet drop in ingress node B !

CCCCC C

C

C

C

C

Pure packet switched system

•Packets arrive reordered at output •Packets may be dropped •Efficient utilization of transmission wavelength

C

C

C

CC CC

C

C

C

A B C

DB

GST lightpath from A→D

Underutilized circuit (wavelength): OpMiGua fills it!

Incoming GST packets destined for node D

Time between packets is unused

– Pure WDM system (circuit) gives low channel utilization

– For 270 Mb/s video on a 1 Gbps, 70 % of capacity is wasted

OpMiGua switch will insert lower quality (Internet) traffic in voids

Transit traffic passes through on optical layer with minimum or no processing

B

C

C

Packet Switch

D

Input Queue Output Queue

A C

D

CD

D

D

C

Comparisons OCS/OBS/OPS/Hybrid

BCP

Packet/burst handling schemes

OPS packet handling schemes

• FLP = Fixed length packets

• VLP = Variable length packets

Contention resolution

• Electronic buffering: Time-domain contention resolution by ”store-and-forward” (large RAM buffers in packet switches) not available without OEO.

• Optical buffering: One FDL gives a fixed delay. Best case is that packet is available on a predetermined number of time instants (by applying many FDLs and circulation of packets).

• Wavelength dimension/conversion used for contention resolution: necessitates use of tunable wavelength converters.

• Deflection routing (or ”hot potato routing”): Send out packet in any free direction if contention. Leads to longer paths and increase in total network traffic.– Packet reordering

Buffering schemes

OPS node design example

Reconfigurable non-blocking design!

OpMiGua node design (Hybrid switch)

Buffer can be electronic (RAM) or based on FDLs (or some future invention?)

Conclusions

• OBS as first step towards OPS due to current limitations in optical buffering.

• ”Reservation” type of OBS is probably to complicated.• The suggested designs rely heavily on use of tunable

wavelength converters and large AWGs. Prices on these components are expected (or hoped?) to drop significantly. (Mass production…?)

• In my view: hybrid solutions (e.g. OpMiGua) has some advantages over OPS also in the long run:- can be tailored better to all types of applications; not all information transport should be divided into (small) packets.– Supports bypass of high-capacity packet switches: Save switch

resources and power