10/3/99 1

Interworking IP and WDM Networks

Malathi Veeraraghavan Mark Karol

Polytechnic University Lucent Technologies

mv@poly.edu mk@lucent.com

Outline:• Provisioned mode

• Switched mode

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WDM as a transmission technology

• Use WDM multiplexers/demultiplexers

• Increased bandwidth - immediate value

IP Router

IP Router DWDM Multiplexer

IP Router

IP RouterDWDM Demultiplexer

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WDM as a networking technology

• Circuit switches– Optical add/drop multiplexers (OADM)– Optical crossconnects (OXC)– Commercially available– We assume that WDM switches are of this

variety for this talk.

• Packet switches– In research laboratories; optical buffering issues

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Types of networks

• A network is defined by its “switching mode” and its “networking mode”

• Circuit switching vs. packet switching– Circuit-switching: switching based on position (space, time, ) of arriving bits

– Packet-switching: switching based on information in packet headers

• Connectionless vs. Connection-oriented networking:– CL: Packets routed based on address information in headers

– CO: Connection set up (resources reserved) prior to data transfer

Packet-switching

Circuit-switching

Switching modesConnectionless Connection-oriented

Networking modes

ATMIPMPLS

IP switch

Telephone network, SONET/SDH, WDM

Shades of gray: provisioned vs. switched modes

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Use of WDM networking technology to carry IP traffic

• For WANs, usage expected to be in provisioned mode - need “CO” service for guaranteed bandwidth– Interconnect IP routers with provisioned (connections set up

a priori) lightpaths

Core networkof OXCs/OADMs

R2

Enterprise 2 LAN

R3

Enterprise 2 LAN

Lightpath

R5Enterprise 1 LAN

R4

Enterprise 1 LAN

Enterprise 1 LAN

R1

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Alternatives

• Alternatives for the core network nodes:– Packet switches with packets of format anything other than the IP

datagram format, e.g. ATM, MPLS (MultiProtocol Label Switching)

– SONET/SDH circuit switches (TDM)

– “IP switches” - resource reservation at the IP layer using RSVP or some network management system

• hardware-based IP forwarding

• variable-length packet switching

– WDM Optical crossconnects and WDM Optical add/drop multiplexers

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Which alternative is “best?”

• Issues/assumptions:– IP traffic even in core measured to be bursty– Protocol layer overhead resulting from protocol

encapsulation– Bandwidth granularity

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Issue 1

• IP traffic even in core measured to be bursty– Implication: need traffic shaping at edge routers

or gateways if circuit-switched alternatives are used

– Is it possible to shape IP (self-similar) traffic to a constant rate?

– Is there a problem if the IP traffic delivered at the far-end router does not replicate burstiness?

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Issue 2

• Protocol layer overhead resulting from protocol encapsulation– 20% in case of ATM (TCP ACKs don’t fit in

one ATM cell with LLC/SNAP encapsulation and ACKs are 45% of packets)

– 4.4% for SONET relative to IP over PPP over fiber/WDM

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Issue 3

• Bandwidth granularity:– In SONET networks, minimum rate is OC1

(~51Mbps)– In WDM networks, issue not at the OXCs but

rather at the transmitter; actual rate used could be less than maximum rate possible

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Which alternative is “best?”

• Alternatives for the core network nodes:– ATM, MPLS: protocol layer overhead issue

– SONET/SDH: all three issues

– IP switches: None

– WDM OXCs/OADMs:

• bursty traffic issue? + granularity issue?

• Answer:– IP switch based solution seems best

– If traffic can be shaped to constant rate and delivery of constant-rate traffic at far-end is acceptable, then WDM OXC/OADM based solution is comparable

• Switch costs could offset transmission cost savings

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Test configuration

R1 R3

R2

Networknode

Core network

R4

OXC, IP switch, ATM switch or SONET XC

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Different cases

Cases Network node Edge routersperformshaping

Separateor mergedchannels

Case 1 OXC No -

Case 2 OXC Yes -Case 3 IP switch Yes -Case 4 IP switch No SeparateCase 5 IP switch No MergedCase 6 ATM switch Yes -Case 7 ATM switch No SeparateCase 8 ATM switch No MergedCase 9 SONET XC No -Case 10 SONET XC Yes -

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Comparison of OXC and IP switch based networks

0

5000

10000

15000

20000

0 5000 10000

# of users supported per edge router

To

tal B

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o b

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lloc

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Case 1:OXC; noedge shaping

Case 2:OXC; edgeshaping

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15000

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0 5000 10000

# of users supported per edge router

Tota

l BW

to b

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loca

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Case 1: OXC;no edgeshaping

Case 2/3:OXC/IP switch;edge shaping

Case 4/5: IPswitch; no edgeshaping;separate/merged channels

Graphs generated by D. Dharmaraju and R. Badri, Polytechnic Univ.

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Comparison of SONET and OXC based networks and IP switch and ATM switch based networks

0

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10000

15000

20000

0 5,000 10,000

# of users supported per edge router

Tota

l BW

to b

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loca

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in M

bps

Case 9: SONET XC;no edge shaping

Case 10: SONET XC;edge shaping

Case 1: OXC; noedge shaping

Case 2: OXC; edgeshaping

0

2000

4000

6000

8000

10000

12000

14000

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# of users supported per edge routerTo

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W to

be

allo

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Mbp

s Case 3: IP switch; edgeshaping

Case 4/5: IP switch; noedge shaping;separate/merged

Case 7/8: ATM switch;no edge shaping;separate/merged

Case 6: ATM switch;edge shaping

Graphs generated by D. Dharmaraju and R. Badri, Polytechnic Univ.

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Classification of optical networks

• One classification (B. Mukherjee’s book)

– Broadcast-and-select local optical WDM networks

– Wavelength-routed (wide area) optical networks

• Second classification (chap. by J. Bannister, M. Gerla, M. Kovacevic, in book on routing) – Optical link networks

– Single-hop networks

– Multi-hop networks

– Hybrid networks

– Photonic networks

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Difference between optical-link and

multihop networks (per ref.)

• Optical-link networks don’t use multiple wavelengths while multihop networks do

• Routing problem in optical-link networks is the simple routing problem in packet-switched networks, while in multihop networks, this problem is tightly coupled with the virtual-topology design problem

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Two-layer routing problem

R1

R2

R3

R5

R6

R7

R4

Virtual Topology Physical Topology

R1

R2

R3

R6

R7

R5

R4

If WDM networks are not efficient when used in provisioned mode, do not create a virtual topology by connecting IP routers with lightpaths that traverse multiple OXCs

Above problem not worth solving if packet switches are IP routers - just build a single-layer IP switch based network

OXC

OXC

OXC

OXC

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How should WDM wavelength-routed networks be used for IP traffic?

• Hybrid network: Single-hop and optical-link– Single-hop: Use WDM circuit switches for large bulk-data

transfers• Operate WDM network in switched mode

• Need a routing protocol and signaling protocol

• Dynamic allocation and removal of lightpaths

– Optical-link network: A packet-switched network (allow WDM mux/demux on links)

• Packet-switched network supports CL and CO services

• In contrast to other hybrid networks, which combine single-hop and multi-hop networks

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Classification of applications

Applications

Non-real-time(stored at sender and receiver ends)

Real-time(consumed or sent live)

Interactive (two-way)(consumed and sent live)

e.g. telephony, telnet, “ftp”

Streaming (one-way)(consumed live;

sent from live or stored source)e.g. radio/TV broadcasts

Recording (one-way)(stored at receiver end;sent from live source)

Short transfers(e.g. DNS query)

Long transfers(e.g. large image,

audio, video or data)

Packet-switched CO networks

Connectionlessnetworks Circuit-switched

networks

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Use of circuit switching for long data transfers

• Scanned from “Fundamentals of Digital Switching,” by J. MacDonald (published 1983 - article written by Miyahara et al. in 1975)

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Conclusions

• Regarding WDM wavelengh-routed (WAN) networks– Value questionable relative to other networking technologies when

used in provisioned mode (pre-established lightpaths) to interconnect IP routers

– In switched mode, ideal for high-bandwidth large file transfers

• Proposed WAN solution: hybrid networks– Optical-link networks interconnecting packet switches that support

connectionless and connection-oriented services

– Single-hop networks of OXCs supporting circuit-switched services for large file transfers