Interworking IP and WDM Networks
Malathi Veeraraghavan
Polytechnic University
mv@poly.edu
Mark Karol
Lucent Technologies
mk@lucent.com
Outline:
• Provisioned mode
• Switched mode
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1
WDM as a transmission technology
• Use WDM multiplexers/demultiplexers
• Increased bandwidth - immediate value
IP Router
IP Router
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IP Router
DWDM
Multiplexer
DWDM
Demultiplexer
IP Router
2
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
Networking modes Shades of gray: provisioned vs. switched modes
Switching modes
Connection-oriented
Connectionless
Packet-switching
Circuit-switching
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IP
MPLS
IP switch
ATM
Telephone network,
SONET/SDH, WDM
4
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
Enterprise 1 LAN
Enterprise 1 LAN
R1
R4
Core network
of OXCs/OADMs
Enterprise 2 LAN
R2
Enterprise 2 LAN
R3
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R5
Enterprise 1
LAN
Lightpath
5
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
R4
R1
Network
node
R3
Core network
OXC, IP switch,
ATM switch or SONET XC
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R2
12
Different cases
Cases
Network node Edge routers
perform
shaping
Case 1 OXC
No
Case 2 OXC
Yes
Case 3 IP switch
Yes
Case 4 IP switch
No
Case 5 IP switch
No
Case 6 ATM switch Yes
Case 7 ATM switch No
Case 8 ATM switch No
Case 9 SONET XC No
Case 10 SONET XC Yes
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Separate
or merged
channels
Separate
Merged
Separate
Merged
13
20000
Case 1:
OXC; no
edge shaping
15000
10000
Case 2:
OXC; edge
shaping
5000
0
0
5000 10000
# of users supported
per edge router
Total BW to be allocated in
Mbps
Total BW to be allocated in
Mbps
Comparison of OXC and IP switch based networks
Case 1: OXC;
no edge
shaping
20000
15000
Case 2/3:
OXC/IP switch;
edge shaping
10000
5000
0
0
5000
10000
# of users supported per
edge router
Case 4/5: IP
switch; no edge
shaping;
separate/merg
ed channels
Graphs generated by D. Dharmaraju and R. Badri, Polytechnic Univ.
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20000
15000
Case 9: SONET XC;
no edge shaping
10000
Case 10: SONET XC;
edge shaping
Case 1: OXC; no
edge shaping
5000
Case 2: OXC; edge
shaping
0
0
5,000
10,000
# of users supported per
edge router
Total BW to be allocated in Mbps
Total BW to be allocated in Mbps
Comparison of SONET and OXC based networks
and IP switch and ATM switch based networks
16000
14000
Case 3: IP switch; edge
shaping
12000
10000
Case 4/5: IP switch; no
edge shaping;
separate/merged
8000
6000
4000
Case 7/8: ATM switch;
no edge shaping;
separate/merged
2000
0
0
5000
10000
# of users supported per edge router
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)
–
–
–
–
–
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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 packetswitched networks, while in multihop
networks, this problem is tightly coupled
with the virtual-topology design problem
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Two-layer routing problem
R
3
R
6
R
1
R
3
R
5
R
4
R
2
OXC
R
1
R
5
OXC
R
7
R
6
OXC
R
2
R
7
OXC
R
4
Virtual Topology
Physical Topology
 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
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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
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)
Non-real-time
(stored at sender and receiver ends)
Short transfers
(e.g. DNS query)
Connectionless
networks
Long transfers
(e.g. large image,
audio, video or data)
Circuit-switched
networks
Packet-switched CO 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
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