1. A HYBRID NoC COMBINING SDM-BASED CIRCUIT SWITCHING WITH
PACKET SWITCHING FOR REAL-TIME APPLICATIONS
Angelo Kuti Lusala Jean-Didier Legat
Microelectronics laboratory, UCL Microelectronics laboratory, UCL
1348 Louvain-la-Neuve, Belgium 1348 Louvain-la-Neuve, Belgium
angelo.kutilusala@uclouvain.be Jean-Didier.Legat@uclouvain.be
Abstract—In this paper we propose a hybrid network-on-chip in the packet-switched sub-network. We use SDM technique
which combines Spatial Division Multiplexing “SDM”-based in order to increase path diversity in circuit-switched sub-
circuit switching and packet switching in order to efficiently network, thereby improving throughput and mitigating low
and separately handle streaming and best-effort traffics resource utilization which affects circuit-switched network.
generated by real-time applications. The SDM technique is In this way multiple connections can use channels in a given
used in circuit-switched sub-network in order to increase path direction; we then take advantage of the abundance of wires
diversity, thereby improving throughput and mitigating low resulting from the increasing integration density of CMOS
resource utilization, while packet-switched sub-network is kept circuits. SDM has been proposed as a valid alternative to
as simple as possible. In this way QoS is simply guaranteed
TDM [2]. SDM technique consists of allocating a sub-set of
without having to share resources, which often leads to a
complex design. The proposed hybrid router architecture has
channel wires to a given connection between a source node
been synthesized in FPGA and ASIC, and results show that a and destination node [2].
practical hybrid network-on-chip can then be built using the In the proposed architecture, the SDM variant used
proposed approach. consists in allocating more than one channel between
adjacent circuit-switched sub-routers. We then define SDM-
Keywords-SDM; TDM; QoS; circuit-switching; packet- Channel as a set of n-bit width sub-channels as shown in Fig.
switching; network-on-chip 1. Only one sub-channel in a SDM-channel is dedicated to a
connection. The SDM-based circuit-switched sub-router is
I. INTRODUCTION configured by the packet-switched sub-router.
The proposed hybrid-router architecture was
Multiprocessor System-on-Chips “MPSoCs” constitute implemented in both ASIC and FPGA technologies. Results
suitable platforms for real-time applications, since they offer show that increasing the number of sub-channels in a SDM-
high-power computing resources and parallelism that require Channel does not greatly affect the size and the maximum
real-time applications. In MPSoC platforms, the performance clock frequency of the proposed hybrid router. A practical
of applications strongly relies on the on-chip interconnection network-on-chip can then be built using the proposed router
network used to carry communications between cores in the architecture.
platform. Since real-time applications generate both The rest of the paper is organized as follows. Related
streaming and best effort traffics, the on-chip interconnection work is explored in section 2. Section 3 introduces the
network must provide quality of service “QoS” for streaming proposed architecture. Section 4 discusses synthesis results
traffic and data completion for best-effort traffic. Several of the proposed router architecture. Finally a conclusion is
networks-on-chips which handle both streaming and best- drawn in section 5.
effort traffics have been proposed in the literature; some of
them are Time Division Multiplexing “TDM”-based II. RELATED WORK
connection-oriented while others are connectionless-oriented Many hybrid Networks-on-Chips have been proposed in
and assign priority to traffic [3][6]. However, handling both the literature. In this paper we focus on those which combine
streaming and best-effort traffics in a network-on-chip by different switching techniques. In [3] ÆTHEREAL NoC is
sharing resources is very hard and often leads to a complex presented. It consists of two disjoint sub-networks: a
design with power consumption and area overhead [1]. Guaranteed Service “GS” sub-network and a Best-Effort
Since streaming traffic is well handled in a connection “BE” sub-network. The GS sub-network is TDM-based
oriented or circuit-switched network and best-effort traffic is connection-oriented while the BE sub-network is packet-
well handled in a connectionless oriented or packet-switched switched based. The BE sub-network is responsible for
network, we thus propose in this paper a hybrid network-on- configuring the GS sub-network. The reserved time slots are
chip which separately and efficiently handles both streaming used to carry streaming traffic while non-reserved time slots
and best-effort traffics. The proposed hybrid network are used to carry best-effort traffic. Store and forward flow
consists of two disjoint sub-networks, a SDM-based circuit- control is used in GS sub-network and wormhole is used in
switched sub-network and a packet-switched sub-network. BE sub-network, this implies the use of buffers in both GS
The streaming traffic is handled in the SDM-based circuit- and BE sub-networks. This often leads to complex design
switched sub-network while the best-effort traffic is handled with area and power consumption overhead [1][2]. In [4], a
978-1-4244-8971-8/10$26.00 c 2010 IEEE

2. technique called Hybrid Circuit Switching “HCS” is to a simple circuit-switched router or TDM connection-
presented; it consists of a circuit-switched network which oriented router.
intermingles circuit-switched flits with packet-switched flits. When a tile needs to send streaming traffic to another tile
A circuit-switched packet is immediately injected in the in the network, a path or connection must first be established
network behind the circuit setup request. If there is no between the two tiles. To establish the connection, the tile
unused resource, the circuit-switched packet is transformed source sends a setup best-effort packet to its packet-switched
to a packet-switched packet and is buffered; it will then keep sub-router. This setup packet reserves an available sub-
its new state until it is delivered. Although this technique can channel in each packet-switched sub-router crossed along its
reduce the circuit setup time overhead in the circuit-switched path from the source to destination. The packet-switched
network, it is still difficult to provide QoS. The NoC sub-router configures the attached SDM-based circuit-
presented in [5] is quite similar to the one presented in [4]. In switched sub-router by indicating the number identifier of
this NoC, a packet can use alternatively a circuit-switched the sub-channel to use for the concerned connection. When
and packet-switched sub-network. The authors claim that in the transaction of transferring streaming traffic is completed,
each router, traffic is split between the two sub-networks in each SDM-based circuit-switched sub-router along the path
such a way that the power and the performance metrics of notifies each attached packet-switched sub-router to release
the NoC are improved. With this technique it is still difficult the concerned sub-channel. When a tile needs to send best-
to provide QoS for streaming traffic. In [2] is presented one effort traffic, it directly sends a “Normal” best-effort packet
of the first works using SDM in a NoC in order to provide to the packet-switched sub-router.
QoS. This NoC covers only streaming traffic.
B. Packet- Switched sub-router
III. PROPOSED NETWORK ARCHITECTURE The Packet-switched sub-router is responsible for
handling best-effort traffic and configuring the attached
A. Router architecture SDM-based circuit-switched sub-router as shown in Fig. 1. It
The proposed router architecture consists of two major uses XY routing algorithm, with cut-through as control flow.
components as illustrated in Fig. 1: a packet-switched sub- We impose that packets coming from a given direction
router and a SDM-based circuit-switched sub-router. cannot return in the same direction. The packet-switched
sub-router has five bidirectional ports as shown in Fig. 1.
Routing is distributed so that up to five packets can be
simultaneously routed when they request different channels.
A best-effort packet consists of five fields. Its structure is
given in Fig. 2. Two bits indicating the type of the best-effort
packet, the source and destination address are 6-bits wide
since we consider a 7x7 mesh topology NoC. The sub-
channel number identifier is 3-bits wide, thus a SDM-
Channel can contain up to 7 sub-channels and the payload is
8-bits wide. The size of the best-effort packets varies
according to the number of sub-channels in a SDM-Channel.
Figure 2. Best-effort packet
Figure 1. Hybrid router architecture
We define three types of best-effort packets for the
The two sub-routers handle traffic independently. The proposed architecture:
SDM-based circuit-switched sub-router is configured by the - A setup best-effort packet, which is responsible for
packet-switched sub-router, while the SDM-based circuit- establishing paths for a streaming traffic through the circuit-
switched sub-router notifies the packet-switched when a switched sub-network between a source and destination. For
transaction of transferring streaming traffic is completed. As a setup best-effort packet, the value of the fields type and
seen previously, an SDM-Channel consists of a set of a given payload are respectively “10” and “00000000”.
number of sub-channels. Each sub-channel is n-bits wide and - An acknowledge “ACK” best-effort packet, which is
is identified by a number called “number identifier”. A generated when a setup packet reaches its destination. It is
connection can only acquire one sub-channel in a SDM- built by swapping the fields destination address and source
Channel. For example, for a SDM-Channel of five sub- address from the setup packet. Its fields type, sub-channel
channels, up to five connections can simultaneously use this number identifier and payload are respectively “01”, “000”
SDM-Channel. With the SDM approach, the router offers and “00000000”.
increased path diversity, improving the throughput compared

3. - A Normal best-effort packet, which carries best-effort channels reserved by this setup packet. The NACK signal in
payload. Its fields type and sub-channel number identifier are the packet-switched sub-router where the setup packet failed
respectively “11” and “000”. is equal to the sub-channel number identifier contained in
this setup packet. The NACK signal indicates to the previous
packet-switched sub-router the sub-channel to release. In the
previous packet-switched sub-router the NACK is equal to
the MSB of the reg_identifier.
Configuring the attached SDM-based circuit-switched
sub-router consists in indicating to each crossbar in the
SDM-based circuit-switched sub-router the number identifier
of the sub-channels to send in the output SDM-Channel.
C. SDM-Based Circuit- Switched sub-router
The SDM-based circuit-switched sub-router is
responsible for carrying streaming traffic. It has five
bidirectional ports. Four bidirectional ports are SDM-based
and are used to connect the sub-router with four adjacent
circuit-switched sub-routers and the fifth bidirectional port
which consists of a sub-channel is used to connect the sub-
router with the local tile as shown in Fig. 1. The SDM
channel consists of a given number of sub-channels. Each
sub-channel is N-bits wide. Streaming data is organized in
Figure 3. Packet-switched sub-router packets like cells in ATM. The streaming packet data unit
structure is shown in Fig. 4.
The packet-switched sub-router consists of input buffers,
link controllers and allocators as shown in Fig. 3. The input
buffers store incoming best-effort packets. The link
controller is responsible for reading packets in the attached
input buffer and deciding to which allocator to send the read Figure 4. Streaming packet data unit
packet according to the destination address. Since no packet
loss is allowed, the link controller keeps the read packet in a The SDM-based circuit-switched sub-router consists of
register until it receives a signal from the allocator indicating five crossbars and five header detectors. A crossbar and
that the packet is successfully sent to the output port. This header detectors are placed in each direction. The crossbar
strategy ensures that no packet is lost in the network. consists of multiplexers. Since crossbars are configured by
The allocators are responsible for writing best-effort the packet-switched allocators, the use of XY routing
packets to the input buffers of the next packet-switched sub- algorithm in the packet-switched sub-router, determines the
router and configuring the SDM-based circuit-switched sub- number of input ports of each crossbar. The crossbar in
router. They first check the type of the best-effort packet. If EAST direction can carry streaming traffic from either
the packet is an ACK or a Normal packet, then the allocator SDM-Channel from west or local tile sub-channel. The
directly sends it to the attached output link without crossbar in WEST direction has the same structure than the
modifying it. If the packet is a setup packet, then the one in EAST direction. The crossbar in NORTH, SOUTH
allocator reserves an available sub-channel in the SDM- and LOCAL can carry streaming data coming from four
Channel in the concerned direction, builds a new setup possible directions. Figure 5 gives the block diagram of the
packet by replacing the field sub-channel number of the crossbar in EAST direction and figure 6 shows its
incoming setup packet by the number identifier of the implementation using multiplexers. Figure 7 gives the block
reserved sub-channel. If the SDM-Channel has four sub- diagram of the crossbar in NORTH direction.
channels, then the number identifier of each sub-channel is
respectively 1, 2, 3 and 4.
The incoming sub-channel number identifier and the
outgoing sub-channel number identifier are concatenated and
stored in a register. The incoming sub-channel number
identifier is the MSB of this register while the outgoing sub-
channel number identifier is the LSB. This register is called
“reg_identifier”. This register helps to find the sub-channel
to release when a setup packet fails. When a setup packet Figure 5. Block Diagram Crossbar East Direction
fails to reserve a sub-channel in a packet-switched sub-
router, a negative acknowledge “NACK” signal is sent back The size of the crossbars depends on the number of input
and propagates to all previous packet-switched sub-routers ports. For a SDM- Channel consisted of three sub-channels,
crossed by this setup packet in order to release all sub- the size of crossbars in directions EAST and WEST is 5 x 3,

4. those in NORTH and SOUTH directions are 11x3 while the The increase of router area is mainly due to crossbars whose
size of the crossbar in local direction is 13x1. Signals “Sel1”, size varies according to the number of input ports. Synthesis
“Sel2” and “Sel3” are provided by the best-effort allocator. results show that the proposed hybrid router architecture can
Each sub-channel of the SDM output channel is connected to be used to build a practical network-on-chip.
a header detector.
TABLE I
ASIC SYNTHESIS RESULTS FOR THE ROUTER
Number of sub-channels Frequency Area Power
in a SDM-Channel GHz µm2 µW/MHz
3 2.8 36352 13.21
4 2.8 40743 14.28
5 2.8 44157 15
TABLE II
FPGA P&R RESULTS FOR THE ROUTER
Number of sub-channels Frequency Total Logic utilization
in a SDM-Channel MHz STRATIX III
3 179 1.4 %
4 164 1.7%
Figure 6. Detailed Crossbar East Direction
5 148 2%
V. CONCLUSION
In this paper, a hybrid network-on-chip architecture
which combines a SDM-based circuit switching with packet
switching for real-time applications is proposed. The
proposed hybrid router architecture efficiently and separately
handles streaming and best-effort traffics respectively in a
SDM-based circuit-switched sub-router and packet-switched
sub-router. The SDM approach is used to allow path
diversity, thereby mitigating low resource utilization in
circuit switching, and improving throughput. QoS is then
easily provided. The packet-switched sub-router is kept as
Figure 7. Block Diagram Crossbar North Direction simple as possible. The proposed router architecture was
implemented in Verilog and synthesized in both ASIC and
The header detector extracts the header of the packet FPGA. Synthesis results show that a practical network-on-
streaming data; if the header is equal to 2 then a signal is sent chip can be built using the proposed approach.
to the best-effort allocator attached to the crossbar to release
the associated sub-channel. The SDM-based circuit-switched REFERENCES
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