International Core Journal of Engineering 2020-26 | Page 119

is used as a relay when the server A and the server B need to
transmit information. Without loss of generality, server A
transmit its packet to the ToR during first timesolt, then
server B transmit its packet to the ToR during second
timesolt in traditional transmission method. In turn, the ToR
forwards packet A to server B and packet B to server A in
two time slots in the same wavelength. In this way, a total of
four time slots are required to complete the transmission. As
shown in fig. 2 (b), by utilizing the PNC, the server A and
the server B can simultaneously transmit their packets to the
ToR during first timesolt, and the two server packets are
encoded at the relay. The encoded packet in ToR is
broadcasted in the same time slot to both servers, and two
server packets are decoded in receiving end. The timeslots
are reduced from four to two. It achieves full-service
communication and doubles throughput.
number of servers. ToR uses a (N+1)h(N+1) coupler to
interconnect the servers in the rack, and the optical network
interface (ONI) is used by the server to send and receive data.
ToR
(b)
(a)
Server A
ToR
Server B
Server B
Server A
Fig. 2. (a) traditional scheduling; (b) OPNC-based network scheduling.
It is shown as a dual fiber interconnect structure. Each
server is connected to a coupler via a dual-port optical
network unit. The two optical fibers are respectively uplink
and downlink transmission signals of the user, which greatly
reduces the insertion loss of the internal communication of
the rack. There is a wavalength selective switch (WSS) in the
coupler to switch the wavalengths to transfer information
between the racks. The traffic inside the rack is broadcast to
all servers by the ToR and Obtained by receivers in the ONIs.
Fig.1 (b) depicts the communication process between the
server A and the server B by using the physical layer
network coding technology. Traditional communication
methods use different wavelengths to distinguish users.
When two users transmit using the same frequency wave, the
signals interfere with each other and the relay is not
recognized properly. However, the relay doesn’t need to
separately identify two users in the PNC structure, and only
needs to mix the information of the two users and broadcast
it to the sender. Therefore, D AB and D BA are transmitted to
the coupler using the same wavelength in one unidirectional
fiber, and the encoded data ( D AB † D BA ) is transmitted to
the server via another unidirectional fiber in PNC structure.
It can reduce half of spectral resources, the transceiver in
each ONIs needs to add a buffer and a decoder as the
expense.
Fig. 3. the encoding and decoding processes: (a) synchronous; (b)
asynchronous data with a time offset of a sub-bit delay Δt.
Fig. 3 (a) depicts the encoding and decoding operation
when packets from server A and B reach the XOR-based
ToR bit-level synchronized. Server A and Server B modulate
the transmitted information and send it to the ToR
simultaneously, and then the ToR broadcasts the encoded
information to the server. After receiving of the encoded data,
each server extracts the other server's packet by performing a
second XOR with a buffer of its own packet. Considering
two servers may not be equal in distance to ToR or due to
network delay, easily a sub-bit mismatch between the two
data packets. As illustrated in Fig.3 (b), where data B is
delayed by a sub-bit time offset Δt. However, the results
show that the correct waveform can still be obtained after the
XOR operation with the damage data. Without the data loss,
indicating that the physical layer network coding can still
perform well in asynchronous situations.
B. Principle
The physical layer network coding is usually used to
study the three-point two-way relay network under the fading
channel. Fig. 2 (a) and (b) depict the scheme for communica-
tion with and without PNC. As shown in Fig. 2(a), The ToR
SMF
Bit rate : 20Gb/s
EDFA
Decoding(Server A)
IP1
Data A
PBC
BC
Server A
CS
Data A
SMF
PR
­
Encoding(ToR)
EDFA
PD
OP2
3dB
Data A
M
CCS
IP1
MZM
PBC
BC
~~~
~~~
~~~
OC ᧛൦‫ؗ‬ਭ
OC ᰬ䫕‫ؗ‬ਭ
Data A ŕData B
PBS
CS
3dB
Data B
MZM
PR
­
MZM
PBC
EDFA
OP2
CCS
PBS
SMF
¸
100Ghz
Server B
Laster
PBS
LPF
MZM
¸
Laster
Data B
Data A ŕData BŕData A
OC ᧛൦‫ؗ‬ਭ
OC ᰬ䫕‫ؗ‬ਭ
OP1
OSC
IP2
PBS
Data B
SMF
EDFA
PBC
OP1
IP2
Fig. 4. Experimental setup of the proposed All-optical physical layer network coding scheme applied to ToR.
pol = 90, are modulated to a sine wave with a phase of 0 and
a subcarrier frequency of 100Ghz. Due to signal uncertainty,
Each server’s transmitter (TX) comprises a Programmable
Pattern Generator (PPG) loaded with a 20 Gb/s NRZ 2 7 -1
Pseudo Random Bit Sequence (PRBS). The signal is
III. E XPERIMENTAL S ETUP
Fig. 4 depicts the all-optical physical layer network
coding scheme applied to the ToR. The Continous Wave
(CW) signals in two servers at O = 1550nm, P = 2.08016mw,
97