diff --git a/.github/workflows/pypi_publish.yml b/.github/workflows/pypi_publish.yml
index a018009..7efd38c 100644
--- a/.github/workflows/pypi_publish.yml
+++ b/.github/workflows/pypi_publish.yml
@@ -4,27 +4,32 @@ name: Upload the ns.py Python Package to PyPI
 
 on:
   release:
-    types: [created]
+    types:
+      - created
 
 jobs:
-  deploy:
+  build_and_deploy:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
-      - name: Set up Python 3.9
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: 3.9
-      - name: Install build dependencies
+          python-version: 3.x
+
+      - name: Install dependencies
         run: |
-          python -m pip install pip
+          python -m pip install --upgrade pip
           pip install setuptools wheel twine
-      - name: Build and publish
-        env:
-          TWINE_USERNAME: ${{ secrets.PYPI_USERNAME }}
-          TWINE_PASSWORD: ${{ secrets.PYPI_PASSWORD }}
-        run: |
-          rm -rf dist/
-          pip wheel . -w dist
-          twine upload --repository pypi dist/ns.py*
+
+      - name: Build package
+        run: python setup.py sdist bdist_wheel
+
+      - name: Publish package to PyPI
+        uses: pypa/gh-action-pypi-publish@release/v1
+        with:
+          user: __token__
+          password: ${{ secrets.PYPI_API_TOKEN }}
diff --git a/ns/__init__.py b/ns/__init__.py
index 771bc6e..3d26edf 100644
--- a/ns/__init__.py
+++ b/ns/__init__.py
@@ -1 +1 @@
-__version__ = "0.3.9"
+__version__ = "0.4.1"
diff --git a/parallel_servers.py b/parallel_servers.py
new file mode 100644
index 0000000..3db34e8
--- /dev/null
+++ b/parallel_servers.py
@@ -0,0 +1,232 @@
+import random
+from functools import partial
+import matplotlib.pyplot as plt
+import simpy
+from collections import defaultdict as dd
+
+from ns.packet.dist_generator import DistPacketGenerator
+from ns.packet.sink import PacketSink
+from ns.port.port import Port
+from ns.packet.packet import Packet
+
+
+class ParallelServers:
+    """
+    K parallel servers (channels) with zero buffer.
+
+    """
+
+    def __init__(
+        self,
+        env,
+        K: int,
+        server_rate: float,
+        zero_buffer: bool = True,
+        zero_downstream_buffer: bool = False,
+        debug: bool = False,
+    ):
+        self.env = env
+        self.store = simpy.Store(env)
+        self.out = None
+        self.zero_buffer = zero_buffer
+
+        # self.packets_available = simpy.Store(env)
+        self.packets_received = 0
+        self.current_packet = None
+
+        self.K = K  # the number of parallel servers
+        self.servers_rate = [
+            server_rate
+        ] * K  # services rate of all servers, in future, could be of different values
+
+        # track the status of busy channels
+        # key: server_index; value: 0 for idle, 1 for busy
+        self.all_channels = dict(zip(list(range(self.K)), [0] * self.K))
+
+        # index of the assigned channel to a pkt
+        self.assigned_channel = dd(lambda: 0)
+
+        # count #pkts sent by each channel
+        self.all_channels_count = dict(zip(list(range(self.K)), [0] * self.K))
+        # count #bytes sent by each channel
+        self.all_channels_byte_count = dict(zip(list(range(self.K)), [0] * self.K))
+
+        self.zero_downstream_buffer = zero_downstream_buffer
+        if self.zero_downstream_buffer:
+            self.downstream_store = {}
+
+        self.upstream_updates = {}
+        self.upstream_stores = {}
+
+        self.debug = debug
+        self.action = env.process(self.run())
+
+    def update_stats(self, packet):
+        """
+        The packet has been sent (or authorized to be sent if the downstream node has a zero-buffer
+        configuration), we need to update the internal statistics related to this event.
+        """
+
+        self.all_channels[self.assigned_channel[packet]] = 0
+
+        if self.debug:
+            print(
+                f"Sent out via channel {self.assigned_channel[packet]}: flow id = {packet.flow_id}, packet id = {packet.packet_id}\n"
+            )
+            print(f"channel status changed to {self.all_channels}")
+
+    def update(self, packet):
+        """
+        The packet has just been retrieved from this element's own buffer by a downstream
+        node that has no buffers.
+        """
+        # With no local buffers, this element needs to pull the packet from upstream
+        if self.zero_buffer:
+            # For each packet, remove it from its own upstream's store
+            # print(f"upstream_stores: {self.upstream_stores.items()}")
+            self.upstream_stores[packet].get()
+            del self.upstream_stores[packet]
+            self.upstream_updates[packet](packet)
+            del self.upstream_updates[packet]
+
+        if self.debug:
+            print(
+                f"Parallel Server: Retrieved Packet {packet.packet_id} from flow {packet.flow_id}."
+            )
+
+    def packet_in_service(self) -> Packet:
+        """
+        Returns the packet that is currently being sent to the downstream element.
+        Used by a ServerMonitor.
+        """
+        return self.current_packet
+
+    def run(self):
+        """The generator function used in simulations."""
+        while True:
+            while sum(self.all_channels.values()) < self.K:
+                if self.zero_downstream_buffer:
+                    packet = yield self.downstream_store.get()
+                    self.current_packet = packet
+
+                    yield self.env.timeout(
+                        packet.size
+                        * 8.0
+                        / self.servers_rate[self.assigned_channel[packet]]
+                    )
+
+                    self.out.put(
+                        packet, upstream_update=self.update, upstream_store=self.store
+                    )
+                    self.current_packet = None
+                    self.update_stats(packet)
+                else:
+                    packet = yield self.store.get()
+                    self.current_packet = packet
+
+                    yield self.env.timeout(
+                        packet.size
+                        * 8.0
+                        / self.servers_rate[self.assigned_channel[packet]]
+                    )
+
+                    self.update(packet)
+                    self.out.put(packet)
+                    self.current_packet = None
+                    self.update_stats(packet)
+
+    def put(self, packet, upstream_update=None, upstream_store=None):
+        """Sends a packet to this element."""
+        self.packets_received += 1
+        flow_id = packet.flow_id
+
+        now = self.env.now
+
+        if sum(self.all_channels.values()) >= self.K:
+            if self.debug:
+                print("All servers are busy")
+        else:
+            # If the packet has not been blocked, record the assigned channel
+            for server_id in self.all_channels.keys():
+                if self.all_channels[server_id] == 0:
+                    self.assigned_channel[packet] = server_id
+                    print(f"selected server_id {server_id}")
+                    break
+            if self.debug:
+                print(
+                    f"\nPacket arrived at parallel servers at time {self.env.now}, \nflow_id = {flow_id}, packet_id = {packet.packet_id}, assigned server_id = {self.assigned_channel[packet]}\n"
+                )
+
+            self.all_channels[self.assigned_channel[packet]] = 1
+
+            print(f"after put, all_channels = {self.all_channels}")
+            self.all_channels_count[self.assigned_channel[packet]] += 1
+            self.all_channels_byte_count[self.assigned_channel[packet]] += packet.size
+
+            if (
+                self.zero_buffer
+                and upstream_update is not None
+                and upstream_store is not None
+            ):
+                self.upstream_stores[packet] = upstream_store
+                self.upstream_updates[packet] = upstream_update
+
+            if self.zero_downstream_buffer:
+                self.downstream_store.put(packet)
+
+            return self.store.put(packet)
+
+
+def const_arrival():
+    """
+    inter-arrival time distribution (sec)
+    """
+    return 0.5
+
+
+def const_size():
+    """
+    packet size distribution (byte)
+    """
+    return 100.0
+
+
+if __name__ == "__main__":
+    # Set up arrival and packet size distributions
+    # arrival_dist = partial(random.expovariate,
+    #                        1)  # arrival rate λ = 0.5 packets per second
+    # size_dist = partial(random.expovariate, 0.01)  # mean size 100 bytes
+
+    server_rate = 800.0  # mean port rate 800 bps per server
+    K_servers = 2  # number of servers
+
+    env = simpy.Environment()  # Create the SimPy environment
+    ps = PacketSink(env, debug=True, rec_arrivals=True)
+    # pg = DistPacketGenerator(env, "pg", arrival_dist, size_dist, flow_id=0)
+    pg = DistPacketGenerator(
+        env, "pg", const_arrival, const_size, flow_id=0, debug=True
+    )
+
+    buffer = Port(env, rate=0, qlimit=None, zero_downstream_buffer=True, debug=True)
+
+    parallel_servers = ParallelServers(
+        env, K_servers, server_rate, zero_buffer=True, debug=True
+    )
+
+    pg.out = buffer
+    # buffer.out = ps
+    buffer.out = parallel_servers
+    parallel_servers.out = ps
+
+    env.run(until=20)
+
+    print(
+        f"\nSimulation Done...\nChannel id : packet count \n {parallel_servers.all_channels_count}"
+    )
+
+    # # Plot the waiting time histogram
+    # plt.hist(ps.waits, bins=50, density=True, alpha=0.7)
+    # plt.xlabel('Waiting Time')
+    # plt.ylabel('Probability Density')
+    # plt.title(f'M/M/{K_servers} Queue System - Waiting Time Histogram')
+    # plt.show()