- Point-to-point link vs. broadcast links
- Point-to-point links
===>
switched networks
- These links typically spans
large geographical distances
(across continents)
- Due to the length:
transmission errors (bit errors)
are
very likely
(Typical: 1 bit in ~10,000 bit for copper and 1 in ~10,000,000 in optical fiber)
Hence:
- The datalink layer
for point-to-point link
deals primarily with:
- reliable transmission using Sliding Windows protocol (in addition to framing)
(E.g.: HDLC --- used in switched networks ---- uses Go-Back-N for reliability)
- These links typically spans
large geographical distances
(across continents)
- Broadcast links
======>
very short distances
(local area network)
Fact:
- Transmissions over short distance are very reliable
Hence:
- The data link layer of broadcast network does not implement any reliable communication protocol
- Point-to-point links
===>
switched networks
- Problem
solve by the Datalink layer of
broadcast networks:
- How to share the broadcast network efficiently
- Property of a
broadcast network:
- A transmission on
a broadcast network will be
received by
all nodes
on the broadcast network
(E.g.: radio !!!)
- A transmission on
a broadcast network will be
received by
all nodes
on the broadcast network
- Consequence:
- When multiple nodes
transmit at the
same time:
- All transmitted frames will be lost !!!
- When multiple nodes
transmit at the
same time:
- Collision:
- Collision =
multiple simultaneous transmission
on a broadcast network
(The simultaneous transmission have collided)
- Collision =
multiple simultaneous transmission
on a broadcast network
- Medium Access Control (MAC):
- Medium Access Control (Protocol) = a protocol (procedure) that nodes on a broadcast network must follow when the node wants to transmit a frame
Graphically:
- There are 2 types of
Medium Access Control
protocols:
- Contention-based protocols
- Contention-free protocols
- Property of
Contention based MAC protocols
- Nodes in
a contention-based
network will
compete to be
the winner
- The winner node will be able to transmit its message on the network !!!
- Nodes in
a contention-based
network will
compete to be
the winner
- Operation of
contention-based MAC
networks:
- Nodes that
want to transmit
a message
will
compete
to be the first node
to start transmitting
its message
- The first transmitting node
will succeed in
completing its
transmission
- When other (= not first) nodes
hears the
transmission of
the first node:
- The other nodes
will refrain from
transmitting
(Until the transmission of the first node is complete)
- The other nodes
will refrain from
transmitting
- When the first node is
finish:
- All active nodes (= nodes that want to transmit a message) will compete again
Another name: Multiple Access protocols
- Competing for
access to the
network can
result in:
- Simultaneous transmissions
- Simultaneous transmissions
are transmission "collisions"
- Result of
a collisions :
- All colliding transmissions from nodes will be lost !!!!
- Nodes involved in a collision
will need to
transmit the frame
again:
- I.e.: nodes need multiple attempts before they will be successful in transmitting
- The contention-based MAC protocols are
also called:
- Multiple Access Protocols
- Nodes that
want to transmit
a message
will
compete
to be the first node
to start transmitting
its message
-
Property of
contention-free
MAC:
- Contention-free
Medium Access Protocols
regulate the
network access
using a special message
called token:
- Contention-free
Medium Access Protocols
regulate the
network access
using a special message
called token:
-
Operation of
content-free MAC
networks:
- A node is
given
the turn
to
transmit a message when
- The node receives a special message called: "token" message.
- When a node
received the token:
- If
the node has some
message
to transmit:
- The node transmits one message and
- The node must "pass" (= send) the token to the next node.
- If the
node does
not
have
any message
to transmit:
- The node must "pass" (= send) the token to the next node.
- If
the node has some
message
to transmit:
- A node is
given
the turn
to
transmit a message when
- Some problems that
contention-free
protocols must resolve:
- Maintain (= keep track of)
the complete list of
nodes:
- How to
add a
new node to the
network
- The new node does not "get" the token without being added !!
- How to
remove a
node from the
network
- If some node sends
the token (message) to
a "deleted" node,
the token (message)
will be lost !!!
(Then the whole network will stop working --- like a broken traffic light that shows red on all intersections)
- If some node sends
the token (message) to
a "deleted" node,
the token (message)
will be lost !!!
- How to
add a
new node to the
network
- Maintain (= keep track of)
the complete list of
nodes:
- The datalink layer for broadcast links consists of
two functional layers:
-
Logical Link Control that
provides
reliable communication
- Medium Access Control (MAC) that provides functions to access (= share) the broadcast network
Graphically:
-
Logical Link Control that
provides
reliable communication
- Logical Link control:
- Provide reliability functionality
(i.e.: sliding window and ACK scheme).
- Provide reliability functionality
-
Real-life
Datalink layer:
- Real-life
datalink layers
(e.g.: Ethernet,
Token ring) for
broadcast networks
do
not implement
the Logical Link control layer (function)
!!!
- Real-life
datalink layers
(e.g.: Ethernet,
Token ring) for
broadcast networks
do
not implement
the Logical Link control layer (function)
!!!
- How to
recover from
transmission errors:
- The error control function is
also implemented in a
higher layer:
- In the Transport Layer (see later !!!)
- The Ethernet/Token Ring protocol
datalink layer
relies (= depends) on the
Transport Layer (TCP !!!)
to recover any
lost messages !!!
(The TCP layer does implement the reliability function)
- The error control function is
also implemented in a
higher layer:
- Medium Access Control:
- The MAC layer implements functions to regulate orderly and fair access to the shared (broadcast) transmission medium.
- We will study a number of commonly used MACs...