Transcript Chapter 2 – X.25, Frame Relay & ATM

Chapter 2 – X.25, Frame Relay &
ATM
Switched Network
• Stations are not connected together
necessarily by a single link
• Stations are typically far apart
• Messages are not broadcast to every station
Three Types of Switched
Communication Network
• Circuit-switched
• Message-switched
• Packet-switched
Circuit-Switched Network
• Before any data can be sent, an end-to-end circuit
must be established
• This circuit is maintained for the duration of the
transfer of all the data
• The data can be digital or analog and the signal
can be either type as well
• Connection is usually full-duplex
• Is inefficient – channel capacity is dedicated for
the duration of the connection
• Example – Pubic telephone system
Circuit-Switched Network –
Cont.
• Bits are transmitted as fast as they are
received – no storage of data at the
intermediate nodes
• Disadvantages
– Both stations must be available at the same time
for data exchange
– Resources in the network are dedicated for the
duration of the transmission
Message-Switched Network
• It is not necessary to establish a dedicated path
between the two stations
• The sending station appends a destination address
to the message
• The message is passed through the network from
node to node
• At each node the entire message is received, stored
briefly, and then transmitted to the next node
Advantages of MessageSwitched Network
• Line efficiency is greater
• Sender and receiver do not have to be
available at the same time
• Duplicate copies of message can be sent to
different destinations
• Message priorities can be established
• Error control and recovery can be built into
the network
Disadvantage of MessageSwitched Network
• Not suited to real-time traffic
• Delay through network is relatively long
and varies considerably
Packet-Switched Network
• Very much like message switching
• Principal external difference is that the length of
the message found internally has a maximum
length
• A typical maximum length is several thousand bits
• Messages above the maximum length are divided
up into smaller units and sent out one at a time
• These smaller units are called packets
• Packets, unlike messages, are typically not filed at
the intermediate nodes
Packet-Switched Network
• The simple rule of limiting the maximum
size of a data unit has a dramatic effect on
performance
• There are two different ways the network
can handle the stream of packets that make
up the message:
– Datagram
– Virtual circuit
Datagram Approach to PacketSwitched Network
• Each packet is treated independently
• The packets may take different paths to the
destination
• The packets might arrive in a different
sequence from the order in which they were
sent
• The packets may have to be reordered at the
destination
Virtual Circuit Approach to
Packet-Switched Network
• A logical connection is established before
any packets are sent
• All packets follow the same path through
the network
• This does not mean that there is a dedicated
path, as in circuit switching
Advantages of the Datagram
Approach
• Call setup phase is avoided
• This is important if a station wished to send only
one or a few packets
• More flexible – incoming packets can be routed
away from congestion when it develops
• Datagram delivery is more reliable – if a node
fails, packets can be sent on an alternate route
Three Examples of PacketSwitched Protocols
• X.25 – Virtual Circuit
• Frame Relay – Virtual Circuit
• ATM – Virtual Circuit
X.25
• Based upon existing analog copper lines that
experience a high number of errors
• Uses the virtual circuit approach
• A set of international protocols approved in 1976
• Provides a way to send packets across a packetswitched public data network
• The redundant error checking is done at each node
• See Figure 2-8 for X.25 encapsulation for IP
datagrams
Frame Relay
• No longer need the overhead associated with X.25
and analog copper wires
• Similar to X.25, but does not have the added
framing and processing overhead to provide
guaranteed data transfer
• Link-to-link reliability is not provided – if a frame
is corrupted, it is silently discarded
• Upper-level protocols such as TCP must detect
and recover discarded frames
• See Figure 2-9 for Frame Relay encapsulation of
IP datagrams
ATM
• Destined to replace most existing WAN
technologies
• Improves on performance of Frame Relay
• Based upon 53-byte cells of fixed size
• 48 bytes of application information together with a
5-byte ATM header
• The standard-sized cells allow switching
mechanisms to achieve faster switching rates
• Rates of 155 – 622 Mbps are achieved with
theoretical rates up to 1.2 Gbps
• Compatible with twisted-pair, coax, and fiber