Computing – Communications and Networks

The Network Core

 mesh of interconnected routers
 the fundamental question: how is data transferred through net?
 circuit switching: dedicated circuit per call: telephone nets
 packet-switching: data sent thru net in discrete “chunks” (IP)


The network core is the mesh of routers that interconnect the end systems. In the figure, we
highlights the network core in the thick, shaded lines.
There are two fundamental approaches towards building a network core: circuit switching and
packet switching. In circuit-switched networks, the resources needed along a path (buffers, link
bandwidth) to provide for communication between the end systems are reserved for the
duration of the session. In packet-switched networks, these resources are not reserved; a
session's messages use the resource on demand, and as a consequence, may have to wait (that
is, queue) for access to a communication link.
The ubiquitous telephone networks are examples of circuit-switched networks. Consider what
happens when one person wants to send information (voice or facsimile) to another over a
telephone network. Before the sender can send the information, the network must first
establish a connection between the sender and the receiver.
In modern packet-switched networks, the source breaks long messages into smaller packets.
Between source and destination, each of these packets can take different communication links
and packet switches (also known as routers). Packets are transmitted over each communication
link at a rate equal to the full transmission rate of the link. Most packet switches use store-andforward
transmission at the inputs to the links. Store-and-forward transmission means that the
switch must receive the entire packet before it can begin to transmit the first bit of the packet
onto the outbound link. Thus store-and-forward packet switches introduce a store-and-forward
delay at the input to each link along the packet's route. This delay is proportional to the
packet's length in bits. In particular, if a packet consists of L bits, and the packet is to be
forwarded onto an outbound link of R bps, then the store-and-forward delay at the switch is
L/R seconds.