1.3 The Network Core

Circuit switching and Packet switching

1.Circuit switching

In circuit-switched networks, the resources needed along a path (buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end systems. (As telephone network)

When two hosts want to communicate, the network establishes a dedicated end-to-end connection between the two hosts.
Because each link has n circuits, for each link used by the end-to-end connection, the connection gets a fraction 1/n of the link's bandwidth for the duration of the connection.

Multiplexing in Circuit-Switched Networks:
A circuit in a link is implemented with either frequency-division multiplexing (FDM) or time-division multiplexing (TDM).

2.Packet switching

In packet-switched networks, these resources are not reserved,a session’s messages use the resources on demand, may have to wait for access to a communication link.
If the stall happened, then the packet will have to wait in a buffer at the sending side of the transmission link and suffer a delay. The Internet makes its best effort to deliver packets without any guarantees.

The source break long messages into smaller chunks of data known as packets. Each of these packets travel through communication links and packet switches(two dominant type:routers and link-layer switches) at a rate equal to the full transmission rate of link.

Most packet switches use store-and forward transmission ,store means it must receive entire packet before transmit which can make store-and forward delay.

For each attached link, the packet switch has an output buffer(output queue ), which stores packets that the router is about to send into that link. Packets may suffer output buffer queuing delays.

When arriving packets find that the buffer is completely filled with other packets waiting for transmission, packet loss will occur, either the arriving packet or one of the already-queued packets will be dropped.

Each routers has a forwarding table that maps destination addresses to outbound links.