EXAMPLES OF CONNECTIONS 5
ATM switch 1
ATM switch 2
A B
ATM switch 3
SETUP SETUP
SETUP SETUP
CONNECT CONNECT
CONNECT CONNECT
Figure 1.3 Successful establishment of an ATM connection.
to the requested QoS, without affecting the QoS of other existing connections. When a connection is accepted, the switch allocates bandwidth on the outgoing link for the
connection. It stops accepting new connections when it runs out of bandwidth, or when it reaches a certain percentage of utilization.
The user starts transmitting ATM cells once it receives the CONNECT message. The ATM cells carry two fields in the header – the virtual path identifier VPI and the virtual
connection identifier VCI – which are used to identify the connection. The ATM switch
uses the combined VPIVCI value to pass a cell through its switch fabric. Specifically, as in the case of an IP router, an ATM switch maintains a table that specifies the next
hop for each VPIVCI value. When a cell arrives at a switch, the virtual path and virtual connection identifiers check the table for the next ATM switch. The cell is then switched
through the switch fabric to the output port that connects to the next ATM switch. The ATM table is considerably smaller than an IP forwarding routing table, since it only
contains the existing ATM connections, rather than an entire set of IP addresses.
When user A completes its transmission to B, it tears down the connection by sending a RELEASE message to ATM switch 1. This message is propagated through the switches
along the path, and each switch releases the bandwidth it had allocated to the connection. As we can see, transmitting packets through the IP network is a lot simpler than trans-
mitting cells through an ATM network, since it is not necessary to establish a connection first. On the other hand, by establishing a connection in an ATM network, the network
can provide QoS guarantees that are not possible in an IP network.
1.2.2 An MPLS Connection
MPLS introduces a connection-oriented structure into the otherwise connectionless IP network. An MPLS-ready IP router does not forward IP packets based on the destination
address in the header. Rather, it forwards them based on a label that is very similar in functionality to the VPIVCI value carried in the header of an ATM cell.
Let us consider an MPLS-enabled IP network that runs over Ethernet. In this case, a special MPLS header, sandwiched between the IP header and the LLC header, is used.
The MPLS header contains a label that is a short, fixed-length connection identifier. The MPLS-ready IP router, known as a label switched router LSR, maintains a table of
labels. When an IP packet arrives at the LSR, the label carried in the MPLS header is cross-referenced to the table of labels to find the next hop. The IP packet is then switched
6 INTRODUCTION
to the destination output port of the LSR that connects to the next hop LSR. The table contains labels for only the existing connections, and therefore it is not as large as the
forwarding routing table in an IP router. The procedure is similar to ATM. In order for a user to transmit over an MPLS-enabled
IP network, it has to first request the establishment of a connection. This is done using a signaling protocol, such CR-LDP or RSVP-TE. The connection is known in MPLS
as a label switched path LSP. As in the case of ATM, an LSR is aware of all of the connections that pass through its switch fabric; therefore, it can decide whether to accept
a new connection or not based on the amount of traffic that will be transmitted and the requested QoS. The LSR allocates a portion of its bandwidth to a new connection, and it
stops accepting new connections when it either runs out of bandwidth or reaches a certain percentage of utilization.
1.2.3 A Telephone Connection
The telephone network is probably the oldest connection-oriented network. A telephone switch, known as the central office, serves many thousands of subscribers. Each subscriber
is directly connected to a central office via a dedicated twisted pair line, known as a local loop
. Central offices are interconnected via time-division multiplexing TDM links, such as SONETSDH links and PDH links i.e., T1, E1, T3, and E3.
Figure 1.4 shows two telephones interconnected via two central offices. For presen- tation purposes, let us assume that the two central offices are connected via a T1 line.
Transmission on a T1 line is organized into frames, with each frame containing 24 time slots. Each time slot is 8 bits long and carries a single voice call. The frame repeats
every 128 µsec, meaning that a particular time slot occurs once every 128 µsec i.e. 8000 times per second. Since it carries 8 bits at a time, the total bit rate of a time slot as it
continuously repeats frame after frame is 64 Kbps.
Transmission on a T1 line is unidirectional; that is, data is routed from central office 1 to central office 2. For a bidirectional transmission between the two central offices, two
separate T1 lines – each transmitting in the opposite direction – are needed. In order for subscriber A to talk to subscriber B, a connection has to be first established.
This connection is set up by the telephone network when A picks up the receiver and dials the number for the called party. A signaling protocol is used to set up a connection that
runs through the central offices that are along the path from subscriber A to subscriber B. The connection involves:
1 a dedicated line from subscriber A to central office 1; 2 a time slot e.g. time slot i on the T1 line from central office 1 to central office
2; and 3 a dedicated subscriber line from central office 2 to subscriber B.
Central office 1
Central office 2
Local loop
Local loop
T1 line Subscriber A
Subscriber B
Figure 1.4 A simple telephone network.
EXAMPLES OF CONNECTIONS 7
