We saw in Chapter 1, “Signalling Protocols in the Circuit-Switched Net-work,” that circuit switching was originally developed for analog transmis-sions. Circuit switching provides a dedicated path from source to destination that is suitable for transmitting continuous analog signals such as voice.Frequency Division Multiplexing (FDM) was the most widespread multiplexing technique in circuit-switched analog networks.
The appearance of digital technology enabled circuit switching to evolve.
Information is transmitted between systems in binary format using 0’s and 1’s. Network nodes can easily store digital data and resend it without degrading the quality of the data. This capability enables the use of multi-plexing techniques that are more efficient than FDM, such as Time Division Multiplexing(TDM).
However, even TDM systems make rather inefficient use of the network resources. Voice transmissions fail to use all the available bandwidth in a circuit, none of which can be reassigned for any other purpose.
As if that weren’t enough, data communications brought new communi-cations patterns into the picture. Data traffic is bursty and non-uniform.
Terminals do not transmit continuously, but are idle most of the time and very busy at certain points. Data rates are not kept constant through the duration of the connection either; they vary dynamically. A particular data transmission has a peak data rate and an average data rate associated to it, and these are not usually the same.
Employing dedicated circuits to transmit traffic with these characteris-tics is a waste of resources, as we can see in Figure 2-1. The circuit in the
figure provides 64 Kbps continuously, but the data rate needed by the sys-tems communicating is rarely 64 Kbps; it is usually less and sometimes more. The figure also shows that the available capacity that is not in use by the system at a certain moment is simply lost.
Packet switching was first designed to fulfill the requirements of bursty traffic presented by data. Although the first papers about packet-switching technology appeared in 1961, the first packet-switching node was not imple-mented until the end of the 1960s. In packet switching, the content of the transmitted data determines how network switching is performed. The source nodes chunk the information that will be transmitted into pieces called packets. Then the information needed to route each packet to its des-tination (such as the desdes-tination address) is appended to the packet, form-ing the packet header. The packet is sent to the first network node; in packet switching, network nodes are referred to as routers. Figure 2-2 illustrates a network of routers. When the router receives the packet, it examines the header and forwards the packet to the next appropriate router. This process of looking up the header of the packet is performed in every router in the path until the packet reaches its destination. After reaching the destina-tion, the destination terminal strips off the header of the packet and obtains the actual data that was originated at the source.
Available capacity provided by the circuit
Kbps Capacity used
at a certain moment
Unused capacity = wasted capacity 64 Kbps
Time Figure 2-1
Inefficient use of the network capacity.
Datagrams Routing in packet-switched networks can be performed with datagrams or virtual circuits. Networks using datagrams make routing decisions based solely on the header of the packet that will be routed. The header is examined and the packet is forwarded to the next proper hop. Dif-ferent packets addressed to the same destination might take difDif-ferent routes through the network, enabling load balancing.Packets can avoid congested points in the network by taking alternative paths. As a conse-quence, packets taking different routes will experience different delays, and it’s entirely possible for packet 5, which was sent later than packet 1, to take a faster route and arrive earlier than packet 1. Out-of-sequence arrival of datagrams is remedied by the destination, which reorders them before delivering them to the user.
Figure 2-3 shows how the datagram approach works in the network.
Datagrams traveling from A to B are routed towards B based on the header
Data Header Data
Data
Header Data Header Data
Header Data
Header Data
Header Data
ROUTER
ROUTER
ROUTER
ROUTER ROUTER
ROUTER ROUTER
Figure 2-2 Network of routers.
of each particular datagram. We’ve already recognized that a particular router might decide to route datagrams towards B using alternative paths.
The result of this approach is that datagrams reaching B have traversed different paths and therefore might have experienced different delays.
Virtual Circuits Virtual circuits behave more like circuit switching.
Routing decisions are made based on the header of the packet that will be routed and on the previous packets that traversed the router.
The source, prior to any data transmission, sends a virtual-circuit estab-lishment request to the network. The network then routes the packet con-taining the request towards the destination and assigns a virtual-circuit identifier to the path. Upon reception, the destination confirms the estab-lishment of the virtual circuit by sending an acknowledgement packet to the source. Once the virtual circuit is established, all the packets carrying the data that will be transmitted will be routed through the same path.
Data Destination: B Data
A
B
Data
Destination: B Data
ROUTER
ROUTER
ROUTER ROUTER
ROUTER ROUTER
ROUTER
Figure 2-3 Network using datagrams.
Packet headers on the path incorporate the virtual-circuit identifier that was assigned previously, and routers forward packets accordingly.
Figure 2-4 shows a network based on virtual circuits forwarding packets from A to B. All packets follow the path of the first packet.
Differences Between Datagrams and Virtual Circuits Perhaps the most important difference between datagrams and virtual circuits is the state that each requires the network to store. Datagram routing requires no state information (besides routing tables) inside the network. Networks handling datagrams are stateless in the sense that once a routing decision is made, the router does not store the result in order to make subsequent decisions. Conversely, networks dealing with virtual circuits must store the state of virtual circuits and identifiers to match incoming packets to the proper vitual circuit. Therefore, virtual-circuit networks implement more intelligence in the network than datagram networks.
Each of these approaches to packet routing presents advantages and dis-advantages. Datagrams do not require any establishment time prior to the
Data
A
B
Data
ROUTER
ROUTER
ROUTER ROUTER
ROUTER ROUTER
ROUTER
Virtual Circuit id: 1 Data
Virtual Circuit id: 1 Data Figure 2-4
Network using virtual circuits.
transfer of user data, whereas virtual circuits have to be requested and established before any user data can transmit. When a terminal wishes to send packets to a number of different destinations, this establishment delay is present every time a new destination address is used. In such situations, datagrams provide more flexibility.
On the other hand, once a virtual circuit is established, it usually pro-vides lower overhead than datagrams because headers have less to carry;
they just carry a virtual-circuit identifier. Thus, for long transmissions, net-works using virtual circuits are a better choice.
Datagrams take advantage of dynamic routing in the network. If a node in the network fails or the network becomes congested, packets can imme-diately take alternative routes to avoid stress points, without needing to establish a new circuit. Load balancing is also enabled; datagrams headed for the same destination can take different paths to distribute the network load more evenly. Virtual circuits can handle some load balancing, but when packets always take the same path, redistribution becomes more complicated.