Multiplexing

FDM & TDM

Multiplexing When two communicating nodes are connected through

a media, it generally happens that bandwidth of media is several times greater than that of the communicating nodes.

Transfer of a single signal at a time is both slow and expensive. The whole capacity of the link is not being utilized in this case.

This link can be further exploited by sending several signals combined into one. This combining of signals into one is called multiplexing.

Frequency Division Multiplexing (FDM)

This is possible in the case where transmission media has a bandwidth than the required bandwidth of signals to be transmitted.

A number of signals can be transmitted at the same time. Each source is allotted a frequency range in which it can transfer it's signals, and a suitable frequency gap is given between two adjacent signals to avoid overlapping.

This is type of multiplexing is commonly seen in the cable TV networks. 

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

This is possible when data transmission rate of the media is much higher than that of the data rate of the source.

Multiple signals can be transmitted if each signal is allowed to be transmitted for a definite amount of time. These time slots are so small that all transmissions appear to be in parallel.

Synchronous TDM: Time slots are preassigned and are fixed. Each source is given it's time slot at every turn due to it. This turn may be once per cycle, or several turns per cycle ,if it has a high data transfer rate, or may be once in a no. of cycles if it is slow. This slot is given even if the source is not ready with data. So this slot is transmitted empty. 

Synchronous TDM

Asynchronous TDM

In this method, slots are not fixed. They are allotted dynamically depending on speed of sources, and whether they are ready for transmission. 

Connectivity Devices

Bridges, Switches, Routers & Gateways

Connecting Devices

Connecting Devices and the OSI Model

Why Bridges?

It operates both the physical & data link layers of the OSI model.

Bridges can divide a large networks into smaller segments.

Bridges contain logic that allows them to keep the traffic for each segment separate.

It can also provide security through this partitioning of traffic.

A Bridge in the OSI Model

A Bridge

Why Bridges?

When a frame enters a bridge, the bridge not only regenerates the signal but checks the address of the destination and forwards a new copy only to the segment to which the address belongs.

As a bridge encounters a packet, it reads the address contained in the frame and compares that address with a table of all the stations on both segments.

When it finds a match, it discovers to which segment the station belongs and relays the packet only to that segment.

Function of a Bridge

Why Bridges?

Example: Last slide shows two segments joined by a bridge. In figure a, A packet from station A addressed to station D

arrives at the bridge. Station A is on the same segment at station D; therefore,

the packet is blocked from crossing into the lower segment. Instead, the packet is relayed to the entire upper segment

and received by station D. In figure b, a packet generated by station A is intended for

station G. The bridge allows the packet to cross and relays it to the

entire lower segment, where it is received by station G.

Bridge Table

Bridge filtering & forwarding is done by bridge table.

The bridge table contains entries for some, not all of the nodes on a LAN. An entry in the tables contains:- The LAN address of the node The bridge interface that leads towards the node. The time at which the entry for the node was

placed in the table.

Types of Bridges

Simple bridge – is the most primitive and least expensive type of bridge. It links two segments and contains a table that

lists the addresses of all the stations included in each of them.

What makes it primitive is that these addresses must be entered manually.

Whenever a new station is added, the table must be modified.

Installation and maintenance of simple bridges are time-consuming.

Types of Bridges

Multi-port bridge – can be used to connect more than two LANs In the next slide figure, the bridge has three

tables, each one holding the physical addresses of stations reachable through the corresponding port.

Multi-port Bridge

Types of Bridges Transparent bridge – or learning bridge builds it table of station

addresses on its own as it performs its bridge functions. The bridge table is initially empty. When a frame arrives on one of the interfaces and the frame’s

destination address is not in the table, then the bridge forwards copies of the frame to the output buffers preceding all of other interfaces.

For each frame received, the bridge stores in its table (1) the LAN address in the frame’s source address field, (2) the interface form which the frame arrived & (3) the current time.

When a frame arrives on one of the interfaces and the frame’s destination address is in the table, then the bridge forwards the frame to the appropriate interface.

The bridge deletes an address in the table, if no frames are received with that address as the source address after some period of time (the aging time).

Types of Bridges

Spanning Tree Algorithm – Bridges are normally installed redundantly, which means that two LANs may be connected by more than one bridge. In this case, if the bridges are transparent bridges,

they may create a loop, which means a packet may be going round and round, form one LAN to another and back again to the first LAN.

To avoid this situation, bridges today use what is called the spanning tree algorithm.

Types of Bridges

Another solution to prevent loops in the LANs connected by bridges is source routing.

In which, the source of the packet defines the bridges and the LANs through which the packet should go before reaching the destination.

Spanning Tree Problem Figure

Br 1 Br 2

Segment 1

Segment 2

Host B

Host A

Two LANs connected by two bridges

Spanning Tree Problem

This situation occurs due to three factors:- Transparent or Learning bridges are being used

that do not have information about the location of host until they receive at least one packet from them.

The bridges are not aware of the existence of other bridges.

Graph has been created instead of tree.

Switches

The switch has a buffer for each link (network) to which it is connected.

When it receives frame, it stores the frame in the buffer of the receiving link and checks the address to find the out-going link.

Switches are based on two different strategies (called fabrics): Store-and-forward – stores the frame in the input buffer

until the whole packet has arrived. Cut-through-switch – forwards the packet to the output

buffer as soon as the destination address is received.

Switch

Routers

Routers relay packets among multiple interconnected networks.

Routers normally act on network layer. Routers take decision on the basis of layer 3

(network layer) logical addresses (IP address).

A Router in the OSI Model

Routers in an Internet

RoutersRouting Concepts: Least-Cost Routing:

Which path does router choose to send packet over a network? The decision of least-cost routing is based on efficiency; which of

the available pathways are the cheapest or shortest? A value is assigned to each links.

The term shortest, in this context, can mean either of two things depending on the protocol: In some cases, it means the route requiring the smallest number

of relays, or hops; for example, a direct link from A to D would be considered shorter than the route A-B-C-D even if the actual distance covered by the latter is the same or less.

In other cases, shortest means fastest, most reliable or most secure etc. etc.

RoutersRouting Concepts:

When shortest means the pathways requiring the smallest number of relays, it is called hop-count routing, in which every link is considered to be of equal length and given the value one.

RoutersRouting Concepts: Non-adaptive versus Adaptive Routing:

Non-adaptive routing: In some routing protocols, once a pathway to a destination has been selected, the router sends all packets for that destination along that one route. In other words, the routing decisions are not made based on the condition or topology of the networks.

Adaptive routing: In which, a router may select a new route for each packet in response to changes in condition and topology of the networks.

Packet Lifetime (or TTL): Each packet is marked with a lifetime in a network, usually the

number of hops that are allowed before a packet is considered lost and, accordingly destroyed.

Each router to encounter the packet subtracts 1 from the total before passing it on, when the lifetime total reaches 0, the packet is destroyed.

A Gateway in the OSI Model

Gateways

Gateways potentially operates in all seven layers of the OSI model.

A gateway is a protocol converter. In contrast with router, a gateway can accept

a packet formatted for one protocol (e.g., AppleTalk) and convert it to a packet formatted for another protocol (e.g., TCP/IPI) before forwarding it.

A gateway is generally software installed within a router.

A Gateway

Multi-protocol Routers

A router by default is a single-protocol device, which means, if two LANs are to be connected through a router, they should use the same protocol at the network layer.

However, multi-protocol routers have been designed to route packets belonging to two or more protocols.

Single-Protocol versus Multi-protocol Router

Brouters

A brouter (bridge/router) is a single-protocol or multi-protocol router that sometimes acts as a router and sometimes as a bridge.

Brouter