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4 The Physical Layer
Chapter Objectives
Identify various types of physical media used to exchange information in computer networks.
Summarize the principles of the physical transmission of data.
Explain why analog and digital signals are translated through encoding.
Define the process and primary types of modulation.
Explain how modulation is used to send multiple bits simultaneously.
Differentiate between hubs, repeaters, switches, bridges, and routers.
Describe the role of a network interface card (NIC), media converters, and firewalls in networks.
Discuss security considerations related to home networking devices.
Connecting Hosts Through Physical Media
The first step when building a network, even a worldwide network such as the Internet, is to connect two hosts together, as illustrated below.
Connecting two hosts together
To enable the two hosts to exchange information, they need to be linked together by some kind of physical media. Computer networks have used various types of physical media to exchange information, notably :
Electrical Cable. Information can be transmitted over different types of electrical cables. The most common ones are the twisted pair cable (that are used in the telephone network, but also in enterprise networks) and coaxial cable (that are still used in cable TV networks, but are no longer used in enterprise networks). Some networking technologies operate over the classical electrical cable.
Optical Fiber.Optical fiber is frequently used in public and enterprise networks when the distance between the communication devices is larger than one kilometer. There are two main types of optical fiber: multi-mode and single-mode. Multi-mode is much cheaper than single-mode fiber because a LED can be used to send a signal over a multi-mode fiber while a single-mode fiber must be driven by a laser. Due to the different modes of propagation of light, multi-mode fibers are limited to distances of a few kilometers while single-mode fibers can be used over distances greater than several tens of kilometers. In both cases, repeaters can be used to regenerate the optical signal at one endpoint of a fiber to send it over another fiber.
Wireless. In this case, a radio signal is used to encode the information exchanged between the communicating devices. Many types of modulation techniques are used to send information over a wireless channel and there is lot of innovation in this field with new techniques appearing every year. While most wireless networks rely on radio signals, increasingly some are using a laser that sends light pulses to a remote detector. These optical techniques allow to create point-to-point links while radio-based techniques can be used to build networks containing devices spread over a small geographical area.
These physical media can be used to exchange information once this information has been converted into a suitable electrical signal. Entire telecommunication courses and textbooks are devoted to the problem of converting analog or digital information into an electrical signal so that it can be transmitted over a given physical link. In this book, we only consider two very simple schemes that allow to transmit information over an electrical cable. This enables us to highlight the key problems when transmitting information over a physical link. We are only interested in techniques that allow transmitting digital information through the wire. Here, we will focus on the transmission of bits, i.e., either 0 or 1.
For an overview of physical cables and when to use each, watch this Lawrence Systems video! August 2021. Creative Commons Attribution license (reuse allowed).
Bit Rates
In computer networks, the bit rate of the physical layer is always expressed in bits per second. One Mbps is one million bits per second and one Gbps is one billion bits per second. This is in contrast with memory specifications that are usually expressed in bytes (8 bits), kilobytes (1024 bytes) or megabytes (1048576 bytes). Transferring one MByte through a 1 Mbps link lasts 8.39 seconds.
To understand some of the principles behind the physical transmission of information, let us consider the simple case of an electrical wire that is used to transmit bits. Assume that the two communicating hosts want to transmit one thousand bits per second. To transmit these bits, the two hosts can agree on the following rules :
On the sender side :
set the voltage on the electrical wire at +5V during one millisecond to transmit a bit set to 1
set the voltage on the electrical wire at -5V during one millisecond to transmit a bit set to 0
On the receiver side :
every millisecond, record the voltage applied on the electrical wire. If the voltage is set to +5V, record the reception of bit 1. Otherwise, record the reception of bit 0
This transmission scheme has been used in some early networks. We use it as a basis to understand how hosts communicate. From our viewpoint, dealing with voltages is unusual. We can rely on models that enable them to reason about the issues that they face without having to consider all implementation details. The physical transmission scheme described above can be represented by using a time-sequence diagram.
A time-sequence diagram describes the interactions between communicating hosts. By convention, the communicating hosts are represented in the left and right parts of the diagram while the electrical link occupies the middle of the diagram. In such a time-sequence diagram, time flows from the top to the bottom of the diagram. The transmission of one bit of information is represented by three arrows. Starting from the left, the first horizontal arrow represents the request to transmit one bit of information. This request is represented by a primitive which can be considered as a kind of procedure call. This primitive has one parameter (the bit being transmitted) and a name (DATA.request in this example). By convention, all primitives that are named something.request correspond to a request to transmit some information. The dashed arrow indicates the transmission of the corresponding electrical signal on the wire. Electrical and optical signals do not travel instantaneously. The diagonal dashed arrow indicates that it takes some time for the electrical signal to be transmitted from Host A to Host B. Upon reception of the electrical signal, the electronics on Host B’s network interface detects the voltage and converts it into a bit. This bit is delivered as a DATA.indication primitive. All primitives that are named something.indication correspond to the reception of some information. The dashed lines also represents the relationship between two (or more) primitives. Such a time-sequence diagram provides information about the ordering of the different primitives, but the distance between two primitives does not represent a precise amount of time.
Time-sequence diagrams are useful when trying to understand the characteristics of a given communication scheme. When considering the above transmission scheme, it is useful to evaluate whether this scheme allows the two communicating hosts to reliably exchange information. A digital transmission is considered as reliable when a sequence of bits that is transmitted by a host is received correctly at the other end of the wire. In practice, achieving perfect reliability when transmitting information using the above scheme is difficult. Several problems can occur with such a transmission scheme.
The first problem is that electrical transmission can be affected by electromagnetic interference. Interference can have various sources including natural phenomena (like thunderstorms, variations of the magnetic field,…) but also other electrical signals (such as interference from neighboring cables, interference from neighboring antennas,…). Due to these various types of interference, there is unfortunately no guarantee that when a host transmit one bit on a wire, the same bit is received at the other end. This is illustrated in the figure below where a DATA.request(0) on the left host leads to a Data.indication(1) on the right host.
With the above transmission scheme, a bit is transmitted by setting the voltage on the electrical cable to a specific value during some period of time. We have seen that due to electromagnetic interference, the voltage measured by the receiver can differ from the voltage set by the transmitter. This is the main cause of transmission errors. However, this is not the only type of problem that can occur. Besides defining the voltages for bits 0 and 1, the above transmission scheme also specifies the duration of each bit. If one million bits are sent every second, then each bit lasts 1 microsecond. On each host, the transmission (resp. the reception) of each bit is triggered by a local clock having a 1 MHz frequency. These clocks are the second source of problems when transmitting bits over a wire. Although the two clocks have the same specification, they run on different hosts, possibly at a different temperature and with a different source of energy. In practice, it is possible that the two clocks do not operate at exactly the same frequency. Assume that the clock of the transmitting host operates at exactly 1000000 Hz while the receiving clock operates at 999999 Hz. This is a very small difference between the two clocks. However, when using the clock to transmit bits, this difference is important. With its 1000000 Hz clock, the transmitting host will generate one million bits during a period of one second. During the same period, the receiving host will sense the wire 999999 times and thus will receive one bit less than the bits originally transmitted. This small difference in clock frequencies implies that bits can “disappear” during their transmission on an electrical cable. This is illustrated in the figure below.
A similar reasoning applies when the clock of the sending host is slower than the clock of the receiving host. In this case, the receiver will sense more bits than the bits that have been transmitted by the sender. This is illustrated in the figure below where the second bit received on the right was not transmitted by the left host.
The physical transmission of information through a wire is often considered as a black box that allows transmitting bits. This black box is commonly referred to as the physical layer service and is represented by using the DATA.request and DATA.indication primitives introduced earlier. This physical layer service facilitates the sending and receiving of bits, by abstracting the technological details that are involved in the actual transmission of the bits as an electromagnetic signal. However, it is important to remember that the physical layer service is imperfect and has the following characteristics :
The Physical layer service may change, e.g. due to electromagnetic interference, the value of a bit being transmitted
The Physical layer service may deliver more bits to the receiver than the bits sent by the sender
The Physical layer service may deliver fewer bits to the receiver than the bits sent by the sender
Encoding and Modulation
Data encoding and modulation are interconnected processes used in computer networks to transmit information efficiently and reliably over communication channels. They are essential for converting digital data into analog signals that can be transmitted over various physical mediums, such as cables, fiber optics, or wireless channels. While they serve different purposes, they are closely related and often used together in practical communication systems.
Encoding is the process of converting digital or analog information into a specific format suitable for transmission. It involves transforming the original data into a signal that can be easily transmitted over a communication channel. Data encoding involves representing digital data as a series of symbols or bits, which can then be transmitted over a communication channel. The primary goal of encoding is to ensure that the transmitted signal can be accurately received and interpreted by the receiving device
Analog Encoding: Analog signals can be encoded using methods like Frequency Modulation (FM) and Amplitude Modulation (AM). These techniques modify the carrier signal’s frequency or amplitude, respectively, based on the input signal.
Modulation is the process of modifying a carrier signal’s properties, such as amplitude, frequency, or phase, to carry the information from the source to the destination. Modulation allows the information signal to be transmitted efficiently over long distances without significant loss or interference. The modulated signal can then be demodulated at the receiver to recover the original digital data.
There are various modulation techniques, such as:
Amplitude Modulation (AM): In AM, the amplitude of the carrier signal is varied in proportion to the amplitude of the modulating signal. This variation carries the information.
Frequency Modulation (FM): In FM, the frequency of the carrier signal is varied based on the amplitude of the modulating signal. Changes in frequency represent the information.
Phase Modulation (PM): In PM, the phase of the carrier signal is changed in response to the input signal. Phase shifts correspond to the transmitted data.
In digital communication, encoding is often done before modulation. For instance, in a digital radio transmission, the original digital data might first undergo encoding (e.g., using error-correcting codes), and then the encoded data is modulated onto a carrier signal for transmission. In analog communication, modulation can be considered a type of encoding, where the original information is encoded onto a carrier signal by modifying certain characteristics of the signal.
These techniques help ensure that the transmitted data can be accurately received despite potential noise and interference in the communication channel. The choice of encoding and modulation techniques depends on factors such as the channel characteristics, data rate, and noise level.
Example: Manchester Encoding
Many types of encodings have been defined to transmit information over an electrical cable. All physical layers are able to send and receive physical symbols that represent values 0 and 1. However, for various reasons that are outside the scope of this text, several physical layers exchange other physical symbols as well. For example, the Manchester encoding used in several physical layers can send four different symbols. The Manchester encoding is a differential encoding scheme in which time is divided into fixed-length periods. Each period is divided in two halves and two different voltage levels can be applied. To send a symbol, the sender must set one of these two voltage levels during each half period. To send a 1 (resp. 0), the sender must set a high (resp. low) voltage during the first half of the period and a low (resp. high) voltage during the second half. This encoding ensures that there will be a transition at the middle of each period and allows the receiver to synchronize its clock to the sender’s clock. Apart from the encodings for 0 and 1, the Manchester encoding also supports two additional symbols : InvH and InvB where the same voltage level is used for the two half periods. By definition, these two symbols cannot appear inside a frame which is only composed of 0 and 1. Some technologies use these special symbols as markers for the beginning or end of frames.
Manchester encoding
All the functions related to the physical transmission or information through a wire (or a wireless link) are usually known as the physical layer. The physical layer allows thus two or more entities that are directly attached to the same transmission medium to exchange bits. Being able to exchange bits is important as virtually any information can be encoded as a sequence of bits. Storage devices such as hard-disks also store streams of bits. Hard-disks store sectors of 512 bytes or more. Unix filesystems group sectors in larger blocks that can contain data or inodes representing the structure of the filesystem. Finally, applications manipulate files and directories that are translated in blocks, sectors and eventually bits by the operating system.
Computer networks use a similar approach. Each layer provides a service that is built above the underlying layer and is closer to the needs of the applications. The data link layer builds upon the service provided by the physical layer. We will see that it also contains several functions.
We discussed physical transmission media above. What about physical networking hardware devices? Computer networking devices are units that mediate data in a computer network and are also called network equipment. Units which are the last receiver or generate data are called hosts, agents, or endpoints. Here are some common devices found in networking.
Hubs connect computers together in a star topology network. Due to their design, they increase the chances for collisions. Hubs operate in the physical layer of the OSI model and have no intelligence. Hubs flood incoming packets to all ports all the time. For this reason, if a network is connected using hubs, the chances of a collision increases linearly with the number of computers (assuming equal bandwidth use). Hubs pose a security risk since all packets are flooded to all ports all the time. If a user has packet sniffing software, they can extract data from the network and potentially decode it and use it. Hubs make it easy to “spy” on users on the same LAN.
A repeater is an electronic device that receives a signal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances without degradation. Because repeaters work with the actual physical signal, and do not attempt to interpret the data being transmitted, they operate on the physical layer, the first layer of the OSI model. Repeaters are majorly employed in long distance transmission to reduce the effect of attenuation. It is important to note that repeaters do not amplify the original signal but simply regenerate it.
Modem (from modulator-demodulator) is a device that turns the digital 1s and 0s of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those sounds back into a form used by a USB, Ethernet, serial, or network connection. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or “bps”.
A network interface card is a computer hardware component designed to allow computers to communicate over a computer network. It is both an OSI layer 1 (physical layer) and layer 2 (data link layer) device, as it provides physical access to a networking medium and provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wirelessly. Most motherboards today come equipped with a network interface card in the form of a controller, with the hardware built into the board itself, eliminating the need for a standalone card.
Media converters are simple networking devices that make it possible to connect two dissimilar media types such as twisted pair with fiber optic cabling. They were introduced to the industry nearly two decades ago, and are important in interconnecting fiber optic cabling-based systems with existing copper-based, structured cabling systems. Media converters support many different data communication protocols including Ethernet, T1/E1, T3/E3, as well as multiple cabling types such as coaxial, twisted pair, multimode and single-mode fiber optics. When expanding the reach of a Local Area Network to span multiple locations, media converters are useful in connecting multiple LANs to form one large “campus area network” that spans over a limited geographic area. As local networks are primarily copper-based, media converters can extend the reach of the LAN over single-mode fiber up to 130 kilometers with 1550 nm optics.
Switches are often confused with bridges because they also operate at the data link layer of the OSI model. Similar to a hub, switches provide a central connection between two or more computers on a network, but with some intelligence. They provide traffic control for packets; rather than forwarding data to all the connected ports, a switch forwards data only to the port on which the destination system is connected. They use a database of MAC addresses to determine where computers are located and very efficiently send packets only where they need to go. The database is created dynamically as computers communicate on the network. The switch simply watches the incoming packets and memorizes the MAC address and port a packet arrives on. If a packet arrives with a destination computer that the switch does not have an address for in its MAC address table, it will flood the packet out all connected ports. A switch creates separate collision domains for each physical connection. A switch will only create separate broadcast domains if separate VLANs (Virtual Local Area Networks) are assigned to different ports on the switch. Otherwise, a broadcast received on one port will be flooded out all ports except the one it came in on.
Bridges can be identified by the fact that they operate at the data link layer of the OSI model. Bridges have intelligence and can “bridge” two of their ports together at very high speed. They use a database of MAC addresses to determine where computers are located and very efficiently send frames only where they need to go. The database is created dynamically as computers communicate on the network. A bridge simply watches the incoming frame and memorizes the MAC address and port a frame arrives on. It uses this information to locate a computer if a packet comes in that must be forwarded to it. If a frame arrives at the bridge and the bridge does not know where to send it, the bridge will flood the frame just like a hub does. Bridging is often inaccurately called switching.
A wireless access point (WAP or AP) is a device that allows wireless communication devices to connect to a wireless network using Wi-Fi, Bluetooth or related standards. The WAP usually connects to a wired network, and can relay data between the wireless devices (such as computers or printers) and wired devices on the network.
A typical corporate use involves attaching several WAPs to a wired network and then providing wireless access to the office Local Area Network. Within the range of the WAPs, the wireless end user has a full network connection with the benefit of mobility. In this instance, the WAP functions as a gateway for clients to access the wired network.
A hot spot is a common public application of WAPs, where wireless clients can connect to the Internet without regard for the particular networks to which they have attached for the moment. The concept has become common in large cities, where a combination of coffeehouses, libraries, as well as privately owned open access points, allow clients to stay more or less continuously connected to the Internet, while moving around. A collection of connected hot spots can be referred to as a lily-pad network.
Home networks generally have only one WAP to connect all the computers in a home. Most are wireless routers, meaning converged devices that include a WAP, router, and often an Ethernet switch in the same device. Many also converge a broadband modem. In places where most homes have their own WAP within range of the neighbors’ WAP, it’s possible for technically savvy people to turn off their encryption and set up a wireless community network, creating an intra-city communication network without the need of wired networks.
Routers operate at the network layer of the OSI model and efficiently route information between Local Area Networks. Since routers operate in the third layer, the network layer, they must understand layer 3 addressing… such as TCP/IP. A router will divide a broadcast domain by not forwarding broadcasts on one connected network to another connected network. Routers operate in two different planes: the control plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations, and the forwarding plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.
A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting outward communication. It is also a device or set of devices configured to permit, deny, encrypt, decrypt, or proxy all computer traffic between different security domains based upon a set of rules and other criteria.
Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet. All messages entering or leaving the Local Area Network pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria. Without proper configuration, a firewall can often become worthless. Standard security practices dictate a “default-deny” firewall ruleset, in which the only network connections which are allowed are the ones that have been explicitly allowed.
These devices play essential roles in setting up and maintaining computer networks, whether they are local area networks (LANs), wide area networks (WANs), or wireless networks.
As you look at the physical equipment in your home, you may locate some smart devices, systems, and/or applications. How do you keep these secure when transmitting data?
Router Settings
First, take a look at your router. Most Internet Service Providers (ISPs) usually provide a router with Wi-Fi. Although there are many things to consider with securing your home network, two of the most important are enabling encryption, and having a unique password to access your router’s settings. Note, the password to access your router’s settings is likely (and should be) different from the password to join your Wi-Fi network.
Some routers come with an unique password configured by the ISP; however, some routers (as well as many other devices that connect to the Internet such as smart doorbells, cameras, etc.) come with a default password that is the same for every device the manufacturer makes. This password is listed in the device’s user manual, and the word “admin” is a common choice. If your device has a standard manufacturer’s password, you should change it. As this is a password that you will not use often, be sure to record the changed password in a logical and secure place. If you were given a user manual with your device, locate where in the documentation the default password is listed, and make a note there about your new password.
Smart Devices
With the advent of home automation, many more devices need to connect to our home network. Typically these “smart” devices work just like their regular counterparts (e.g. a smart light switch works like a regular light switch, you can physically press the switch to turn the light on or off), and they also have smart features (e.g. you can program the switch to turn lights on or off at certain times, or use your computing device to turn the light on or off). Here are some examples of smart devices, with some features that these devices may have:
Video Doorbells. Someone rings your doorbell, and you get a notification on your smartphone. You can see who is standing at your door, and talk to the person even if you aren’t home. These video doorbells can double as a security system, taking a picture of anyone who comes in range of the doorbell’s camera.
Smart Locks. Set codes for the keypad, so a key is no longer required. Create different codes for different people, and you can track who is unlocking the door, and when they do it. Want to remove someone’s access to your home, then just delete their access code. Worried you left home and forgot to lock the door – start the smart lock app on your phone and lock the door remotely. Do you forget to lock your door frequently – then set it to automatically lock after a certain time period has elapsed.
Smart TVs. Video streaming services such as Netflix provide on demand access to movies, documentaries, etc. A smart TV will have apps to access common streaming services, and also allow you to connect your phone or computer to the TV so you can view your photos or computer screen on the larger TV screen.
Smart Lighting. Smart wall light switches work like a normal light switch, but can be turned on or off using your computing device, and also can be programmed to automatically turn lights on or off at certain times of the day. Smart light bulbs are similar, but you leave the old light switch always turned on, and turn the smart bulb on and off through the app or a secondary switch.
Smart Thermostats. Do you like to cool your home down just before bed, and would like it to warm up just before you get up in the morning – then install a smart thermostat.
Smart Smoke Alarms. Battery low in your smoke detector – then get a message sent to your phone reminding you to change it. Did one of your smoke detectors go off – get a text message with the details.
Smart Appliances. Is your washer or dryer finished with your clothes – then get a text message reminding you to take out your clothes before they get wrinkled.
Smart Speakers (with microphones). Don’t want to use your phone or your watch to control your home automation, then give it a voice command instead. “Answer the phone” when it rings, or just to “Set the timer for 3 minutes” are just two of many possibilities.
Mass-Market Home Automation Ecosystems
Home automation has been around for many years; however, many early systems were often very expensive. With the introduction of the mass-market systems, home automation prices have dropped substantially.
Some of the more dominant companies with home automation ecosystems (and corresponding voice assistants) are:
Apple HomeKit (Siri)
Google Home Assistant (Hey Google)
Amazon Alexa (Alexa)
Ring Doorbell
These ecosystems have corresponding apps for your smartphone, as well as voice assistants. So if you have your bedroom light on a smart light switch, then to turn off the light you could either:
Go to the light switch and turn it off, or
Go to the app on your smartphone or smartwatch and turn off the light, or
Issue a voice command, e.g. “Siri, turn off the bedroom light”
When you purchase a smart device (such as a video doorbell, smart lock, etc.) remember to check which ecosystems it will work with. Some devices will work with all three of the systems listed above, others only work with one or two of those ecosystems.
Smart Home Security Settings
One of the conveniences of a smart home is that it can be set up to be accessed remotely (i.e. you don’t need to be in your home to control it). So if you are sitting down to dinner at a restaurant and then wonder if you remembered to lock your home’s front door, an app on your smartphone can be used to see if the door is unlocked, and if so, you can lock it without needing to leave your restaurant seat. This convenience (remote access), also means you want to give consideration to a number of things to keep your home automation system secure. Consider the following:
Change the Default Password. If any of your home automation devices come with a “default” password (one set at the factory and it’s the same password for every device), make sure you change this password.
Use a Variety of Strong Passwords. Don’t use the same password you use for all your accounts, and don’t use something that is easily guessed. You want to keep hackers out of your home automation devices (especially important if you have home automation cameras inside your house.
Keep Firmware Updated. For each of your smart devices, if you see on the corresponding app that a firmware update is available, be sure to install it, it may be an update that fixes a security vulnerability.
Consider a Second Wi-Fi Network. If your router allows you to set up two different Wi-Fi networks, consider setting one up for your computing devices (phone/tablet/computer), and a second network for all the home automation devices to connect to. Separating your home automation devices on a separate network from your computing devices will increase security.
Know Your Background Listening Settings: If you can give a command to your smart speaker, phone or tablet without pushing a button (to activate the voice assistant), then your device has been set up to always listen to what you say (this feature can also be turned off). When it is always listening, the device waits for you to say the activation word (e.g. the voice assistant name, such as Siri or Google or Alexa), and then the device assumes the next words you say will be some sort of command it can execute. If you want to test if this feature is enabled, try saying something like “Alexa, what time is it? Background listening can be considered a security issue as sometimes logs of everything that was said within the device’s listening range are kept for the purpose of increasing the accuracy of the voice recognition commands; however, there have been instances of law enforcement and intelligence agencies requesting access to specific people’s logs.
Used in wireless telecommunications for transmission of sound waves in communications or navigation. Britannica. “Electromagnetic Radiation - Radio Waves.” Encyclopædia Britannica, 2019, www.britannica.com/science/electromagnetic-radiation/Radio-waves. Accessed 12 Sept. 2023
A device or node that connects to the LAN or WAN and accepts communications back and forth across the network. Examples include a modem, hub, bridge, switch, router, printer, workstation, etc.
A loss of signal strength from networking cables or connections. An example of attenuation is when a home Wi-Fi signal strength becomes weaker the further a device is from the router. Attenuation affects both wired and wireless connections. Factors that can cause attenuation are physical surroundings such as a wall, distance, and large amounts of frequency and noise.
Also known as an "intermediary" or "middle-man", a proxy serves as a go-between for a client and server. Traffic that is proxied has requests and responses relayed to their respective endpoints. Since a proxy typically maintains its own IP address, the IP of the actual requester provides a surface level anonymity to the server returning the original requester's response as it must be forwarded through the intermediary.
