The Media Access Control (MAC) data communication Networks protocol sub-layer, also known as the Medium Access Control, is a sub-layer of the data link layer specified in the seven-layer OSI model. The medium access layer was made necessary by systems that share a common communications medium. Typically these are local area networks. The MAC layer is the “low” part of the second OSI layer, the layer of the “data link”. In fact, the IEEE divided this layer into two layers “above” is the control layer the logical connection (Logical Link Control, LLC) and “down” the control layer The medium access (MAC).
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The LLC layer is standardized by the IEEE as the 802.2 since the beginning 1980 Its purpose is to allow level 3 network protocols (for eg IP) to be based on a single layer (the LLC layer) regardless underlying protocol used, including WiFi, Ethernet or Token Ring, for example. All WiFi data packets so carry a pack LLC, which contains itself packets from the upper network layers. The header of a packet LLC indicates the type of layer 3 protocol in it: most of the time, it is IP protocol, but it could be another protocol, such as IPX (Internet Packet Exchange) for example. Thanks to the LLC layer, it is possible to have at the same time, on the same network, multiple Layer 3 protocols.
In LAN nodes uses the same communication channel for transmission. The MAC sub-layer has two primary responsibilities:
Data encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception. Media access control, including initiation of frame transmission and recovery from transmission failure.
FollowingProtocols are used by Medium Access Layer :
ALOHA : ALOHA is a system for coordinating and arbitrating access to a shared communication channel. It was developed in the 1970s at the University of Hawaii. The original system used terrestrial radio broadcasting, but the system has been implemented in satellite communication systems. A shared communication system like ALOHA requires a method of handling collisions that occur when two or more systems attempt to transmit on the channel at the same time.
In the ALOHA system, a node transmits whenever data is available to send. If another node transmits at the same time, a collision occurs, and the frames that were transmitted are lost. However, a node can listen to broadcasts on the medium, even its own, and determine whether the frames were transmitted.
Carrier Sensed Multiple Access (CSMA) : CSMA is a network access method used on shared network topologies such as Ethernet to control access to the network. Devices attached to the network cable listen (carrier sense) before transmitting. If the channel is in use, devices wait before transmitting. MA (Multiple Access) indicates that many devices can connect to and share the same network. All devices have equal access to use the network when it is clear.
Even though devices attempt to sense whether the network is in use, there is a good chance that two stations will attempt to access it at the same time. On large networks, the transmission time between one end of the cable and another is enough that one station may access the cable even though another has already just accessed it. There are two methods for avoiding these so-called collisions, listed here :
CSMA/CD (Carrier Sense Multiple Access/Collision Detection) : CD (collision detection) defines what happens when two devices sense a clear channel, then attempt to transmit at the same time. A collision occurs, and both devices stop transmission, wait for a random amount of time, and then retransmit. This is the technique used to access the 802.3 Ethernet network channel.
This method handles collisions as they occur, but if the bus is constantly busy, collisions can occur so often that performance drops drastically. It is estimated that network traffic must be less than 40 percent of the bus capacity for the network to operate efficiently. If distances are long, time lags occur that may result in inappropriate carrier sensing, and hence collisions.
CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) : In CA collision avoidance), collisions are avoided because each node signals its intent to transmit before actually doing so. This method is not popular because it requires excessive overhead that reduces performance.
Ethernet : IEEE 802.3 Local Area Network (LAN) Protocols : Ethernet protocols refer to the family of local-area network (LAN)covered by the IEEE 802.3. In the Ethernet standard, there are two modes of operation: half-duplex and full-duplex modes. In the half duplex mode, data are transmitted using the popular Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) protocol on a shared medium.
The main disadvantages of the half-duplex are the efficiency and distance limitation, in which the link distance is limited by the minimum MAC frame size. This restriction reduces the efficiency drastically for high-rate transmission. Therefore, the carrier extension technique is used to ensure the minimum frame-size of 512 bytes in Gigabit Ethernet to achieve a reasonable link distance.Four data rates are currently defined for operation over optical-fiber and twisted-pair cables :
The main disadvantages of the half-duplex are the efficiency and distance limitation, in which the link distance is limited by the minimum MAC frame size. This restriction reduces the efficiency drastically for high-rate transmission. Therefore, the carrier extension technique is used to ensure the minimum frame-size of 512 bytes in Gigabit Ethernet to achieve a reasonable link distance.Four data rates are currently defined for operation over optical-fiber and twisted-pair cables :
10 Mbps – 10Base-T Ethernet (IEEE 802.3)
100 Mbps – Fast Ethernet (IEEE 802.3u)
1000 Mbps – Gigabit Ethernet (IEEE 802.3z)
10-Gigabit – 10 Gbps Ethernet (IEEE 802.3ae).
100 Mbps – Fast Ethernet (IEEE 802.3u)
1000 Mbps – Gigabit Ethernet (IEEE 802.3z)
10-Gigabit – 10 Gbps Ethernet (IEEE 802.3ae).
The Ethernet System consists of three basic elements :
(1) The physical medium used to carry Ethernet signals between computers,
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(2) a set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel, and
(3) an Ethernet frame that consists of a standardized set of bits used to carry data over the system.
As with all IEEE 802 protocols, the ISO data link layer is divided into two IEEE 802 sub-layers, the Media Access Control (MAC) sub-layer and the MAC-client sub-layer. The IEEE 802.3 physical layer corresponds to the ISO physical layer.
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Each Ethernet-equipped computer operates independently of all other stations on the network: there is no central controller. All stations attached to an Ethernet are connected to a shared signaling system, also called the medium. To send data a station first listens to the channel, and when the channel is idle the station transmits its data in the form of an Ethernet frame, or packet.
After each frame transmission, all stations on the network must contend equally for the next frame transmission opportunity. Access to the shared channel is determined by the medium access control (MAC) mechanism embedded in the Ethernet interface located in each station. The medium access control mechanism is based on a system called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
As each Ethernet frame is sent onto the shared signal channel, all Ethernet interfaces look at the destination address. If the destination address of the frame matches with the interface address, the frame will be read entirely and be delivered to the networking software running on that computer. All other network interfaces will stop reading the frame when they discover that the destination address does not match their own address.
IEEE 802.4 Token Bus : In token bus network station must have possession of a token before it can transmit on the network. The IEEE 802.4 Committee has defined token bus standards as broadband networks, as opposed to Ethernet’s baseband transmission technique. The topology of the network can include groups of workstations connected by long trunk cables.
These workstations branch from hubs in a star configuration, so the network has both a bus and star topology. Token bus topology is well suited to groups of users that are separated by some distance. IEEE 802.4 token bus networks are constructed with 75-ohm coaxial cable using a bus topology. The broadband characteristics of the 802.4 standard support transmission over several different channels simultaneously.
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The token and frames of data are passed from one station to another following the numeric sequence of the station addresses. Thus, the token follows a logical ring rather than a physical ring. The last station in numeric order passes the token back to the first station. The token does not follow the physical ordering of workstation attachment to the cable. Station 1 might be at one end of the cable and station 2 might be at the other, with station 3 in the middle.
While token bus is used in some manufacturing environments, Ethernet and token ring standards have become more prominent in the office environment.
IEEE 802.5 Token Ring : Token ring is the IEEE 802.5 standard for a token-passing ring network with a star-configured physical topology. Internally, signals travel around the network from one station to the next in a ring. Physically, each station connects to a central hub called a MAU (multi-station access unit). The MAU contains a “collapsed ring,” but the physical configuration is a star topology. When a station is attached, the ring is extended out to the station and then back to the MAU .
If a station goes offline, the ring is reestablished with a bypass at the station connector. Token ring was popular for an extended period in the late 1980s and 1990s, especially in IBM legacy system environments. IBM developed the technology and provided extensive support for connections to SNA systems. More recently, Ethernet, Fast Ethernet, and Gigabit Ethernet technologies have pushed token ring and other LAN technologies to the sidelines.
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Transmit 5 is available for purchase from panic.com as well as via subscription from the Mac App Store. How do you know which one is best for you? Let’s take a look at the differences--
Mac App Store
Subscription pricing, $24.99 USD/year.
You can use the app on all of your Macs, provided you are the only user.
View and mange your subscriptions: https://support.apple.com/en-us/HT202039
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Benefits
- You get all of the benefits of the Mac App Store: Security, one-click install, easy updates, etc.
- If we ever release a new major update (like a “6.0”), active subscribers would get that version at no additional cost.
Drawbacks
The Mac App Store version of Transmit:
- Does not include the Open In Terminal feature (which was removed at Apple’s request)
- Has minor sandboxing-related limitations
- No longer functions if your subscription expires
- Is only supported on macOS 10.14 (Mojave) or later
Direct from panic.com
One-time purchase, $45 USD
You can use Transmit 5.X on all of your Macs, provided you are the only user.
Benefits
- Buy the app once and you can use Transmit 5.X indefinitely (provided your Mac can run the app, of course!)
- No sandboxing limitations
- Supports the Open In Terminal feature
- Supports macOS versions 10.13+
Drawbacks
- The app must be downloaded and updated from panic.com. We have a built-in updater so once you’ve installed Transmit for the first time, you should never have to worry about it. It’s almost as easy as getting it from the App Store.
- If we ever release an all-new major update (like a “6.0”) there would likely be an upgrade price involved.
FAQs
Wait, so I have to subscribe to Transmit now?
No, my post-skimming friend, you don’t. While Transmit in the App Store is a subscription, you can still buy Transmit 5 directly from us at full price and own it forever for just $45.
I already bought Transmit 5. Can I switch to the App Store version?
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Apple doesn’t give us a way to do this. If you bought Transmit 5 in the last 60 days, we can refund your purchase if you’d like to subscribe on the App Store instead. Simply e-mail our support team with your purchase details and we can take a look. Beyond refunding your purchase, it’s not possible to transfer your license to the App Store.
Why am I being asked to subscribe to Transmit when I’ve already bought a copy of Transmit 5 in the past?
This means you’ve downloaded Transmit 5 from the App Store rather than from panic.com. Unfortunately, the App Store version and the version from panic.com are not 100% the same and are not interchangeable.
The Mac App Store version of Transmit is only available via subscription, so if you have already purchased a non-subscription copy of Transmit 5 from panic.com in the past, you’ll need to download the version from panic.com instead of using the App Store version.
If you have found yourself in this situation, delete the copy of Transmit 5 you’ve got in your Applications folder, then download a copy from our website instead. Once you’ve got the correct version, you will be able to enter your serial number to unlock the app when prompted.
I’m subscribed to Transmit on the App Store but want to purchase the app from panic.com instead. Can I transfer my license?
It is not possible to transfer your license from the App Store directly. As the App Store uses a subscription model and the version from panic.com does not, you would need to first cancel your subscription from the App Store, the purchase a copy from panic.com/transmit. Users can manage App Store subscriptions via the following link: https://support.apple.com/en-us/HT202039
Unfortunately, Apple provides us no way to refund subscriptions in any way. If you’re looking for a subscription refund, we recommend reaching out to Apple support.
Is there a free trial?
Absolutely. When you install Transmit from the App Store, on first launch you’re given 7 days to use it as much as you want, without restriction, until the subscription kicks in, and you can cancel during the trial (here) if it doesn’t work for you. It’s really simple and should give you more than enough time to demo the app and make sure you love it.
What’s included with the subscription?
Transmit and Panic Sync, of course. But you also automatically get any major updates we release in the future if you remain subscribed.
Hey, what if I love the App Store but strongly dislike subscriptions?
Yeah, that’s the one bummer zone in our plan — we’re sorry. But we believe the App Store is suited particularly well for subscriptions, and we’re always looking for a sustainable future building our complex applications.
If my subscription lapses, do I lose my sites/favorites?
Absolutely not. While Transmit won’t function without a subscription, if you use Panic Sync, your sites are safely stored in our spacious, welcoming cloud. You can sync them down in the future, even to a non-subscription copy of Transmit. And if you don’t use Panic Sync, as long as you don’t delete Transmit’s application support files, they’ll be there waiting for when you re-subscribe.
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Now that Transmit 5 is in the App Store, will Transmit support iCloud for sync?
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We’re not planning to. We know it’s a bit of a bother to manage another account, but we really value the ability to debug syncing problems directly.
I was charged for a subscription, but I’d like a refund
Apple gives us no control over refunds in any way whatsoever. You will need to submit a request to Apple for a refund for any App Store app, including Transmit. This can be done by signing into reportaproblem.apple.com with your Apple ID.