Ethernet (802.3)


Important Note: Ethernet dalam istilah Computer Network (IEEE 802.3) adalah teknologi layer 2 (data link) untuk menghantarkan data dalam suatu Local Area Network (LAN) dengan membaca MAC Address (dalam bentuk frame), jadi bukan sebuah mesin, alat, atau kabel yang seperti kebanyakan orang bilang

Ethernet itu adalah sebuah protocol

Ethernet Frame Field (di dalam data Ethernet yang dikirim Switch…ada apa aja sih??)

warning: most of this article words are in english…wkwkwk

The Preamble (7 bytes) and Start Frame Delimiter (SFD) (1 byte) fields digunakan untuk sending dan receiving. 8 byte pertama dari frame Ethernet digunakan untuk memberitahu lawan “bicara” bahwa mereka siap untuk menerima atau mengirim frame.

The Destination MAC Address field (6 bytes) is the identifier for the intended recipient. Seperti yang kita tahu bahwa alamat ini digunakan layer 2 (data link) untuk membantu devices menentukan apakah sebuah frame yang ditujukan kepada mereka. The address in the frame is compared to the MAC address in the device. If there is a match, the device accepts the frame.

Length field (or Type Field) defines the exact length of the frame’s data field. This is used later as part of the FCS to ensure that the message was received properly. If the purpose of the field is to designate a type as in Ethernet II, the Type field describes which protocol is implemented. (apakah Ethernet II ini lebih baik daripada Ethernet biasa ataukah Ethernet II ini maksudnya adalah FastEthernet yang digunakan sekarang ini, gw kurang tau…feel free to provide me with the Information)

The Data and Pad field (46 – 1500 bytes) contains the encapsulated data from a higher layer, which is a generic Layer 3 PDU, or more commonly, an IPv4 packet. All frames must be at least 64 bytes long. If a small packet is encapsulated, the Pad is used to increase the size of the frame to this minimum size.

The Frame Check Sequence (FCS) field (4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame.

*gw ga ajarin CRC, out of field gw…liat aja di Wikipedia

Initially, Ethernet was implemented as part of a bus topology. Every network device was connected to the same, shared media. In low traffic or small networks, this was an acceptable deployment. The main problem to solve was how to identify each device. The signal could be sent to every device, but how would each device identify if it were the intended receiver of the message?

using MAC Address Structure

MAC Address = 48 bit size

The MAC address value is a direct result of IEEE-enforced rules for vendors to ensure globally unique addresses for each Ethernet device (kita bisa bilang…ini nomor/alamat Mesin). The rules established by IEEE require any vendor that sells Ethernet devices to register with IEEE. The IEEE assigns the vendor a 3-byte code, called the Organizationally Unique Identifier (OUI) (ada 1 pertanyaan CCNA Exam tentang ginian loh…wkwkw kampret kan..ginian jg ditanyain).

IEEE requires a vendor to follow two simple rules:

  • All MAC addresses assigned to a NIC or other Ethernet device must use that vendor’s assigned OUI as the first 3 bytes.
  • All MAC addresses with the same OUI must be assigned a unique value (vendor code or serial number) in the last 3 bytes.

The MAC address is often referred to as a burned-in address (BIA) because it is burned into ROM (Read-Only Memory) on the NIC. This means that the address is encoded into the ROM chip permanently – it cannot be changed by software. (makanya kadang disebut Physical Address)

However, when the computer starts up, the NIC copies the address into RAM. When examining frames, it is the address in RAM that is used as the source address to compare with the destination address. The MAC address is used by the NIC to determine if a message should be passed to the upper layers for processing. (makanya ada software2/hardware2 buat duplikat MAC address…contohnya aja TP-LINK…ada tuh buat duplicate mac address…biasanya buat Internetan pake Cable modem kek FastNet)

Figure 1 Taken from Wikipedia.org

The source device sends the data through the network. Each NIC in the network views the information to see if the MAC address matches its physical address. If there is no match, the device discards the frame. When the frame reaches the destination where the MAC of the NIC matches the destination MAC of the frame, the NIC passes the frame up the OSI layers, where the decapsulation process take place.

All devices connected to an Ethernet LAN have MAC-addressed interfaces. Different hardware and software manufacturers might represent the MAC address in different hexadecimal formats. The address formats might be similar to 00-05-9A-3C-78-00, 00:05:9A:3C:78:00, or 0005.9A3C.7800 (klo lo liat2 sama…Cuma beda penempatan titk, dash, sama titik dua aja). MAC addresses are assigned to workstations, servers, printers, switches, and routers any device that must originate and/or receive data on the network.

Hexadecimal Numbering on Ethernet

View MAC Address

Ketik di command prompt > ipconfig /all

Sekarang….coba search…di http://standards.ieee.org/develop/regauth/oui/public.html , MAC address lo siapa yang bikin (LAN Card/ NIC lo manufakturnya siapa?? Dari ketik ipconfig /all, alamat MAC uda diketahui kan ?? nah…3 HURUF pertama adalah kode dari vendor / manufaktur LAN Card / NIC lo)

Oh iya…sebelum lupa….

Ketika host send frame…untuk bisa sampai ke tujuan, tergantung tipe koneksinya (Unicast, Broadcast, Multicast)

  1. Klo UNICAST= switch/router akan baca DESTINATION MAC & IP Address
  2. Klo Multicast = switch/router akan baca DESTINATION MAC Address & IP Multicast (
  3. Klo Broadcast = switch/router akan baca BROADCAST MAC & IP Address (FF-FF-FF-FF-FF-FF (dan ga ada IP sama sekali) & 192.168.1. [255] ) – 255: broadcast (kalau subnet mask/prefix nya adalah /24, klo /23, /25, itu itung lagi..bisa dengan VLSM)

Media Access Control in Ethernet (CSMA/CD) – The Process

Collision Domain

Karena metode CSMA/CD sewaktu2 bisa saja terjadi collision, maka kita perlu tahu…daerah mana yang sering terjadi collision

Hub (Multi Port Repeater) = yaitu alat untuk menghubungkan device satu dengan yang lain, 1 Hub = 1 Collision Domain (dikarenakan Hub adalah network devices tipe lama), kalau ga ngerti…penjelasannya ada di bawah ini (Switch)

Switch = fungsi sama dengan Hub, hanya saja karena Switch adalah teknologi baru, dia bisa menghubungkan device yang 1 dengan yang lain dengan resiko ZERO collision, kenapa ?? karena tiap port dalam switch adalah 1 collision domain tersendiri, koq bisa tiap 1 port adalah 1 domain collision sedangkan Hub adalah 1 collision domain tanpa perduli jumlah portnya ??? karena switch bisa menggunakan teknologi multiplexing (memecah data dan mentransfernya tanpa harus tabrakan dengan packet lain), kelemahan dari multiplexing adalah, lebih lambat transfer datanya (kan dipecah2 dulu / segmentasi)

The electrical signal that is transmitted takes a certain amount of time (latency) to propagate (travel) down the cable

Ethernet with throughput speeds of 10 Mbps and slower are asynchronous. An asynchronous communication in this context means that each receiving device will use the 8 bytes of timing information to synchronize the receive circuit to the incoming data and then discard the 8 bytes.

Ethernet implementations with throughput of 100 Mbps and higher are synchronous. Synchronous communication in this context means that the timing information is not required. However, for compatibility reasons, the Preamble and Start Frame Delimiter (SFD) fields are still present.

Type of Ethernet (lebih lengkap)

ARP Process – Mapping IP to MAC Address

ARP means Address Resolution Protocol (Mapping IP to MAC Address), ARP inilah yg dipakai ketika suatu host ingin menemukan host lainnya untuk mengirim datanya

ARP-How it works:

NO ARP Entry at First

Broadcast ARP Frame

Unicast Frame from (owner of destination frame)

Adding MAC Address of Destination Host to its ARP Cache


Standar ARP “ping” (di windows sih) itu dalah 2 menit, jika “tetangga” menjawab, akan ditambah jadi 10 menit, untuk bisa ping2an lagi

As a broadcast frame, an ARP request is received and processed by every device on the local network. On a typical business network, these broadcasts would probably have minimal impact on network performance. However, if a large number of devices were to be powered up and all start accessing network services at the same time, there could be some reduction in performance for a short period of time. For example, if all students in a lab logged into classroom computers and attempted to access the Internet at the same time, there could be delays. (Overhead on the Media)

In some cases, the use of ARP can lead to a potential security risk. ARP spoofing, or ARP poisoning, is a technique used by an attacker to inject the wrong MAC address association into a network by issuing fake ARP requests. An attacker forges the MAC address of a device and then frames can be sent to the wrong destination. (Security)

Manually configuring static ARP associations is one way to prevent ARP spoofing. Authorized MAC addresses can be configured on some network devices to restrict network access to only those devices listed.

*untuk Spoofing dan ARP Poisoning nanti akan kita pelajari

Network Basic Theory 8 (Data Link)

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Hmmm….Layer 5,6,7 udah…Transport yang Layer 4 udah, Network di Layer 3 udah

Kali ini kita akan belajar yang namanya Data Link…dimana tuh ?? Layer 2

Data Link menyediakan pengaturan untuk exchanging data over local media (Layer Data Link menyediakan service untuk mensupport proses komunikasi dari setiap medium ketika data di transmit/dikirim)

2 Service Basic/Dasar dari Data Link Layer adalah:

  • Allows the upper layers to access the media using techniques such as framing
  • Controls how data is placed onto the media and is received from the media using techniques such as media access control (MAC) and error detection

Terminologi2 yang ada di Layer 2 – Data Link

  • Switch: Alat ini BIASA nya ada di layer 2 (walaupun switch2 “canggih” bisa di Layer 3), contoh Layer 2: Switch Cisco Catalyst 2950, Layer 3 Switch / Multilayer Switch: Switch Cisco Catalyst 3650.
  • Frame: PDU dari Data Link Layer (remember *encapsulation)
  • Node: istilah Layer 2 untuk network devices yang terkoneksi dengan media yang sama (media penghantar yang sama)
  • Media/medium (physical) – The physical means for the transfer of information between two nodes
  • Network (physical) – Two or more nodes connected to a common medium

Data Link Layer – Frame

Protocol Data Link layer require control information untuk membuat protocol bekerja. Control information may tell:

  • Which nodes are in communication with each other
  • When communication between individual nodes begins and when it ends
  • Which errors occurred while the nodes communicated
  • Which nodes will communicate next

The Data Link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.

Data Link layer frame includes:

  • Data – The packet from the Network layer
  • Header – Contains control information, such as addressing, and is located at the beginning of the PDU
  • Trailer – Contains control information added to the end of the PDU

Bahasa mudahnya adalah : Data Link layer bertugas menghubungkan antara physical/hardware layer dengan logical/software layer (network layer keatas) dengan menyediakan protocol, encapsulasi, dll

Data Link Sub Layer

  1. Upper Layer : defines the software processes that provide services to the Network layer protocols –> LLC (Logical Link Control)
  2. Lower Layer : defines the media access processes performed by the hardware –> MAC (Media Access Control)

Memisahkan Data Link Layer kedalam sublayer memungkinkan 1 tipe frame di define oleh upper layer (seperti Network Layer) untuk mengakses different type of medium yang di define oleh lower layer (Physical layer), Such is the case in many LAN technologies, including Ethernet

Let’s see these sublayers in brief view:

  1. Logical Link Control

    Logical Link Control (LLC) menaruh informasi di frame yang mengidentifikasikan Network layer protocol mana yang sedang digunakan di frame. This information allows multiple Layer 3 protocols, such as IP and IPX, to utilize the same network interface and media.

  2. Media Access Control

    Media Access Control (MAC) provides Data Link layer addressing and delimiting of data according to the physical signaling requirements of the medium and the type of Data Link layer protocol in use. Mengatur placement data frame kedalam media

Media Access Control (MAC) berguna untuk mendefinisikan proses2 apa sajakah dari setiap network device yang bisa mengakses network media dan mentransmit frames di lingkungan network yang berbeda.

Media Access Control tuh kek ibarat Aturan2 Lalu Lintas yang mengatur keluar masuknya kendaraan di jalan raya. Ketidak hadiran dari MAC ini akan berakibat kendaraan2 yang ada memasuki jalan raya tanpa perduli dengan kendaraan lain (bodo amat beybeh)

Tapi, tidak semua jalan raya dan entrance itu sama. Traffic di jalan raya (kendaraan2) dapat memasuki jalan dengan merging (1 jalan digabung), pake lampu lalu lintas (biar ga macet). Pengemudi mengikuti set aturan2 dari jalan2 yang ada (klo jalannya hanya bole 1 arah…ya satu arah…jgn ngelawan arah 😛 )

The method of media access control used depends on:

  • Media sharing – If and how the nodes share the media
  • Topology – How the connection between the nodes appears to the Data Link layer

Standard2 dalam Data Link Layer

There are two basic media access control methods for shared media:

  • Controlled – Each node has its own time to use the medium
  • Contention-based – All nodes compete for the use of the medium

CSMA: Carrier Sense Multiple Access, Sebuah protocol dimana node yang mo mentransmit data harus men-“listen” sinyal carrier (yaitu sinyal yang menandakan bahwa ada node lain yang lagi transmit data), jika ada…maka node tersebut nunggu sampe sinyal itu hilang/tidak ada

  • CD : Collision Detection

    Contohnya adalah CSMA itu sendiri

  • CA : Collision Avoidance

    Bedanya adalah, ketika sinyal carrier ga ada, host yang intent to send data, kirim pesan “HOI, gw mo kirim data, jgn ada yang make yaaaa” wkwkkw (CSMA/CD = jika sinyal ga ada…langsung try sending, ga pake tegor2an dulu)

There are two basic media access control methods for non-shared media:

  • Half Duplex: (pake gambar aja de..males gw ngetiknya)

  • Full Duplex:

Membahas data link berarti membahas pula tentang topology, khususnya Logical Topology

  • The physical topology is an arrangement of the nodes and the physical connections between them. The representation of how the media is used to interconnect the devices is the physical topology (configurasi kabel, PC, dan alat2 jaringan)
  • A logical topology is the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network independent of their physical layout. These logical signal paths are defined by Data Link layer protocols. The Data Link layer “sees” the logical topology of a network when controlling data access to the media. It is the logical topology that influences the type of network framing and media access control used.

P2P Connection

*Sory untuk Multi Access (pake flash…susah T_T)

Ring Topology Connection (sama ini juga…pake Flash -_-; )

Data Link – Header & Trailer

Typical frame header fields include:

  • Start Frame field – Indicates the beginning of the frame
  • Source and Destination address fields – Indicates the source and destination nodes on the media
  • Priority/Quality of Service field – Indicates a particular type of communication service for processing
  • Type field – Indicates the upper layer service contained in the frame
  • Logical connection control field – Used to establish a logical connection between nodes
  • Physical link control field – Used to establish the media link
  • Flow control field – Used to start and stop traffic over the media
  • Congestion control field – Indicates congestion in the media

The field names above are nonspecific fields listed as examples. Different Data Link layer protocols may use different fields from those mentioned. Because the purposes and functions of Data Link layer protocols are related to the specific topologies and media, each protocol has to be examined to gain a detailed understanding of its frame structure.

Data Link Frame Trailer

How Data Link Works

Pake gambar aja….

  1. Laptop Ngirim Data ke Komputer yang paling ujung…Laptop terhubung ke wireless Access Point (AP)…Data Link yang dipakai adalah data link Wireless Frame

  1. AP terhubung ke Router…Data Link Frame yang dipakai adalah PPP Frame (Point-to-Point Protocol) ke Router

  1. Router kirim Frame ke Router dengan Frame HDLC (High-level Data Link Control, propiertary Cisco, its seems…Both router are Cisco then 😛 )

  1. Router yang ke 2, setelah menerima data link frame HDLC, mengirimkan frame over internet (cloud = Internet) dengan metode Frame Relay

  1. Sisanya ke Router à Ke Switch à Ke Komputer memakai Ethernet Frame

Sebenernya gw mau bahas tentang Ethernet…tapi di Cisco sendiri ada 1 BAB KHUSUS untuk ini..so nanti aja gw jelasin

Next…gw mo kasi mini-guide (bergambar) tentang Follow Data Through Internetwork

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