CCNA Data Center - opening remarks

I am about to complete my CCNA Data Center certification.  I have a CCNA Route & Switch due to expire in August, and with the change in my career focus to data center technologies, I felt it was more appropriate to continue this certification path in the data center instead of route/switch.

I'm beginning with the DCICN 640-911, "Introducing Cisco Data Center Networking" exam.  it covers many of the same topics as the CCNA R/S so I'm figuring I'll have a leg up.

So I purchased the Cisco certification guide and will begin there.

The Cisco guide begins with Networking fundamentals, including the OSI model, ethernet LANs, WAN and IPv4 addressing and routing.  It also goes into the fundamentals of TCP/IP transport and applications.

While I have to admit I don't use it every day, I do recall that the OSI model consists of:

1. Physical Layer - the actual wiring and connectors
2. Data Link Layer - physical addressing, error detection (CRC), Protocols include PPP, Frame Relay, HDLC
3. Network Layer - logical addressing network devices, IP addresses, protocols include IP, IPX, RIP
4. Transport Layer - end-to end communication and defines buffering, window size and flow control, error correction.  Protocols include UDP, TCP, SPX
5. Session Layer - Establish, maintain and tear down sessions. Protocols include SQL, RPC, NFS
6. Presentation Layer - Compression, encryption and decryption happen at L6.  Here protocols include ASCII, JPEG, GIF
7. Application Layer - makes data available to software, protocols include FTP, telnet, HTTP(S), SMTP

Where TCP/IP combines some of these into:

1. Link - OSI L1-L2
2. Internet - OSI L3
3. Transport - OSI L4
4. Application - OSI L5-L7

As data moves from one layer to the next, it encapsulates the data it receives from the previous layer, adds some info to the header and passes it to the next layer.  The terminology for the encapsulated data at each layer is:

1. L1 - bits
2. L2 - frame
3. L3 - packet
4. L4 - segment
5. L5-L7 data

Instead of bits-frames-packets-segments-data, the OSI model uses "Protocol Data Units" such as L7PDU, L6PDU, which consist of the data encapsulated in that particular layer's header and trailer.  Someone should have thought of that before everyone started with the commonly used terminology above...while it's more clear, let's face it...nobody uses that.

And a few other reminders for myself from chapter 1:

Adjacent-layer communication happens within the same computer between different layers, while same-layer communication occurs between different computers.  Adjacent-layer communication is when one layer requests or provides services to the layer above or below it.

The TCP/IP model in recent years shows the Link Layer at level 1 split into 2 layers, the Data Link and Physical layers.